Idle help

[sphooch]

Hi guys, i'm trying to get my roadster to run better. here is what i did. I adjusted the air to 1&1/2 turns out as per the Less Andrews , adjusted the idle speed to around 700rpm,in doing so i found an air leak and tightened the manifold bolts, running not bad but when i shut it down about 3 sec. later i get a back fire?? this seems to be my stumbling block. what am i missing?? any help? thanks THE HOOCH

[Patrick L.]

Did you fix the air leak ? In other words, did you check it after you tightened the bolts ?
Setting the idle mix screw to 1.5 turns is a starting point. That screw adjusts air, not fuel.

[Rob Doe]

X2 on Patrick's comments. IMO, 700 seems a bit fast on the idle. When turning the key off, raw fuel may be reaching the exhaust and igniting. Our coupe idles around 550. Our engine has a high compression head and good compression numbers. Too fast an idle can cause shifting problems.

Check for leaks at the block to manifold, carb to manifold, hairline cracks in the intake manifold, windshield wiper line and fitting, throttle plate shaft, and a more obscure location is the throttle plate itself. (inside the upper half of the carb. It should completely close off the air when idle screw is backed off completely. Use a pen light to check for the fit in the bore? This info learned recently from Marshal Daut)

I would reduce the idle speed at the idle rpm adjustment screw, then move the spark advance all the way up and close off the GAV just before turning the key off to see if the issue clears.

When fuel related issue, Backfiring in the exhaust or muffler is sometimes referred to as after fire and is often a too rich mixture. Backfire in the carburetor is often a lean mixture.

[Marshall V. Daut]

Sponsored Links (Register now to hide all advertisements)

Until you have found the problem, always retard the spark lever before turning off the engine. That will reduce backfires dramatically. Push the spark lever all the way to the top of the quadrant, allow the engine to run for two or three seconds at idle, and then turn off the ignition. The late spark inside the combustion chamber will reduce the tendency for remaining unignited gas to fire = no backfire. In fact, for all Model A drivers with the original distributor and manual spark control, it's a good idea to retard the spark before killing the engine anyway. This way, the ignition timing is already retarded in the distributor for the next time you start the engine. No forgetting to retard the spark next time and breaking a Bendix spring upon engine kickback.
Admittedly, retarding the spark to stop backfiring is only a band-aid masking the cause of chronic backfiring. But it's still better than hearing backfiring and possibly blowing out the muffler! Follow all the suggestions offered so far and you'll probably snare the cause of your problem.
Marshall