72 MGB Morning Sickness
Discussion
Won't start first thing in the AM. Turn on key, pump strokes once or twice. Crank engine but no start. Fires but quits cold in about 2 sec. Obvious fuel starvation. Detached fuel line at fwd SU and proved pump good by output measure at carbs. Attached bicycle pump to carb fuel inlet and pumped twice to pressurize carbs. Fuel bubbles visible at piston bases. Reattached fuel line, cranked and engine started immediately. Runs, starts and pulls fine all day. Next AM, same thing, same cure. Have changed original needle-seat float valves to ball-type from Moss with no change. Any ideas?
Have primed with raw fuel, both carbs, runs 2 seconds and quits. Have used cockpit choke, no help. Pump strokes only once or twice when ignition first turned on, as though the float bowls were already full. The bicycle pump trick works every time. My symptoms don't yet make complete sense. E.g. Yesterday no start as usual, drained one float chamber by loosening fwd float chamber cover [HIF SUs], did nothing else [no bicycle pump], turned on ign and pump stroked about 5-10 times to fill the empty chamber, engine started right up. Stuck float I guess, but both carbs at the same time every tiime??
In this Northern California climate my engine has never required use of the choke to start. By test and parts replacement I have ruled out the pump and the float valves. My current working theory is that due to an accumulation of some kind of gummy foreign matter, BOTH COLD PISTONS ARE STUCK ALL THE WAY DOWN first thing in the AM, not rising off their seatings when the engine is cranked, leaning the mixture to the point that a cold engine won't start and run. The pistons must come off their seating surface a little to enrich the mixture. If the pistons don't come off their seating, the resulting mixture is at its leanest and a cold start is unlikely. Later in the day, after running some and with the sun up, the pistons are warm and moving freely and thus the engine starts and runs fine. This squares with the fact that the bicycle pump trick works with a COLD ENGINE because one of the things it's doing is RAISING THE PISTONS OFF THEIR SEATS. This AM the first thing I did was to raise both pistons briefly using the pushbuttons that are in the carbs for that purpose and then let both pistons drop. The engine then started promptly and normally, and idled smoothly from the first. Later, with a warm engine idling nicely, I forced both pistons down on to their seating. The engine all but died due to the very lean mixture. When I released the pistons they rose slightly, as expected, the mixture was enriched and the idle returned to normal.
Proof positive will be to catch the cold engine refusing to start and then raising the pistons briefly to break the sticky, doing nothing elst to the mill. I'd bet it will then start just fine.
If that happens, the obvious cure will be to thoroughly clean pistons, chambers, carb inlet passages, jet faces and needles to get rid of whatever foreign matter is causing the sticky, and it would probably be a good idea to flush out the entire fuel system including the tank. But before I do that, I want to catch the rascal in the act.
Proof positive will be to catch the cold engine refusing to start and then raising the pistons briefly to break the sticky, doing nothing elst to the mill. I'd bet it will then start just fine.
If that happens, the obvious cure will be to thoroughly clean pistons, chambers, carb inlet passages, jet faces and needles to get rid of whatever foreign matter is causing the sticky, and it would probably be a good idea to flush out the entire fuel system including the tank. But before I do that, I want to catch the rascal in the act.
Well, so much for my high-blown theories. Sure enough, no start this AM as usual. Lifted the pistons per plan, hit the key and....no start. Drat! The bicycle pump trick worked again thank God but the mystery still is unsolved. My 'sticking pistons' idea was bs, I guess. Realizing I needed to understand SUs better, I found this excellent discussion
http://www.zparts.com/zptech/articles/mal_land/ml_...
According to this fellow, the pistons don't move during a start thus enriching the mixture, not leaning it. Goodbye Sticking Piston Theory.
Because the bicycle pump trick works every time I am now going to assume the jets become blocked after a long layoff and cool temperatures, the air blast from the bicycle pump opens them and the engine runs. So the next plan is to attempt a start and if no start stop right there and pull the carbs. Then, very carefully disasseble them and check the jet inlets for blockage of any kind.
If I find crud, 'good news, bad news'. Good news: Cause found. Bad news: Where is it coming from and how to eliminate it?
More when I know it.
http://www.zparts.com/zptech/articles/mal_land/ml_...
According to this fellow, the pistons don't move during a start thus enriching the mixture, not leaning it. Goodbye Sticking Piston Theory.
Because the bicycle pump trick works every time I am now going to assume the jets become blocked after a long layoff and cool temperatures, the air blast from the bicycle pump opens them and the engine runs. So the next plan is to attempt a start and if no start stop right there and pull the carbs. Then, very carefully disasseble them and check the jet inlets for blockage of any kind.
If I find crud, 'good news, bad news'. Good news: Cause found. Bad news: Where is it coming from and how to eliminate it?
More when I know it.
PositronicRay:
Thanks for listening and trying to help. But regarding your 'choke' suggestion...
1. Over the many, many years I have owned and maintained this car, in this climate my engine has seldom, virtually never, needed choke of any sort to start.
2. The choke feature is known to work. I have applied it to a warm, running engine. The engine runs roughly, belches black smoke and the idle speeds up. Choking the cold engine from the cockpit has not helped with my present difficulties.
3. I haven't touched the carb setup, mixture or linkage, over the history of this symptom.
4. This problem came on gradually a few months back, intermittently at first and then, with no change to the system setup, consistently every AM.
5. The suggestion that the bicycle pump is simply flooding the carbs is opposed by the fact that priming through the carbs with squirts of raw gas causes only a few seconds, two or three at the most, of running and then the mill dies of fuel starvation.
6. No, I have not tried the manual choke idea primarily because I'm alone working on this car. Now it is time to try it if for no other reason than to possibly rule out the 'needs more choke' theory. I will enlist the aid of another geezer who lives down the street. The downside of that will be that he is an unstoppable storyteller and I will have to invest about an hour listening to irrelevancies for 30 seconds of useful help.
Just this morning the engine failed to start again but rather than dismantling the carbs as I had planned I kept at it because the engine ran a second or two on an early try. I did not use the choke at all. It showed more and more signs of life as I continued to give it short bursts of cranking. Eventually it staggered into normal operation. One way to interpret that is that I had gradually cleared the morning crud from the jets until the thing sprang into life. But that's just a theory, like Darwin's Evolution.
Thanks for listening and trying to help. But regarding your 'choke' suggestion...
1. Over the many, many years I have owned and maintained this car, in this climate my engine has seldom, virtually never, needed choke of any sort to start.
2. The choke feature is known to work. I have applied it to a warm, running engine. The engine runs roughly, belches black smoke and the idle speeds up. Choking the cold engine from the cockpit has not helped with my present difficulties.
3. I haven't touched the carb setup, mixture or linkage, over the history of this symptom.
4. This problem came on gradually a few months back, intermittently at first and then, with no change to the system setup, consistently every AM.
5. The suggestion that the bicycle pump is simply flooding the carbs is opposed by the fact that priming through the carbs with squirts of raw gas causes only a few seconds, two or three at the most, of running and then the mill dies of fuel starvation.
6. No, I have not tried the manual choke idea primarily because I'm alone working on this car. Now it is time to try it if for no other reason than to possibly rule out the 'needs more choke' theory. I will enlist the aid of another geezer who lives down the street. The downside of that will be that he is an unstoppable storyteller and I will have to invest about an hour listening to irrelevancies for 30 seconds of useful help.
Just this morning the engine failed to start again but rather than dismantling the carbs as I had planned I kept at it because the engine ran a second or two on an early try. I did not use the choke at all. It showed more and more signs of life as I continued to give it short bursts of cranking. Eventually it staggered into normal operation. One way to interpret that is that I had gradually cleared the morning crud from the jets until the thing sprang into life. But that's just a theory, like Darwin's Evolution.
GeordieMGmike...
Arguing against the 'misadjusted float' theory is the fact that after running beautifully for years, this symptom began out of the blue while the SUs still had the original needles and seats installed. Thinking like you = sticking needle valves [smart man!] I replaced them with aftermarket ball-closure [Grose Jet] valves. No change in behavior.
Also thinking like you [smarter still] I early on gave the float chambers a few smart raps. No sound of the pump filling empty chambers and no change in behavior. Furthermore, if the floats were set incorrectly, how to explain that once I get the thing to run in the AM it starts and runs like a champ all day, until wakeup time tomorrow.
But please keep thinking. This thing will yield its secrets some day and then we'll all be embarrassed at how simple it was. Especially me.
Arguing against the 'misadjusted float' theory is the fact that after running beautifully for years, this symptom began out of the blue while the SUs still had the original needles and seats installed. Thinking like you = sticking needle valves [smart man!] I replaced them with aftermarket ball-closure [Grose Jet] valves. No change in behavior.
Also thinking like you [smarter still] I early on gave the float chambers a few smart raps. No sound of the pump filling empty chambers and no change in behavior. Furthermore, if the floats were set incorrectly, how to explain that once I get the thing to run in the AM it starts and runs like a champ all day, until wakeup time tomorrow.
But please keep thinking. This thing will yield its secrets some day and then we'll all be embarrassed at how simple it was. Especially me.
Latest news flash: This morning I planned to pull the carbs for a close inspection as I mentioned a couple of days ago. I was ready to go ahead with that plan regardless of whether the engine started or not, simply so that I could rule out serious carburetor contamination and crud as the root of my issues. I hoped that the thing would refuse to start so I could catch it redhanded. No such luck. The bloody thing fired right up! [I'm beginning to talk like a ruddy Brit, God forbid!]
But off they came, per the plan. I disassembled them both finding nothing of note in the way of crud, but I did discover evidence of my poor workmanship of the distant past. Both floats were quite a bit out of adjustment, both set to permit a lot more fuel into the float bowls than the book calls for. The height adjustment per the book is 0.04 +/- 0.02" between the machined surface of the float chamber and a spot on the float [look it up in your shop manual for the HIF SUs] Mine were more like 1/8 - 1/4" lower than that, a setting that would result in much more fuel in the bowl and they were different from each other. Deplorable. I have no idea how they ever got that way but I'm the one who must have done it years ago. Nobody else has ever seen the inside of my carbs. I'm surprised that the thing ran as well as it has for decades with the floats set to that level. But, it has. The alert reader would ask why I didn't catch the misadjustment when I replaced the original needles and seats with the Grose Jet units a little while back. The answer is that I stupidly neglected to even check them at the time, assuming the Grose valves were drop-in replacements and how could anything be amiss? Nobody's perfect.
I also checked the several and various ports and passages the HIFs have for emission control, finding nothing out of the ordinary. They all were open and functioning.
So I put everything back together, turned on the key, heard the pump filling both bowls, and then hit 'Start'. The thing lit off and ran smoothly immediately, like it has for many years. I now will run it to warm it up and check the mixture adjustments now that the floats are in spec. Then I'll wait until tomorrow AM to see what if anything I have improved.
Bottom line: I discovered the floats but their misadjustment has existed for a long, long time so that can't have been the cause of my recent difficulties.
One last thing. There is a short run of rubber fuel line in the engine compartment, from the steel delivery tube near the left fender through an in-line fuel filter and on to the carburetors, plus a short length of the same stuff between the carbs. Just to say I've ruled that out too, I replaced both sections and the filter. Neither rubber section was deteriorating internally, I cut them to check, and the filter was wide open to the breath test. No smoking gun there, sorry to say.
Stay tuned.
But off they came, per the plan. I disassembled them both finding nothing of note in the way of crud, but I did discover evidence of my poor workmanship of the distant past. Both floats were quite a bit out of adjustment, both set to permit a lot more fuel into the float bowls than the book calls for. The height adjustment per the book is 0.04 +/- 0.02" between the machined surface of the float chamber and a spot on the float [look it up in your shop manual for the HIF SUs] Mine were more like 1/8 - 1/4" lower than that, a setting that would result in much more fuel in the bowl and they were different from each other. Deplorable. I have no idea how they ever got that way but I'm the one who must have done it years ago. Nobody else has ever seen the inside of my carbs. I'm surprised that the thing ran as well as it has for decades with the floats set to that level. But, it has. The alert reader would ask why I didn't catch the misadjustment when I replaced the original needles and seats with the Grose Jet units a little while back. The answer is that I stupidly neglected to even check them at the time, assuming the Grose valves were drop-in replacements and how could anything be amiss? Nobody's perfect.
I also checked the several and various ports and passages the HIFs have for emission control, finding nothing out of the ordinary. They all were open and functioning.
So I put everything back together, turned on the key, heard the pump filling both bowls, and then hit 'Start'. The thing lit off and ran smoothly immediately, like it has for many years. I now will run it to warm it up and check the mixture adjustments now that the floats are in spec. Then I'll wait until tomorrow AM to see what if anything I have improved.
Bottom line: I discovered the floats but their misadjustment has existed for a long, long time so that can't have been the cause of my recent difficulties.
One last thing. There is a short run of rubber fuel line in the engine compartment, from the steel delivery tube near the left fender through an in-line fuel filter and on to the carburetors, plus a short length of the same stuff between the carbs. Just to say I've ruled that out too, I replaced both sections and the filter. Neither rubber section was deteriorating internally, I cut them to check, and the filter was wide open to the breath test. No smoking gun there, sorry to say.
Stay tuned.
Well, no cure today. This AM I hit the starter and, again, no start. All that effort with the carbs was for naught. The bicycle pump trick worked as usual, so at least I have wheels for the day.
The mantra is 'air, fuel, ignition'. We have air. I do have the poppet valve anti-runon throttle plates so maybe too much air has been suggested but why then IMMEDIATELY after the first start of the day all is well? How about ignition? Same question. Just to be sure, and before the bicycle pump trick, I pulled the plugs and checked to see that they are all firing. They are. Just leaves fuel, right?
It helps me to write things down. The knowns so far seem to be:
1. Long layoffs, i.e. many hours such as overnight, are definitely bad about 90% of the time. In the past two or three weeks I have achieved a normal AM start only 2 or 3 times.
2. Cool i.e. ambient engine and carb temperature MAY be bad. E.g, a few nights ago, after the car had sat outside in cool weather for hours, it refused to start, the only time that has happened except in my own garage. The bicycle pump, which I carry with me always now, got me going. Was it the layoff? Was it the temperature? Both?
3. Raw fuel squirted into the carb inlets does not achieve a start. Never.
4. Pressurizing the float chambers to cause fuel to emerge from the jets achieves a start every time. Always, no exceptions.
4a. After that start, no further problems all day except for #2.
5. I believe the float bowls are full in the AM based on having drained them once after failing to start and the fact that the pump strokes only once or twice before a non-start. [See #7]
5a. After draining a float bowl and securing it, the engine started immediately!
6. The fuel pump is known to be good. It delivers a plentiful flow. I do not know its output pressure but it supplies everything the engine needs once I get the mill to run.
7. The pump will fill an empty float bowl, and when the bowl is full the pump stops, exactly as expected. E.g. I ran the pump until it filled the bowls and stopped, removed the fuel line at the forward carb, plugged it with a drill bit and ran the engine for 90 seconds at 1500RPM with only the fuel in the bowls before it died. As it died it ran for a few seconds on only one carb so I know what that sounds and feels like. When I reattached the line the pump refilled the bowls in about 5 seconds and stopped pumping.
I am reluctantly focusing again [still] on the carburetors, looking for a phenomenon that affects them BOTH after a long layoff. I guess I will have to strip them both to their constituant parts, examine them more closely, clean them meticulously and hope for the best.
How about some of you smart guys suggest a perceptive test or two that will help me draw a better bead on the likely culprit[s]? E.g. I could have shut the engine down half way during the test described in #7 and tried a restart to prove that the engine would/would not start with only 1/2 a bowlfull of fuel. Keep in mind that I already have ruled out faulty choke behavior by warm-engine test. The choke function is alive, causing rough running, black smoke and increased idle speed.
The mantra is 'air, fuel, ignition'. We have air. I do have the poppet valve anti-runon throttle plates so maybe too much air has been suggested but why then IMMEDIATELY after the first start of the day all is well? How about ignition? Same question. Just to be sure, and before the bicycle pump trick, I pulled the plugs and checked to see that they are all firing. They are. Just leaves fuel, right?
It helps me to write things down. The knowns so far seem to be:
1. Long layoffs, i.e. many hours such as overnight, are definitely bad about 90% of the time. In the past two or three weeks I have achieved a normal AM start only 2 or 3 times.
2. Cool i.e. ambient engine and carb temperature MAY be bad. E.g, a few nights ago, after the car had sat outside in cool weather for hours, it refused to start, the only time that has happened except in my own garage. The bicycle pump, which I carry with me always now, got me going. Was it the layoff? Was it the temperature? Both?
3. Raw fuel squirted into the carb inlets does not achieve a start. Never.
4. Pressurizing the float chambers to cause fuel to emerge from the jets achieves a start every time. Always, no exceptions.
4a. After that start, no further problems all day except for #2.
5. I believe the float bowls are full in the AM based on having drained them once after failing to start and the fact that the pump strokes only once or twice before a non-start. [See #7]
5a. After draining a float bowl and securing it, the engine started immediately!
6. The fuel pump is known to be good. It delivers a plentiful flow. I do not know its output pressure but it supplies everything the engine needs once I get the mill to run.
7. The pump will fill an empty float bowl, and when the bowl is full the pump stops, exactly as expected. E.g. I ran the pump until it filled the bowls and stopped, removed the fuel line at the forward carb, plugged it with a drill bit and ran the engine for 90 seconds at 1500RPM with only the fuel in the bowls before it died. As it died it ran for a few seconds on only one carb so I know what that sounds and feels like. When I reattached the line the pump refilled the bowls in about 5 seconds and stopped pumping.
I am reluctantly focusing again [still] on the carburetors, looking for a phenomenon that affects them BOTH after a long layoff. I guess I will have to strip them both to their constituant parts, examine them more closely, clean them meticulously and hope for the best.
How about some of you smart guys suggest a perceptive test or two that will help me draw a better bead on the likely culprit[s]? E.g. I could have shut the engine down half way during the test described in #7 and tried a restart to prove that the engine would/would not start with only 1/2 a bowlfull of fuel. Keep in mind that I already have ruled out faulty choke behavior by warm-engine test. The choke function is alive, causing rough running, black smoke and increased idle speed.
As I looked at my list of 'knowns' I concluded that one potential cause that fits all the data is blockage of both jets by some mysterious goo that loosens when the bicycle pump does its thing, somehow stays out of the way during the day, but then reappears [congeals?] overnight to create at least partial blockage of the jets the next morning. I have no smoking gun for the origin of the mystery goo but a bad tank of gas, perhaps more than one, could do the trick.
I further reasoned that the bicycle pump doesn't have the oomph to blow the jets permanently clear with the metering needle taking up cross sectional area at the face of the jet but does at least clear the jet enough that the engine runs. So this AM I approached the day with a new plan. Whether or not the engine started [it did] I intended to remove the domes and pistons and flush out the jets with the metering needles out of the way.
With the domes, pistons and metering needles off, the bicycle pump attached to the fuel inlet pipe and rags draped over the engine to catch fuel, I pressurized the float chambers. A stream of fuel emerged from each jet, of course. [Have you ever been surprised by your baby boy while changing his diaper? If so, you get the picture.] Interestingly, they were different in quantity and arc range at first. I pumped until the bowls were emptied and only air was coming from the jets. Then I twice more filled the bowls and emptied them. By the third interation the streams of fuel were looking much more the same in quantity and length of arc. I think I have now flushed the jets as well as I can and to do better will require replacement of the jets. Not quite ready to do that yet.
With the domes and pistons handy, I cleaned them thoroughly and gave them each Mr. Twist's 'drop test', which is also described in the MG Shop Manual. My pistons each took 7-8 seconds to fall per the procedure. The book says 4-6 is the spec for my HIF-4s. Close enough, I guess, especially when the book's alternative is to replace both the domes and the pistons, which I am NOT going to do.
With everything back in place, the engine lit off like it always had until this mysterious malady struck. Now we wait and see.
I further reasoned that the bicycle pump doesn't have the oomph to blow the jets permanently clear with the metering needle taking up cross sectional area at the face of the jet but does at least clear the jet enough that the engine runs. So this AM I approached the day with a new plan. Whether or not the engine started [it did] I intended to remove the domes and pistons and flush out the jets with the metering needles out of the way.
With the domes, pistons and metering needles off, the bicycle pump attached to the fuel inlet pipe and rags draped over the engine to catch fuel, I pressurized the float chambers. A stream of fuel emerged from each jet, of course. [Have you ever been surprised by your baby boy while changing his diaper? If so, you get the picture.] Interestingly, they were different in quantity and arc range at first. I pumped until the bowls were emptied and only air was coming from the jets. Then I twice more filled the bowls and emptied them. By the third interation the streams of fuel were looking much more the same in quantity and length of arc. I think I have now flushed the jets as well as I can and to do better will require replacement of the jets. Not quite ready to do that yet.
With the domes and pistons handy, I cleaned them thoroughly and gave them each Mr. Twist's 'drop test', which is also described in the MG Shop Manual. My pistons each took 7-8 seconds to fall per the procedure. The book says 4-6 is the spec for my HIF-4s. Close enough, I guess, especially when the book's alternative is to replace both the domes and the pistons, which I am NOT going to do.
With everything back in place, the engine lit off like it always had until this mysterious malady struck. Now we wait and see.
Yes, I did give the bores and piston diameters a good cleaning, they needed it badly, and used WD40 on the piston rod for the test.
As for dampener oil, I've been using engine oil, 30wt, forever with satisfactory results. Fact is, I so seldom check the dampener that I'm sure I've run hundreds of hours with no oil whatever in the piston rod and can't say I've noticed a difference. But if Mr. Twist says 90wt gear oil, I might give it a try. What's the worst that can happen?
However, others like SU itself say differently, http://sucarb.co.uk/technical-hif-type-carburetter... and my original 43-year-old MGB Handbook, pg. 51, says explicitly in bold, 'Under no circumstances should heavy-bodied lubricant be used'. Anyway, they've got 30wt in them now, the engine is running splendidly, and I have no complaints about the dampeners.
As for dampener oil, I've been using engine oil, 30wt, forever with satisfactory results. Fact is, I so seldom check the dampener that I'm sure I've run hundreds of hours with no oil whatever in the piston rod and can't say I've noticed a difference. But if Mr. Twist says 90wt gear oil, I might give it a try. What's the worst that can happen?
However, others like SU itself say differently, http://sucarb.co.uk/technical-hif-type-carburetter... and my original 43-year-old MGB Handbook, pg. 51, says explicitly in bold, 'Under no circumstances should heavy-bodied lubricant be used'. Anyway, they've got 30wt in them now, the engine is running splendidly, and I have no complaints about the dampeners.
Thanks for all the free advice and especially interesting was the lubricant chart. I was unaware that 'gear oil' viscosity range had a correlation to 'engine oil'. Since 30wt is in the viscosity neighborhood of 80-85wt gear oil and the dashpot action is considerable, I intend to continue to use it.
The good news is that the engine is back to normal, meaning Morning Sickness is gone, at least for now. It starts readily under all the conditions prevailing here in Northern California, morning, noon and night, hot or cold, and no choking is required, just has been the case for decades.
The bad news is that I have no solid understanding why. The ONLY overt actions I ever took, aside from the bicycle pump trick, was to replace rubber fuel lines and the inline filter, open and clean the float chambers, adjust the float levels, replace the original float needle valves and flush the jets as I described a few days ago. Notice I never touched the 'starter' valve, aka the choke. None of those actions revealed a smoking gun. Not even the jet flush because the next morning the engine again refused to start even using the choke. However, the morning after that I used the choke again and the engine started instantly. It has ever since without the aid of the choke, just as I had been experiencing lo these many years.
Some of you continue to focus on the choke function and rigging. I haven't touched the choke setup and it is known to be correctly rigged and functioning. Nor have I adjusted the mixture screws, even when disassembling the floats and removing the jets, which all can be accomplished without disturbing the mixture scews.
So, I'm left perplexed as to 'why' but for now I going to declare victory and wait to see if my luck holds.
The good news is that the engine is back to normal, meaning Morning Sickness is gone, at least for now. It starts readily under all the conditions prevailing here in Northern California, morning, noon and night, hot or cold, and no choking is required, just has been the case for decades.
The bad news is that I have no solid understanding why. The ONLY overt actions I ever took, aside from the bicycle pump trick, was to replace rubber fuel lines and the inline filter, open and clean the float chambers, adjust the float levels, replace the original float needle valves and flush the jets as I described a few days ago. Notice I never touched the 'starter' valve, aka the choke. None of those actions revealed a smoking gun. Not even the jet flush because the next morning the engine again refused to start even using the choke. However, the morning after that I used the choke again and the engine started instantly. It has ever since without the aid of the choke, just as I had been experiencing lo these many years.
Some of you continue to focus on the choke function and rigging. I haven't touched the choke setup and it is known to be correctly rigged and functioning. Nor have I adjusted the mixture screws, even when disassembling the floats and removing the jets, which all can be accomplished without disturbing the mixture scews.
So, I'm left perplexed as to 'why' but for now I going to declare victory and wait to see if my luck holds.
PositronicRay: Yes, I am running a little rich and have been told the exhaust does smell a bit like fuel, which is the stuff you call petrol. Years ago I carefully set up the carbs per the Shop Manual and found the result to be lean. So lean the engine staggered at hiway speeds. So, in small increments I richened the mixtures until it smoothed out. It didn't take much and it's been there ever since. At the time California required that I smog check the car. Even rich as it was it passed easily. Since the result was a smooth running machine that started easily and passed smog check, I figured I had a good solution. Mileage is OK, I get about 20mpg around town, 23 on long stretches. What's your experience?
By the way, I installed an overdrive transmission a few years ago. It greatly improved the driving comfort at hiway speeds but did nothing for fuel economy. Nor should it. Overdrive or not, it takes the same BHP, thus gph, to move the car at 65mph. Any savings would come from a tiny reduction in friction HP, not noticeable in the big scheme of things.
By the way, I installed an overdrive transmission a few years ago. It greatly improved the driving comfort at hiway speeds but did nothing for fuel economy. Nor should it. Overdrive or not, it takes the same BHP, thus gph, to move the car at 65mph. Any savings would come from a tiny reduction in friction HP, not noticeable in the big scheme of things.
It's been over a month since I last checked in about my MGB Morning Sickness. What's been happening over that time is a long story which I won't relate. The short form is that I have been trying a lot of things with no real solution found. Until now.
HIF pistons, perhaps uniquely, have a tiny plastic button inserted into the bottom of the piston. My 43-year-old buttons had worn to nothing, virtually flush with the piston face. I replaced them, $8,50 from Mr. Joe Curto, http://joecurto.com. The new buttons project 0.030" from the piston face and serve to hold the piston face off the bridge when everything is at rest and, presumably, during a start.. It seems that's important, otherwise why is the button there? My engine's starting behavior immediately changed. For several days now I have been able to reliably start the engine first thing in the morning, sometimes without using the choke but always if the choke function is used. The bicycle pump is no longer needed but I'm carrying it in the car just in case. I'm calling this a solution without knowing exactly what is happening or why. Can anybody shed light on why these buttons are there and how they make a difference during the starting event? I have a theory but I'll let the rest of you weigh in with yours.
Take a look at Mr. Solihull's posting at http://www.mg-cars.net/mgb-technical-bbs/hif-carb-... [May not be available on your server] Scroll down to the next-to-last entry to see a sketch of the HIF bridge and portings. The sketch accurately shows my '72 carburetors.
HIF pistons, perhaps uniquely, have a tiny plastic button inserted into the bottom of the piston. My 43-year-old buttons had worn to nothing, virtually flush with the piston face. I replaced them, $8,50 from Mr. Joe Curto, http://joecurto.com. The new buttons project 0.030" from the piston face and serve to hold the piston face off the bridge when everything is at rest and, presumably, during a start.. It seems that's important, otherwise why is the button there? My engine's starting behavior immediately changed. For several days now I have been able to reliably start the engine first thing in the morning, sometimes without using the choke but always if the choke function is used. The bicycle pump is no longer needed but I'm carrying it in the car just in case. I'm calling this a solution without knowing exactly what is happening or why. Can anybody shed light on why these buttons are there and how they make a difference during the starting event? I have a theory but I'll let the rest of you weigh in with yours.
Take a look at Mr. Solihull's posting at http://www.mg-cars.net/mgb-technical-bbs/hif-carb-... [May not be available on your server] Scroll down to the next-to-last entry to see a sketch of the HIF bridge and portings. The sketch accurately shows my '72 carburetors.
If the link I provided in the last entry for a picture of my '72 HIF pistons and bridge areas doesn't work for you, try mg-cars.net, click on MG, click on MGB Technical, click on HIF carb setting, scroll down to the next-to-last entry. As for the latest news, I am finding that use of the choke is now necessary in the morning, but at least the engine starts and runs smoothly. It also seems that a little attention to mixture tuning is now called for with the installation of the little buttons. I will do that next and see how, if at all, the starting behavior is affected..
Several days have passed since I installed new buttons and the results are good. With full choke on the first start of the day the engine lights off 100% of the time and for the rest of the day no choke is needed. That is a real change in cold starting behavior and I think I'm done with this issue. But what accounts for the change?
Here's my explanation of my starting issues and the role the buttons play in mitigating them. First I'll describe how I understand the SU carbs to work in general. In particular I visualize the carb in very low air flow such as when starting. If you can poke a hole in this explanation, go ahead.
1. As air flows across the bridge the piston is caused to rise [or fall] to maintain a constant, pre-established pressure at the bridge, or venturi section. 'Constant depression', so-called. That pre-established pressure is determined by the spring in the dome which is balanced by the difference of venturi pressure and ambient pressure acting across the piston, venturi pressure on top, ambient below.
2. There must be sufficient air flowing over the bridge, thus lowering venturi pressure, to cause fuel to flow past the needle and into the carb throat.
3. If the piston is resting flat and tight on the bridge, air flow area does not exist, air will not flow between the bridge and the piston face, the necessary 'constant depression' conditions will not be established and fuel will not be drawn from the jet and into the carb throat.
4. At constant engine RPM [thus constant air flow rate] raising the piston manually will weaken the mixture, not richen it, because the 'depression' pressure is increased due to a larger venturi area having been created between the piston face and the bridge surface. When the depression pressure is increased, fuel flow will decrease, weakening the mixture.
5. So, to establish correct fuel/air mixture ratio for starting, a] there must be SOME venturi area created for air to flow and fuel to be drawn from the jet, but b] too much area will so weaken the resulting mixture that it will not ignite. It is necessary for some venturi area to exist at the bridge but not too much. The buttons do that job at very low air flow.
6. With my old buttons worn to nothing, insufficient venturi area at the bridge prevented the necessary 'depression' to exist during a low-air-flow event like cranking at starting revs. Sufficient fuel did not flow from the jet and the mixture was too weak to ignite UNTIL a generous prime was provided [the bicycle pump]. The prime allowed the engine to spin however briefly. Once the engine began to spin a lot more air flowed past the bridge, pressure at the bridge was reduced [aka 'depression'], the piston was raised to establish the correct depression pressure, fuel was drawm from the jet and normal carburetion occurred. After the first start of the day a warm engine would start on a weaker mixture, or sufficient residual fuel/air mixture remained in the manifold to permit a start for at least a few turns until the carbs began to function. A dead-cold engine required such a rich mixture to start that not even the starting valves were sufficient without the contribution of the main jet, which was lacking due to worn buttons.
7. With the buttons in place, the pistons are now raised 0.030" off the bridge, sufficient venturi area exists to allow normal carburetion, and with a little choke to finish the job the engine now starts reliably.
8. The fact that this problem came on gradually over months is consistent with worn/wearing buttons. It doesn't take much lift of the piston off the bridge to allow a start but as my buttons wore to nothing, eventually no lift was happening and a cold start was not possible.
I'll stick with this explanation until someone refutes it or the car develops Morning Sickness again.
Here's my explanation of my starting issues and the role the buttons play in mitigating them. First I'll describe how I understand the SU carbs to work in general. In particular I visualize the carb in very low air flow such as when starting. If you can poke a hole in this explanation, go ahead.
1. As air flows across the bridge the piston is caused to rise [or fall] to maintain a constant, pre-established pressure at the bridge, or venturi section. 'Constant depression', so-called. That pre-established pressure is determined by the spring in the dome which is balanced by the difference of venturi pressure and ambient pressure acting across the piston, venturi pressure on top, ambient below.
2. There must be sufficient air flowing over the bridge, thus lowering venturi pressure, to cause fuel to flow past the needle and into the carb throat.
3. If the piston is resting flat and tight on the bridge, air flow area does not exist, air will not flow between the bridge and the piston face, the necessary 'constant depression' conditions will not be established and fuel will not be drawn from the jet and into the carb throat.
4. At constant engine RPM [thus constant air flow rate] raising the piston manually will weaken the mixture, not richen it, because the 'depression' pressure is increased due to a larger venturi area having been created between the piston face and the bridge surface. When the depression pressure is increased, fuel flow will decrease, weakening the mixture.
5. So, to establish correct fuel/air mixture ratio for starting, a] there must be SOME venturi area created for air to flow and fuel to be drawn from the jet, but b] too much area will so weaken the resulting mixture that it will not ignite. It is necessary for some venturi area to exist at the bridge but not too much. The buttons do that job at very low air flow.
6. With my old buttons worn to nothing, insufficient venturi area at the bridge prevented the necessary 'depression' to exist during a low-air-flow event like cranking at starting revs. Sufficient fuel did not flow from the jet and the mixture was too weak to ignite UNTIL a generous prime was provided [the bicycle pump]. The prime allowed the engine to spin however briefly. Once the engine began to spin a lot more air flowed past the bridge, pressure at the bridge was reduced [aka 'depression'], the piston was raised to establish the correct depression pressure, fuel was drawm from the jet and normal carburetion occurred. After the first start of the day a warm engine would start on a weaker mixture, or sufficient residual fuel/air mixture remained in the manifold to permit a start for at least a few turns until the carbs began to function. A dead-cold engine required such a rich mixture to start that not even the starting valves were sufficient without the contribution of the main jet, which was lacking due to worn buttons.
7. With the buttons in place, the pistons are now raised 0.030" off the bridge, sufficient venturi area exists to allow normal carburetion, and with a little choke to finish the job the engine now starts reliably.
8. The fact that this problem came on gradually over months is consistent with worn/wearing buttons. It doesn't take much lift of the piston off the bridge to allow a start but as my buttons wore to nothing, eventually no lift was happening and a cold start was not possible.
I'll stick with this explanation until someone refutes it or the car develops Morning Sickness again.
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