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Location: Auckland NZ
Car type: 36 Nippy, 31 RM, 38 Special, 24 Works Rep
Mick I would suggest you have the wrong needle profile, almost all SU needles are the same diameter in the idle position, you set your idle mixture by adjusting the position of the main jet, however the mixture throughout the rest of the rev range is controlled by the Needle profile. What happens so often when people fit an SU is the get a needle which is close enough for the car to run then adjust the cruising mixture by moving the main jet, more often than not this results in an over rich idle mixture. The fact that your engine dies when the throttle is opened quickly suggests the mixture is too weak, as a check richen the mixture by lowering the main jet if this helps you need a richer needle. Alans suggestion that the damper is not function correctly may also have a bearing, the damper slows down the rate of piston rise helping control the mixture transition as revs increase.
What has been done to the engine other than open up the inlet tract?
Black Art Enthusiast
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Car type: Ruby sports trial special .
If it runs on choke but not without
then it's got to be running very weak?
Ideally I'd put old needle back in and check
For a good spark as a poor one will course
The engine to run very rich ?
My problem I ask questions that other people don't like?
Like have you got that for an investment or for fun?
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Location: Auckland NZ
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I forgot to add make sure you have fitted the needle correctly, the shoulder must be flush with the face of the damper, too far in or out will have a bearing on the overall mixture same as moving the main jet will.
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JonE - Yes, the Zeniths seemed to act the same. The original Zenith needed a good clean out but still couldn't get a consistent tick-over, and the newly refurbished Zenith was all over the place, but I put that down to the settings being out having never been fitted before.
Following advice on facebook I tried the oil test on the carb. inlet tract but nothing dramatic happened - changed the carb to inlet manifold gasket anyway. I'd like to think there was a hint of an improvement, but really just being overly optimistic - it still doesn't tick over without the choke. It seems an air leak could be part of the problem, but I can't detect it.
Chris - A lot of information and advice given - thanks. I'll reset the gaps to those you quote. It took nearly an hour to leave Woolaton last year on what was a red-hot day, nudging forward a yard at a time. My friend's RN boiled up in sympathy. The piston falls easily enough and I've used 3-in-1 oil in the damper. I'll follow yours and Hugh's advice and go back to the EB needle - I only changed it in the hope of weakening the mixture, but I'll adjust the main jet as Reckless suggests.
It looks like I can rule out the ignition side of things, apart from setting the timing more accurately, and focus on carb and mixture.
Fantastic response chaps - I hope I can do it all justice.
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If it helps, I have successfuly run on an SU with a GG needle but have subsequently changed to an AN. They are very close to each other in profile... for clarity, that is a 1" OM series from an pre-war Morris Minor (on both cars)
Have the points opened up? I know there has been a discussion about the plugs, I wasn't sure whether you had looked at the points as well..
Hugh
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Does the pump deliver enough fuel ?
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13-05-2019, 11:10 AM
(This post was last modified: 13-05-2019, 11:16 AM by Mick Hanna.)
Ian - Thanks for the suggestions. I will adjust the jet to make it richer and see if this helps. The needle is fitted to the piston correctly so I can rule that out. The charts show the EB has a richer profile than the GG so, as suggested by yourself and Spannerman, I can go back to it. I'll also check the damper function as Alan pointed out.
Ian, other than open out the inlet tract on the manifold and smooth the curves at the base of the throat, there is little different to the standard set up. The block has been rebored to .060, all valves and valve gear renewed, and it was running very well. The electronic distributor pick-up gave up the ghost while trying to overcome this problem, but the manual distributor has been refitted and works well.
Hugh, points and plug gaps were set at .015" and .025", but it has since been suggested gaps of .012-.014" for the points and .020 for the plugs might be better.
It looks like the consensus is forming around the fuel side. I think I have an AN needle as well, but I should get it idling and running before I begin experimenting.
Austin, the pump is working well. I think I can rule that part of the system out. Thanks.
Austin, the pump is working well. I think I can rule that part of the system out. Thanks.
Austin, the pump is working well. I think I can rule that part of the system out. Thanks.
Austin, the pump is working well. I think I can rule that part of the system out. Thanks.
Hugh, points and plug gaps were set at .015" and .025", but it has since been suggested gaps of .012-.014" for the points and .020 for the plugs might be better.
It looks like the consensus is forming around the fuel side. I think I have an AN needle as well, but I should get it idling and running before I begin experimenting.
Ian - Thanks for the suggestions. I will adjust the jet to make it richer and see if this helps. The needle is fitted to the piston correctly so I can rule that out. The charts show the EB has a richer profile than the GG so, as suggested by yourself and Spannerman, I can go back to it. I'll also check the damper function as Alan pointed out.
Ian, other than open out the inlet tract on the manifold and smooth the curves at the base of the throat, there is little different to the standard set up. The block has been rebored to .060, all valves and valve gear renewed, and it was running very well. The electronic distributor pick-up gave up the ghost while trying to overcome this problem, but the manual distributor has been refitted and works well.
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Location: Auckland NZ
Car type: 36 Nippy, 31 RM, 38 Special, 24 Works Rep
AN is the richest followed by EB and weakest is GG, if you have an AN start with this. Sounds like the EB was close but to get mid range mixture right the main jet was lower than ideal for idle mixture, hence the reason it fouled plugs trying to get out of Autokana. With the AN being slightly richer you will be able to maintain the midrange mixture but with a leaner idle if that makes sense. People seem to get very confused about SU needles, remember almost all .90 jet needles have the same diameter at idle, set the jet for correct mixture here, if it is lean or rich anywhere else DO NOT alter the main jet to compensate choose a different needle. If you alter the main jet you will simply stuff up the mixture elsewhere along the needles range and performance will suffer as a result. It is hard selecting needles unless you have experience and a lot of alternative profiles to choose from, using a proven needle such as the AN is a good starting point but even so this may not be ideal in your car.
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Just a thought; have you checked the float level in the float chamber and the operation of the needle valve?
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Hello again Mick
Just re-reading your posts again, do I understand it that your problems show themselves with the SU, a Zenith and another Zenith ?
It would seem to me that if this is the case it's extremely unlikely that all 3 carbs are faulty.
I'd be inclined to look elsewhere. For example any kind of misfire or weak spark under load will also tend to soot up plugs even with a correct mixture. If your engine isn't fully run in it could also be burning a little oil (not enough to give tailpipe smoke). This also promotes plug fouling unless you can get their insulator tips hot enough to burn off the deposits, which will only happen on a decent fast run. Idling and pootling around (especially in cool weather) don't seem to be enough in my experience.
Cheers John
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