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MG MGB Technical - Carburetors not returning to normal idle speed
|Carburetor Problem 1964 MGB|
A problem has just developed with my SU carburetors on my 1964 MGB engine. During normal driving and when coming to a stop the idle speed does not return to normal. The engine idles between 1000 and 1500 RPM. If I give the pedal a quick goose it will return to the normal 700 RPM. I have checked all of the linkage and find that it is not hanging up and it does come to a firm stop. Even if I apply additional pressure to the linkage it still remains at a high idle. But again, after a quick goose the idle returns to normal. I have recently rebuilt the carbs completely including new throttle shafts.
Any ideas for what may be causing this problem will be greatly appreciated.
|Frank. A few ideas to check out. First, check the throttle plates for being loose. The screws securing them may have slipped, allowing the plates to shift a little. I had this happen, on a two year old set of new Burlen carbs, and your symptom is how I realized there was a problem. High idle after a run, with "blipping" the throttle causing the idle to drop back down to where it should be. Did not, however, find the problem quickly enough and one of the throttle plates had one of its holes worn oval, requiring replacement.|
Second thing is to make sure the needles are perfectly centered. Blipping the throttle makes the piston rise up quickly and drop back quickly--more quickly than it does when coming up to a stop sign or in normal driving. A slight rubbing of a needle on a jet may be a factor in your problem.
Third, throttle return springs. Are they hooked up correctly and are they relatively new? Springs stretch with age and a new set is inexpensive.
Fourth, the linkage, especially the two brackets which connect the shaft to the carbs. There should be some slight side to side movement when these are properly adjusted. If not, they can cause a binding to take place, inside the carbs, with the results you have noted. Normally, however, you can push the linkage down to restore the correct idle, something that you note you cannot do. So, I consider this may be a contributing factor, but is, most probably, not the primary cause of the problem. But, still check it out.
|I had a similar problem a few years ago a new throttle cable fixed it. I greased the inner with lithium grease before fitting it. It's also worth checcking the return spring and pivot on the accelerator pedal.|
|If pressing on the linkage doesn't change it then it could be an air-leak on the throttle spindles due to wear, as well as the loose butterfly Les mentions. 64 should be too early for poppet valves in the butterflies, but they also cause it. Wiggling each in turn, or opening one slightly without moving the other, might reveal it which one if not where is the cause. A couple of weeks ago someone else had this and someone suggested an air-leak 'tween carb and manifold. Didn't seem that likely to me but that is apparently what it turned out to be. Propane or possibly carb cleaner sprayed round the gaskets may reveal that.|
|You should also check that the pistons fall with a clunk on to the venturi bridge. You can use the lifting pin, or take off the air filters and dampers and lift with a finger. If they don't "clunk" it will be, 1. Jets not centred, 2. Dashpots not seating properly on carb body.(trapped muck or a burr usually) 3. Burr or muck again, on the piston or damper.|
|FRANK: I would initially try spraying some lithium grease into the throttle cable sheath as Stan mentions...I've had this issue on a few cars; some of htem I just spray the grease into the cable on a regular basis (1 time per year)...if it doesnt even slightly improve, then try other culprits: springs, needles, shafts. |
LES: what are your thoughts on the Burlen carbs?
|Could be a simple carb adjustment. probably one of the solutions mentioned above, but keep this one in mind.|
|It's the butterfly opening, mixture getting past it when it shouldn't, or air getting in where it shouldn't that will cause a high idle. If the piston it stuck up it is more likely to cause stumbling and stalling, try lifting the piston right up and see. Also I can't see how it will be cable or linkages *intermittently* sticking if it can't be 'corrected' by pressing the butterflies home by hand. If the idle screws are on their stops it can't be cable or linkages.|
This thread was discussed between 21/10/2008 and 22/10/2008
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