Carb conversion rough idle and misfire
Discussion
Hey guys, posted in the mini section but seems to be inactive. new to the forums so I’ll give a overview before I start my rant. Got a 1995 mini Mayfair around 2 years ago for my first car, restored it all and finally got it back to where I needed it to be. I started to learn to drive in it and straight away the spi started giving me problems, it wouldn’t start, it would cut out and had very bad hesitation when accelerating. The car has a high lift cam fitted along with a straight through decat exhaust and lcb manifold. So after all the problems with the spi I decided to do a carb conversion. I completed this about 2 weeks ago, I have a facet fuel pump delivering ~6psi to the carb which is a Nikki downdraught twin choke carb. The car runs worse than it did before frown it usually starts up fine but has a very rough idle at around 1200 rpm which I heard is about right for a mini fitted with a sportier cam, it hunts anywhere from 1100-1500 rpm no matter the temp of the engine. When just taking it easy around the car wants to stall, as soon as it gets up in the revs it’s perfect but revving it everywhere gets a bit tiring. Also the car seems to have a flat spot when accelerating. The ecu is still currently fitted to the car just running the timing although I have an A+ Dizzy to go on hopefully this weekend. Also the car overruns everytime I turn it off, dropping the clutch in 4th gear getting tiring. Any help of the problem would be hugely appreciated
catman said:
Most carbs are supplied for a standard engine, so was yours jetted to take account of the mods that you made?
Have you done all the work yourself? Running on can be bad timing, weak mixture, air leaks etc. If you haven't got a distributor fitted, what have you done with the vacuum take off point on the carb?
I believe the carb was taken off a standard engine but I’m not too sure, there wasn’t much of a description of it before I purchased it. Yeah I’ve done it all myself, I have a dizzy fitted but it’s still connected to the ecu controlling the timing as it was before the carb conversion, all the vacuum lines are still connected in the same way when it had the spi throttle body and manifold.Have you done all the work yourself? Running on can be bad timing, weak mixture, air leaks etc. If you haven't got a distributor fitted, what have you done with the vacuum take off point on the carb?
catman said:
A high idle speed will also cause an engine to run on. What cam have you put in? I don't know whether you can buy different jets for a Nikki carb, but I seem to remember that they used to suffer from icing up. I think that you would be lucky to buy a second hand one that would be right for your modified engine.
Your carb could also be worn, if second hand. You can have problems if the butterfly spindles are worn, for example. I used to use Weber 28/36 carbs back in the days of Mk 1 Cortinas.
They always ran beautifully and were easy to set up. You may need to have it set up on a rolling road.
I have a 264 Evo Fast Road Cam installed in the engine. Parts for the Nikki carbs seem to be almost impossible to come by, starting to regret my decision of purchasing one and wish I maybe stuck with a simple SU. The butterflies seemed to be okay when I rebuilt the carb and had little to no play in fully closed and fully open positions. I’m installing the A+ dizzy tomorrow so this will allow me to rule out any problems with the timing.Your carb could also be worn, if second hand. You can have problems if the butterfly spindles are worn, for example. I used to use Weber 28/36 carbs back in the days of Mk 1 Cortinas.
They always ran beautifully and were easy to set up. You may need to have it set up on a rolling road.
Edited by catman on Monday 16th November 20:27
oilrag1 said:
Im firstly thinking fuel pressure is too high ,from my own experience with an spi conversion the fuel pump we used was a facet cube 1.5 to 4 psi . im also assuming the brake servo(if still fitted) vacuum pipe has no leaks,and is the ignition vacuum advance working.
Yeah after looking around realised it was quite high, could this alone be causing my problems? I have a FPR but it’s not the best and ranges from 0-140psi and have read that these can give a quite large difference in the pressure set. Should I install this and see if I can get the pressure down a bit? The servo isn’t fitted at the mo so no leaks from there although will be fitted soon, at least I know that’s not leaking for now. The vacuum advance should be working fine as it is the same set up as what was in the spi just with a different manifold. Although even with the spi it seemed like it had timing issues so I’m hoping getting rid of the ecu and setting the timing manually may help me with my problemscatman said:
Yes, Su carbs are great and there are lots of spares around too. I agree that the fuel pressure is high, which may well give you a higher level in the carb. Didn't they have a see through panel to check?
Regarding the distributor, they used to use different springs on the weights, to suit the power curve of the many states of tune. It may be worthwhile in talking to one of the experts on your particular engine.
It's always been a problem in changing things like cams etc, but then trying to get the engine to run on other parts that were only designed for the standard set up.
Just to add, the ignition timing will probably be different to standard too, due to the cam. The manufacturer should be able to advise you on that.
Yeah the float level in the carb is perfect, stays right on the line which is across the glass viewing window of the carb which is why I’m thinking it’s not the high fuel pressure. Okay I’ll have a look on how to set timing with a sportier cam, would this mean I need to advance the timing a bit more?Regarding the distributor, they used to use different springs on the weights, to suit the power curve of the many states of tune. It may be worthwhile in talking to one of the experts on your particular engine.
It's always been a problem in changing things like cams etc, but then trying to get the engine to run on other parts that were only designed for the standard set up.
Just to add, the ignition timing will probably be different to standard too, due to the cam. The manufacturer should be able to advise you on that.
Edited by catman on Wednesday 18th November 23:42
NMNeil said:
When you replaced the FI with the carb, did you have any electrical plugs left over?
If the PCM doesn't get an input from a sensor they will often go into 'limp home' mode, which could possibly explain some of your symptoms.
If the PCM doesn't get an input from a sensor they will often go into 'limp home' mode, which could possibly explain some of your symptoms.
Uh oh..... I had many electrical plugs left over
. I had read online though that the ecu controls the fueling and timing of the engine completely separately and only uses the vacuum from the manifold (which is still being provided to the ecu) to advance the timing. If it were in limp mode would this affect the timing in anyway?? Hoping to fit the new dizzy tomorrow so fingers crossed this will fix most of the problemsEdited by Callump on Thursday 19th November 00:08
NMNeil said:
I know that most limp modes limit the RPM, and it's possible they do that by retarding the timing.
As you have a different distributor on the way, that would remove the PCM from the list of suspects completely.
Fitted the dizzy today, ready to try and start.... flat battery As you have a different distributor on the way, that would remove the PCM from the list of suspects completely.
will try and get her started tomorrow and get the timing sorted. PhilF329 said:
That’s a very high idle speed. I’ve had a lot of modified minis and never had one idle as fast as that, albeit on SU’s or Weber’s. Back to basics I think - hope you get it sorted
That’s what I was thinking, the only way it would idle was to have it that high, hopefully when I get the timing properly sorted with the A+ dizzy I’ll be able to drop the idle a bitDarkslider said:
Think you should go back to basics on this one, check compression hot and cold, valve clearances (if they're adjustable) timing, fuel pressure at the carb, spark strength and condition (accuspark make a set of testers you can bridge the cap to plug with to eyeball while you're revving the engine) exhaust rich or lean? (Presuming no lambda on these despite the SPI?)
Too much chance of you chasing a more and more obscure seeming fault because you've overlooked something basic that you weren't to know as the car obviously has an interesting past!
Cheers for all the help guys pretty much sorted now car was running earlier but timing is a bit off, just done to the point where it’ll run atm. Compression i know is fine as car was running a few weeks back. Valve clearances were set maybe a month or two ago when rebuilding the engine. Timing is the thing I need to set now. Pressure is ~6psi which is too much from what I’ve heared although the fuel fills to the float line and stops perfectly, is this okay or could a too high pressure do any other things than force the float open? Spark seems quite strong took all four out yesterday and checked. Spark plugs seemed perfect Sandy colour although 2 were quite white on parts of the spark plug so possibly a bit hot?? Too much chance of you chasing a more and more obscure seeming fault because you've overlooked something basic that you weren't to know as the car obviously has an interesting past!
Did some more work today, no luck, the car will start after a while but cuts out straight away and is definitely not firing on all 4 cylinders despite having strong spark in all 4. I checked out my dizzy and I think it has a 45d point in it, but the dizzy is a 59d dizzy, I think this could be causing my problems.... I checked out the electronic ignition dizzy after this and it was gross inside, rotor arm and points were badly visible under all the cr*p on top of them. I’m thinking I’m going to put the ecu back in, clean up the electronic ignition dizzy and see how it runs as it’s fairly easy to install. I have read online that the timing and fueling on the ecu are not linked in anyway, so not having the fi throttle body connect won’t throw any issues with timing from the ecu. I will try and get either a 59d dizzy rebuild kit or buy another dizzy with the correct points and try to set it all up without the ecu soon but due to it atleast being close to driveable before when the ecu was controlling the timing I’m going to use this in the meantime, hopefully how gunked up the dizzy was was the main cause of my problems. Will report back tomorrow with my findings.
MB140 said:
Erm your post at 19:16 yesterday implies it was running okay and the fault was fixed. Which is it.
Or has it started having problems again since yesterday.
It was running yesterday same as today, but yesterday I got it running to the point where it sounded normal and then it went bad again, I’m not too sure if the points moved in the dizzy or what but it was cranking, fired up nicely for maybe 5 seconds then cut out and wouldn’t really start again. So I thought it was just something I have to mess about with a bit now to get it all good but after finding it has the wrong points in the dizzy I think it would be better to wait and get all the correct parts. Please correct me if I’m wrong and if the 45d dizzy points can be used in the 59d dizzy but they just need a bit of tweaking, any help to get it running properly now Or has it started having problems again since yesterday.
GreenV8S said:
Does it have a vac advance on the dizzy? If so, confirm it's connected to the base plate properly. If that has come adrift you will have random and inconsistent timing.
All the vac lines were connected securely, I tried to check it was working by sucking on the advance and the plate shifted, I didn’t really know any ways to test it. The plate inside was all secure so I assumed it was okay.Okay, cleaned up the electronic dizzy today, fitted, fired straight up no issues running great. Was idiling a bit high so took it down to maybe 1200 which is still high but it hunts quite a lot. Also when accelerating it pulls to maybe 3000 rpm then there seems to be no power after this point. Any suggestions on what my next move should be? I’m still going to fit an A+ dizzy eventually, but anything I can check out in the mean time to help sort the hunting idle and lack of power would be great. Cheers guys
catman said:
Hi, regarding the idle, are you adjusting the mixture screw as well as the throttle adjustment?
The loss of power higher up could be many things, including having the wrong jets in the carb, incorrect timing for the new cam, incorrect advance for the new cam.
Fairly basic, but have you checked how much the carb opens when you floor the gas pedal? How does the engine actually behave higher up?
I’m only adjusting the idle screw, should I be doing both? My colour tune got delivered the other day so I’m going to get that on ASAP and see how it’s looking. Problem is spare parts for the Nikki carbs are hard to come by and the timing is all controlled by the ecu The loss of power higher up could be many things, including having the wrong jets in the carb, incorrect timing for the new cam, incorrect advance for the new cam.
Fairly basic, but have you checked how much the carb opens when you floor the gas pedal? How does the engine actually behave higher up?
does anyone know anywhere I can get some Nikki jets or would any other jet work aslong as its the correct size? When the throttle is fully open both chokes on the carb are fully open. It just seems to have no power, like after say 3000 rpm I put the clutch in by carry on building the rpms of the engine, but less noticeable.Edited by Callump on Monday 23 November 17:26
catman said:
Getting the idle right can only be done by using the mixture screw and the throttle stop screw. When the engine hunts, slowly turn the mixture screw in or out until it runs smoothly, then slow the engine down with the throttle stop screw. Keep repeating until you get a steady idle at sensible revs.
As mentioned though, you have put several parts on that don't really match. You may spend hours trying to find a missing fault, but it may just be that the parts fitted don't work together. It may be worth talking to a Mini specialist, to find the best way to achieve what you want.
Will do this tomorrow thank you, sure I’m pretty sure the cam was for a carb’d engine as I was planning to do a carb conversion as soon as my car was running due to all the things I had heard about the spi. Sometimes a tight budget comes back to bite you in the a*se. Although this is my first car and I haven’t even passed my test yet, as much as i want it to be running the best I can get it I’m not looking to get any crazy figures from the car just want a fun (and hopefully reliable) car. As mentioned though, you have put several parts on that don't really match. You may spend hours trying to find a missing fault, but it may just be that the parts fitted don't work together. It may be worth talking to a Mini specialist, to find the best way to achieve what you want.
Edited by Callump on Monday 23 November 21:09
Hey guys. I fitted a brand new 59d dizzy today (going to get it recurved to suit my need soon but for now it’s fine) the car runs perfectly, much more smooth idle and doesn’t seem to hunt anymore. Although when I tried to set the timing with a light it was advanced to roughly 40°, way below the gauge to see how advanced the timing is.... may be a stupid question but if the cam was installed one notch wrong would the car still even run?? Although i triple checked everything I did when rebuilding the engine but I can’t think of why the timing would be so “advanced” but still run perfectly. If I try to retard the timing to where it’s even on the gauge the car cuts out. The timing light is a cheap product so could this be affecting the results I’m getting from it or is that unlikely? Also where the timing is set now the car runs great, idles and Revs up perfectly. I’ve just set the timing by ear now and found a spot where the engine seems to be the happiest. Any help much appreciated. Cheers guys.
Skyedriver said:
Are you getting cam timing and ignition timing confused.
If the cam timing is out by one tooth it will probably still run but badly and you are unlikely to get it to run right no matter what you do. Can't remember my A Series days but one tooth out may cause valves to hit pistons. (There were some engine builders that set cam timing out to actually gain power but that is on a very limited engine spec and not for you).
Have you rechecked compression? Plugs out, foot on the accelerator, spin engine over a few times to check compression is OK and the same across all 4 cylinders. Post your readings please.
High tick over. An A Series should tick over at 800 -1000 no matter what cam.
Carb, never played with the one you have but a HIF44 on a decent manifold should work a treat unless you've a 286 or 296 cam
Have you got an air leak at the manifold? Sucking in air would make the mixture weak, cause running on and high tick over.
Vacuum advance. remove it from the dizzy and blank the hole - too much advance can cause problems but also what are you setting the ignition timing at? And static or at a certain revs?
If you are suffering from Dieselling (Running on) don't continue to use it, you'll hole a piston or destroy piston rings (been there etc).
That’s great cheers mate, I was worried I had reinstalled the camshaft incorrectly, but if it could cause valves to hit pistons or to run badly then it should be out of the question, as it is running quite well only on the crankshaft pulley the timing seems to be way out although running well. I haven’t checked compression yet although the cars only done 50000 miles, I will check this ASAP and post what I find. The car was idiling nicely today at around 1000 which I could never get it down to without cutting out with the old dizzy so that’s good. I’m not too sure if there is a leak although I did spray some easy start around the carb a few weeks back and there seemed to be no change in rpm which makes me think it’s okay, I’ll have another proper check ASAP. I’ve been setting the timing at idle speed. I’m not using the car at all at the moment due to the issues going to get it all sorted before it goes back on the road so hopefully won’t damage the engine in anyway. Cheers for all the help I’ll get back with my findings hopefully soon.If the cam timing is out by one tooth it will probably still run but badly and you are unlikely to get it to run right no matter what you do. Can't remember my A Series days but one tooth out may cause valves to hit pistons. (There were some engine builders that set cam timing out to actually gain power but that is on a very limited engine spec and not for you).
Have you rechecked compression? Plugs out, foot on the accelerator, spin engine over a few times to check compression is OK and the same across all 4 cylinders. Post your readings please.
High tick over. An A Series should tick over at 800 -1000 no matter what cam.
Carb, never played with the one you have but a HIF44 on a decent manifold should work a treat unless you've a 286 or 296 cam
Have you got an air leak at the manifold? Sucking in air would make the mixture weak, cause running on and high tick over.
Vacuum advance. remove it from the dizzy and blank the hole - too much advance can cause problems but also what are you setting the ignition timing at? And static or at a certain revs?
If you are suffering from Dieselling (Running on) don't continue to use it, you'll hole a piston or destroy piston rings (been there etc).
Skyedriver said:
Someone with more recent memories of the A Series will be able to confirm if valves and pistons will collide.
How far out is the timing mark on the pulley? My 1330 A Series was set at about 6 degrees static IIRC. Aldon distributor vacuum advance disconnected IIRC
Got rid of my last hateful crock of s
t about 6 or 7 years ago and have been trying to erase the memories ever since.
Have you tried asking on the Mini Classic forum on PH you might get more recent knowledge.
Did some more research and pretty sure the piston wouldn’t collide with the valves and wouldn’t even run so I’m pretty sure I could rule this out which is a big relief. The timing mark is about an inch below the timing gauge when idling (so maybe 40° of advance) as the lowest on the gauge is 20° of advance. I asked on the classic mini section a while back but seems to be pretty inactive. The car seems to run quite well and revs up nicely although the revs do drop quite slowly...? I still need to take the car for a drive but if I were to just advance the timing untill I get pinking then retard untill the pinking goes away would this be my best bet of getting the timing sorted as with the timing light it’s way outHow far out is the timing mark on the pulley? My 1330 A Series was set at about 6 degrees static IIRC. Aldon distributor vacuum advance disconnected IIRC
Got rid of my last hateful crock of s
t about 6 or 7 years ago and have been trying to erase the memories ever since.Have you tried asking on the Mini Classic forum on PH you might get more recent knowledge.
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