backfiring through carb when engine cold [Archive] - FSB Forums

: backfiring through carb when engine cold


stewarad
02-13-2010, 01:44 PM
ok guys i have searched other threads and tried a few things, now im hoping by starting a new thread ill get down to the issue more direct.

here is the situation: 79 bronco with 400, 4bbl intake and a carter afb carb. when i crank the motor and let it idle everything seems fine. when i stomp the gas for a second the motor will hesitate and sometimes it will backfire through the carb. and i mean backfire with fire coming out of the carb when the air filter/breather is removed.

looking through some past threads i have deduced that it is probably either the carb is running too lean, or a timing issue. when i adjust the accelerator pump from the lowest setting (the top hole) to the mid or highest setting (the bottom hole) i still get the backfire. and it seems like it backfires a larger ball of flame. so i tried adjusting the timing by turning the distributor clockwise, tightening it up, and trying again. when i adjusted the timing it seemed to reduce the amount of hesitation that was there, but i still get a slight amount of backfire through the carb.

now before everyone jumps all over me and tells me what an idiot i am, let me lay out a few things that may or may not be effecting this (i dont know, thats why im asking you guys who know more). ok, i really dont know that by turning the distributor clockwise if im advancing the time or not. i have a timing light but its old and you have to hold it in such a way to get it to work. and no i dont have the cash to buy a new one right now (still laid off in this economy). also i dont have a way to pressure check the fuel going into the carb, nor do i know how to do this (i just read some things suggesting this in some threads). also, i have been fighting my exhaust falling apart for the past several months and have just been piecing things back together the best i can. so part of the dual exhaust has holes rusted thorugh and the other part i had to go from 2.5 pipe to 2.25 pipe back to 2.5 pipe.

like i said i dont know how much any of this is effecting everything, but i wanted to lay out as much as i can to assist you guys in whatever advice you can give me. this is my daily driver and i have to use it to work several gigs coming up. i admit that im a little ignorant when it comes to the stuff with the carb. but i have rebuilt a 1bbl in the past, just never messed with a 4bbl before.

thanks in advance to all the credible advice i receive.

njtinman
02-13-2010, 01:49 PM
carbed engines did it all the time when cold they like to be warmed up. and you did not say if you have a stick or an auto trans

ol"blue79ford
02-13-2010, 08:45 PM
Let it WARM up. The manifold needs to be warm to run good.

mickaila
02-13-2010, 10:28 PM
out of tune engines with carbs can backfire. I wouldn't say "carbed engines back fired all the time" or that the solution is "let the engine warm up"

it sounds like your timing is off. you should probably buy a timing light and do a 6L tune up.

is your distributor vacc advance, and is it working properly?

www.edelbrock.com has tech videos on tuning carbs.. a bit different (eddelbrock) but the principles are the same.

also, is it choked? electric or manual?

waltman
02-13-2010, 10:52 PM
So what if you have to hold the timing light a certain way, do it! If the RPMs increase when you rotate the dizzy, you are advancing the timing and if the backfiring problem improves then it sounds like a timing issue and yes a cold carbed motor will back fire when you punch it

stewarad
02-13-2010, 11:10 PM
ok, apparently i forgot to mention a few things in my haste.....first off it is a manual, not an auto trans......and it is a manual choke and not an electric choke but i have disconnected the manual choke cord due to it sticking really bad.

from what i am able to gather from what you guys posted it is more of a timing issue than a carb issue.....so i will work on adjusting the timing. as for holding the light in a funny way, im able to do that and the marks line up fine. but if this is common for a cold motor, then ill just be satisfied with what i got. im still learning a lot about this as i go along, so i appreciate what you can tell me. but give me a break cuz im a poor phuck who is trying to get by too. thanks for the info.

stewarad
02-13-2010, 11:16 PM
mickaila- oh, one thing, the vacuum advance on my distributor is just capped off and there isnt anything going from there to the carb. i thought this was strange, but this is the way i have seen many old fords with out this issue.

waltman
02-13-2010, 11:24 PM
you need vacuum advance.

Runnin'OnEmpty
02-13-2010, 11:57 PM
Sounds like the timing is a bit too high; that'll make one
backfire through the carb for sure. I'd turn the distributor
counter clockwise about 1/2 inch, and see if that helps.

Turning dist CCW retards timing, while turning it CW advances.

I'd also buy a couple feet of vacuum hose and enable the
vacuum advance. You'd be surprised how much better it will
drive with vacuum advance working.....

Bill3753
02-14-2010, 12:02 AM
Sounds like the timing is a bit too high; that'll make one
backfire through the carb for sure. I'd turn the distributor
counter clockwise about 1/2 inch, and see if that helps.

Turning dist CCW retards timing, while turning it CW advances.

I'd also buy a couple feet of vacuum hose and enable the
vacuum advance. You'd be surprised how much better it will
drive with vacuum advance working.....

+1. I agree.

stewarad
02-14-2010, 12:50 AM
Sounds like the timing is a bit too high; that'll make one
backfire through the carb for sure. I'd turn the distributor
counter clockwise about 1/2 inch, and see if that helps.

Turning dist CCW retards timing, while turning it CW advances.

I'd also buy a couple feet of vacuum hose and enable the
vacuum advance. You'd be surprised how much better it will
drive with vacuum advance working.....

ok, ill try hooking up the vacuum advance....i assume it goes to the vacuum port on the front right (passenger) side of the carb.

ill also try retarding the timing a bit too.

thanks

mickaila
02-14-2010, 12:58 AM
vacuum advance. do it :}
switch to working choke. do it :}
6L tune up. do it :}

backfire of a cold carbed engine. never personally had it happen, but i keep mine tuned well too.

about 5.5 y ago, i spent a few months in PCB. fun town. Hoo Yah America!

rlgbronco
03-01-2010, 11:37 PM
engine running to lean when cold, choke pull off on carburetor needs adjusted. also adjust choke thermostat tighter a notch or two. this will fix it

outlawbob
03-24-2012, 02:45 PM
there may also be a timing chain issue...it could be worn that is is throwing off the timing...another issue could be a sticky lifter or valve..

Fetcherr
03-24-2012, 02:56 PM
take the dist cap off and bump the motor over if it rolls over before the rotor starts moving you have some slack in the chain if not i would try the timing, hook your vac advanced back up then get a timing gun hook it to the number 1 cylinder mark your 10 degree before top dead center with a yellow marker or something and mark the timing meter unplug your vac advanced and set it to 10 degrees btc and hook your vac advanced back up, if theres a intake valve sticking you would have the same problem alot of people run sea foam in there oil and its suppose to help free up sticky valves.

JKossarides
03-24-2012, 03:02 PM
Becarefull with backfiring you don't blow out the "power valve" at the meterting block inside and get an electric adjustable choke and hook it to the coil for direct power when you turn the key on for now.


Good Luck ~ :thumbup

psychlopath
03-30-2012, 12:02 PM
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.