The Stovebolt Site Forums The Shop Area Engine Shop Flame outt'a the carburetor

I have just reinstalled a Carter BBD-4925 two bbl on my Chrysler. Engine was timed and running smoothly before rebuilding the carb. Now I am at the on-engine fine tuning phase of the rebuild. The carb won't curb idle - it putters to a stop. Starts right up, but can't take m' foot off the gas pedal to idle.

Anyway, I was punching the pedal to check that the acceleration jets and the pump were working ok and got a couple of thru-carb backfires, spitting flame up about 4-6 inches.

Any ideas what causes this? I'm prepping to get back under the hood this Saturday.

Cheers from NC

Flame coming out of the carburetor indicates a cylinder is firing while the intake valve is open. Possible causes are many (stuck or burned intake valve; valve timing; timing chain skipped tooth; spark plug wires on on correct wires; defective vacuum advance; cracked or carbon track in distributor cap; defective carbon spark plug wires; more).

Would suggest starting with a compression test, which will tell you the condition of the valves.

It doesn't take many backfires to melt the venturi cluster in the carburetor.

Jon.

If it runs with the foot on the pedal, I'd say it sounds like it's too far advanced. Maybe a bit too rich too. Back off the timing, lean it out, get it running and tuned up, then adjust the timing again.

-Boyo

its lean and most likly retarded due to a worn out chain
the fire up the carb is a slow acelerator pump causing a lean mixture
I know this because it took me 3 weeks to fix mt dodge van once

if it won't idle, I'd say the culprit is a vacuum leak. This could also lead to a lean backfire.

I'd start searchin for loose, cracked or broken vacuum lines. If that doesn't fix it, then start sprayin carb cleaner around the carb, especially the vacuum lines and the base of the carb where it fastens to the manifold. If the engine speed increases when ya spray the stuff in that area, you've probably located the source of the vacuum leak.

Now my 64 C10 spits thru the carb....bit since it doesn't have a choke and it's a little cold blooded, I figger that it's just a lean backfire and I don't worry about it, cuz it goes away when it warms up. And it's gettin an electric choke AFB carb here pretty soon anyway. Like I might start workin on it tomorrow, if the good Lord's willin and the creek don't rise. Hmmm...I might actually have a hopped up 292 by the end of the weekend. How bout dat?

Thank you all for the input.

Yeah, this is a tough carburetor to tune, I'll say! There are two mixer nozzles in the front base of the carb and a third in the back that seems to be for curb idle mixture (that's three mixtures).

I may have the thermostatic coil too lean, but I think I have the three nozzles too rich.

I finally found a MOPAR Master Technicians' Service Conference bulletin for 1968 Carter BBDs and will have to read that. I do know now my carb is a 68 on a 66 car.

Mybe you should just get rid of the chrysler and drive one of your real vehicles