Too rich on my DCOEs
Discussion
I'm running a pair of sidedraft webers with my 1700 x-flow and I'm strugging to lean off my idle circuit. Before everybody tells me to whizz off and find a rolling road specialist, I have already invested in a session and although some progress was made I'm still running too rich. The car is brilliant if I start of with fresh plugs, fire her up and steam off over the horizon. A plug check suggests the car is running rich - not unusual i guess when running these carbs but the trouble starts if I let the car idle for any length of time because the plugs will foul up and the car misfires when I pull away.
At the moment the carbs are fitted with 30mm chokes which from what I can gather are great for increasing air flow through the carbs at low revs but might be one reason why I'm having trouble leaning off the mixture. I don't really want everything to come in over 4000rpm so I don't necessarily want massive air flow from big chokes but I wonder if I went up a size if would help me rain in the fuel ratio mix at low and medium revs. Also any thoughts on emulsion tubes. F16 is what I have but there seems to be a school of thought that F11 tubes perform well when mated to a cooking crossflow?
Just for info I'm currently running 30 chokes, 120 mains, 195 air correctors, 45F8 idles jets. I know this is a massive can of worms but it's all part of the fun! Any thoughts/advice from any weber gurus out there?
At the moment the carbs are fitted with 30mm chokes which from what I can gather are great for increasing air flow through the carbs at low revs but might be one reason why I'm having trouble leaning off the mixture. I don't really want everything to come in over 4000rpm so I don't necessarily want massive air flow from big chokes but I wonder if I went up a size if would help me rain in the fuel ratio mix at low and medium revs. Also any thoughts on emulsion tubes. F16 is what I have but there seems to be a school of thought that F11 tubes perform well when mated to a cooking crossflow?
Just for info I'm currently running 30 chokes, 120 mains, 195 air correctors, 45F8 idles jets. I know this is a massive can of worms but it's all part of the fun! Any thoughts/advice from any weber gurus out there?
Thanks guys, lots to think about. The Aux chokes are 4.5s I think.
I realise that every engine has different characteristics and I think I will do some "suck it and see" trials as Frank has suggested. I would be surprised Neil if you were running 26mm or 28mm chokes on a crossflow unless you were on a standard head with little valves, which would be unlike you! The standard set up for the Caterham super sprint had 32mm chokes. Quite a few fast road xflow owners seem to get good results from twin 45s and these must be running at least 34mm chokes and probably bigger otherwise there is no advantage in the bigger barreled carb. The 30mm chokes certainly give the car plenty of low down grunt and the tick over and pick up is smooth following some recent fettling but I need to lean off the mixture a tad through the idle and progression phase and perhaps a set of bigger chokes will be a starting point, possibly along with some bigger air holes in my idle jets.
I know the original downdraft twin choke carb was always going to be less of a headache but my car came with the twin 40s and I rather like the sound and the look as well as that little extra punch. Not quite so keen on the mpg!
I realise that every engine has different characteristics and I think I will do some "suck it and see" trials as Frank has suggested. I would be surprised Neil if you were running 26mm or 28mm chokes on a crossflow unless you were on a standard head with little valves, which would be unlike you! The standard set up for the Caterham super sprint had 32mm chokes. Quite a few fast road xflow owners seem to get good results from twin 45s and these must be running at least 34mm chokes and probably bigger otherwise there is no advantage in the bigger barreled carb. The 30mm chokes certainly give the car plenty of low down grunt and the tick over and pick up is smooth following some recent fettling but I need to lean off the mixture a tad through the idle and progression phase and perhaps a set of bigger chokes will be a starting point, possibly along with some bigger air holes in my idle jets.
I know the original downdraft twin choke carb was always going to be less of a headache but my car came with the twin 40s and I rather like the sound and the look as well as that little extra punch. Not quite so keen on the mpg!
Yes lots of variables, even on engines with the same basic build formula.I guess that's why the consensus is always a session with an expert on a rolling road with emissions kit.
I have already gone for leaner idle jets but it didn't seem to make much difference. I will try one stage at a time and then go back and try a few different combinations and if still no progress will stump up for a second opinion - possibly "throbnozzle". Thanks for the tip off Richard. Don't think I will google them on my wife's computer just in case I get more than I bargained for!
I have already gone for leaner idle jets but it didn't seem to make much difference. I will try one stage at a time and then go back and try a few different combinations and if still no progress will stump up for a second opinion - possibly "throbnozzle". Thanks for the tip off Richard. Don't think I will google them on my wife's computer just in case I get more than I bargained for!
I think the air correctors have very little impact on the primary circuit and regulate the fuel ratio as the revs climb above the mid range. I'm told the big danger is that if you select bigger (numbers) air correctors you run the risk of leaning off the mix just when you really don't want to to put additional stresses on the engine. I'm quite happy if my car runs a tad rich over 4000rpm because it will only be for short duration during acceleration. Probably better to be too rich than too lean although I guess the engine response tells you long before any damage is inflicted.
Will update once I've had a play.
Will update once I've had a play.
David Vizard, who knows a little bit about carbs, suggests that emulsion tubes can play a very significant role in regulating the air fuel mix depending on the number of holes and their distribution. Emulsion tubes are not cheap but I think I will give a set of F11s a turn around the block. F11s have more holes and should lean out the mix when combined with the increased flow that will come with bigger chokes.
My strategy will be to make some single changes and run checks on the plugs over low, medium and high revs - I might even give my old colour tune plug an outing. I know this isn't a substitute for a rolling road diagnosis but it should be fun and will be a real buzz if I make some progress. I can always go back to the current settings if my fettling makes things worse.
My strategy will be to make some single changes and run checks on the plugs over low, medium and high revs - I might even give my old colour tune plug an outing. I know this isn't a substitute for a rolling road diagnosis but it should be fun and will be a real buzz if I make some progress. I can always go back to the current settings if my fettling makes things worse.
Goran - your English is better than mine! I realise that the idle circuit is regulated by the idle jets and mixture screws for each barrel.I have gone for f8 idles jets with bigger air holes than the f9s which were originally in the carbs. This hasn't made much difference, perhaps because of the comparatively small choke size. The current 30mm chokes have not been machined out. The choke size will effect everything and perhaps a lower air flow on tick-over (bigger choke) will also give me a bit more adjustment on my idle screws.
Everything I've read and researched seems to suggest that the 30mm chokes are small for a tuned crossflow and will make the whole circuit richer to the point that if you try and throttle the jets sizes down too much to try and moderate the fuel/air mix then you will just nullify any advantages the
twin side-drafts give over a single twin-choke down-draft.
With regard to the carbs, they are 40DCOE48s which are a bit of a rarity I believe. These are old Italian carbs and do not have the additional air bleed screw fitted to later carbs which I imagine makes life much easier when setting up idle circuit mixture.
I've no reason to think that they are faulty and I had them stripped and cleaned by Southern Carbs. The cam is a Kent 234 which I installed and set up using an adjustable timing gear and crankshaft protractor, using a clock on the piston to measure dwell angle. I spent ages on this and I'm pretty confident I got it right (says I confidently).I am currently running 12 degrees advance at 1000 rpm on an upgraded non vac Alden distributor which came with the Vulcan built engine.
I know a gas analyzer is the kit to have but my thoughts were that I will be able to see if the idle, mid-range and fast running circuit is out by a simple plug test. What I won't know is what the mixture is doing in the progression phase from idle to opening the throttle. If I can get these intermediate stages about right then I'm sure driving the car with the engine under load will tell me if I've got the rest hopelessly wrong!
I have now got a choke/jet/air prescription for the 135 hp Vulcan built engine. I can try this, set up the slow running and carb balance (I have a carb sync vac gadget)and see how she goes. If no improvement I can always go back to the settings I currently have and throw the remaining bits in a bag and go and book a session with a professional. I know this would be the sensible approach and is one I will not hesitate to take if a few simple changes by me bring no improvement.
This process has made me think a lot about how these carbs actually work. They are not as scary as I thought and if nothing else it has enabled me to learn a lot more about how the carb set up impacts on performance and economy. Its all good fun!
Everything I've read and researched seems to suggest that the 30mm chokes are small for a tuned crossflow and will make the whole circuit richer to the point that if you try and throttle the jets sizes down too much to try and moderate the fuel/air mix then you will just nullify any advantages the
twin side-drafts give over a single twin-choke down-draft.
With regard to the carbs, they are 40DCOE48s which are a bit of a rarity I believe. These are old Italian carbs and do not have the additional air bleed screw fitted to later carbs which I imagine makes life much easier when setting up idle circuit mixture.
I've no reason to think that they are faulty and I had them stripped and cleaned by Southern Carbs. The cam is a Kent 234 which I installed and set up using an adjustable timing gear and crankshaft protractor, using a clock on the piston to measure dwell angle. I spent ages on this and I'm pretty confident I got it right (says I confidently).I am currently running 12 degrees advance at 1000 rpm on an upgraded non vac Alden distributor which came with the Vulcan built engine.
I know a gas analyzer is the kit to have but my thoughts were that I will be able to see if the idle, mid-range and fast running circuit is out by a simple plug test. What I won't know is what the mixture is doing in the progression phase from idle to opening the throttle. If I can get these intermediate stages about right then I'm sure driving the car with the engine under load will tell me if I've got the rest hopelessly wrong!
I have now got a choke/jet/air prescription for the 135 hp Vulcan built engine. I can try this, set up the slow running and carb balance (I have a carb sync vac gadget)and see how she goes. If no improvement I can always go back to the settings I currently have and throw the remaining bits in a bag and go and book a session with a professional. I know this would be the sensible approach and is one I will not hesitate to take if a few simple changes by me bring no improvement.
This process has made me think a lot about how these carbs actually work. They are not as scary as I thought and if nothing else it has enabled me to learn a lot more about how the carb set up impacts on performance and economy. Its all good fun!
No worries Frank. Thanks Goran, that set up is almost identical to mine except the F11 tubes. My carbs do have an idle mixture screw per barrel but that only regulates the amount of premixed flow into the head on the idle circuit. I think the 151s have an addition air bleed screw which can be used to lean off the mixture.
This might be lost on you guys but I'm working on my Noddy Holder accent before booking my next holiday to Sweden!
Although I haven't got a wide band lambda sensor I do have an old Gunsons colour tune so I'm going to have a play with my idle circuit once the sun dips down! Thanks again for all the helpful suggestions. This forum is a very good place for info. Long may it be so.
Although I haven't got a wide band lambda sensor I do have an old Gunsons colour tune so I'm going to have a play with my idle circuit once the sun dips down! Thanks again for all the helpful suggestions. This forum is a very good place for info. Long may it be so.
I don't think my budget will run to a new pair of 40s and I'm a long way away from giving up on the ones I have. I hate them and love them all at the same time!
Andrew, you are dead right that ignition timing plays a huge role in how well these carbs work and for my 234 cam I'm running 14 degrees advance at 1000 RPM. This should be about right for this cam and I can't hear any pinking but I'm a tad mutton (deaf to our continental friends) and most of the noise I do hear is induction hiss and the exhaust bark when I I boot it. I'm very aware that you can easily wreck an engine through running too lean or too much advance but a combination of test runs and plug inspections should get me very close.
I did make a little bit of progress yesterday in the fact that I found that the new "leaner" idle jets (F845) that I fitted in an attempt to lean the idle were much less likely to do the job than the F945s I removed! The F9s should be richer but when I closely inspected it was obvious that the air holes had been drilled out. I've run a 1.5mm drill into the airhole just to ream up the hole and get them all the same size and in effect they are now f545s. I also found out that the hotter plugs I thought I had fitted, were in fact cooler. As Richard has said, a slightly warmer plug might overcome the low speed fouling. Being a knob, I thought that the larger plug number equated to a warmer plug and it is the opposite! So, have fitted the BP6s, balanced the air flows and tickover and I will be off for a blat to see how she runs.
Andrew, you are dead right that ignition timing plays a huge role in how well these carbs work and for my 234 cam I'm running 14 degrees advance at 1000 RPM. This should be about right for this cam and I can't hear any pinking but I'm a tad mutton (deaf to our continental friends) and most of the noise I do hear is induction hiss and the exhaust bark when I I boot it. I'm very aware that you can easily wreck an engine through running too lean or too much advance but a combination of test runs and plug inspections should get me very close.
I did make a little bit of progress yesterday in the fact that I found that the new "leaner" idle jets (F845) that I fitted in an attempt to lean the idle were much less likely to do the job than the F945s I removed! The F9s should be richer but when I closely inspected it was obvious that the air holes had been drilled out. I've run a 1.5mm drill into the airhole just to ream up the hole and get them all the same size and in effect they are now f545s. I also found out that the hotter plugs I thought I had fitted, were in fact cooler. As Richard has said, a slightly warmer plug might overcome the low speed fouling. Being a knob, I thought that the larger plug number equated to a warmer plug and it is the opposite! So, have fitted the BP6s, balanced the air flows and tickover and I will be off for a blat to see how she runs.
Cracked it. Smooth tick-over, flat spot-free and plugs the colour of rich-tea biscuits! I'm glad I have stuck with the 30mm chokes because the car pulls so well, even from very low revs and under load - it absolutely flies uphill and a few times today I had to remind myself that I'm already carrying points on my licence! It still occasionally pops on the over-run but overall a massive improvement right through the range. Obviously the warmer plugs have helped when the car is just ticking over in combination with slightly more airy idle jets and the F11 emulsion tubes.
A big thank you to all for the helpful advice.
A big thank you to all for the helpful advice.
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