AFB idle issue 340 SHP
I recently installed a newly restored 3269 AFB. The restoration appears to be done well but was not done by me. The idle was set at 850-900 RPM at rest. When driving the car the idle will stick at 1200 RPM. Blipping the throttle will sometimes return the idle to 900. If I stop and manually push the throttle against the idle set screw it will return to 900 RPM. Something is sticking holding the throttle open slightly. The throttle return spring and bracket are correct for this 340 SHP. What could be sticking and how can I correct this problem. Carb otherwise works well.
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Re: AFB idle issue 340 SHP
The problem can be caused by a worn throttle shaft bore. Contact the vendor and find out if they bushed the throttle shaft bore.
Duke -
Re: AFB idle issue 340 SHP
Greg;
I have had three different AFB's on my 63 since new. The original 3461S was removed and replaced (stolen) by a Corvette Service shop with a 3720SA back in 92, and I replaced that one soon therafter with a new service replacement 3721SB. With all these three Carbs I have never had a problem associated with worn throttle shafts, but perhaps I never ran one long enough for that to occur (car has 44k+ miles). However, I always had unstable idle speeds. Now, yours being a 62, we can discount a vacuum advance problem. That leaves throttle return spring(s) and choke as the two main culprits that I have witnessed.
For a long time I used the so called correct 63 white return spring, but they never were correct as is. I always had to modify one end. Also, I found I needed a helper spring which I chose carefully so as not to be too stiff, yet strong enough to give me a stable idle speed. Recently I went with a later C-2 black spring which fits correctly, but gives me back the unstable idle. With the spring conditions, however, I am only seeing a + or - 100 to 150 RPM change, depending on ambient conditions. If I have had a situation where it was running say up to 1100 or 1200 RPM at idle, it was almost always a hanging choke. The AFB is infamous for this, at least they have been for me. That little piston pull-off has a tendency to stick for a variety of reasons, but usually a slight amount of corrosion of the housing vs. the brass piston (no wonder many mfgs. went to vacuum pull offs).
So in addition to the throttle shafts mentioned by Duke, I'd check the return spring situation, possibly using a light booster spring, and I would check the choke. Take a look/feel of the butterfly when the engine is fully warm and see if it is being pulled into full open position. Make sure all you carb linkages (choke, high idle, throttle and pump) are all free and in adjustment based on the specs.
Good Luck. You have to enjoy a love/hate relationship with AFB's.
Stu FoxComment
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