Join Date: Aug 2005
Location: Shreveport, LA
Bronco Info: 1979 Freewheel'n, PowrLok, ARB, OBA, and anything else I can cram in
This thread is about 2 years old, so I HOPE he isnt fiddling with cold backfires still.
BUT, for reference the carb he talked about doesnt have a power valve to blow out, and is pretty indestructible.
And carbed vehicles didnt do this all the time. It's not just a carb thing, it's a maladjusted carb thing. Just like maladjusted EFI can do this.
While vacuum advance IS, in fact important and having it hooked up to the spark port OR full manifold vacuum will REALLY depend on what happens with the vehicle and the build of the engine, but having it unhooked at idle will not affect anything as far as back firing. Especially if you were using the spark port. And having it unhooked when he stomps the gas, as mentioned. All your vacuum rushes down the tail pipe and it's the same as having that vacuum advance unhooked.
The timing chain isnt the culprit, as faulty valve timing will cause all kinds of odd issues as well as back firing ALL the time, IF it's far enough off. But when it's far enough off to cause back fires, youre probably not going to be able to shift the vehicle into gear and go down the road. As a side note, isnt it kind of funny how older vehicles with non-interference valve trains tend to have timing chains and gears and newer interference engines ALWAYS seem to have nice, stretchy belts??
It's unlikely to be a choke adjustment. Although I can just about bet his choke is not adjusted properly (anyone remember back when you had to adjust those with the weather change??) having it running too fat (at least to the extent the choke can provide) isnt going to cause an issue, and if it IS, then there are other problems. Why just mask symptoms when we can fix problems in this board??
It COULD be carb mixture. With a TON of fuel going in, you could, possibly, be having a little back firing through the carb. You can try resetting the idle mixture. Since I'm long winded already, AND there are a million and one writeups out there, I wont cover it here. If it gets any better (And this will be ONLY at idle below about 800RPM) then you probably have your culprit. Reason being is that circuit deals with not JUST idle, but also the transition out of idle. BUT, if it wasnt popping before and it just started one day, this is another bandaid.
My bet would be that the carb needs to be rebuilt.
This, of course, all assumes that it was running "Fine," before.
If it were running fat enough to be dumping SO much fuel down the intake to back fire, you'd probably have very wattery eyes from all the unburned fuel floating around in the air. Makes me cough.