Massive backfire- HELP
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From: Calgary Alberta
Massive backfire- HELP
My 1970 f100, w/ 302, 3 speed manual has recently started backfiring, REALLY loud. It never used to do this. I have changed two things. One; I tweaked the idle screws on the carb to get a little more rpm out of it at idle.
Two; I purchased a new gas cap, (because I lost the old one, oops). The reason I suspect the gas cap might have somthing to do with the problem is because it seals really well, and when I take it off after driving around for a while I get a whoosh of air being sucked into the fuel tank.
The backfire occurs when decellerating, and occasionally when lugging. It is an exhaust backfire. The truck accellerates really nice after adjusting the idle screws, so I dont know what to figure.
Any help would be great, I dont want a noise violation ticket, I have already been pulled over by the cops once but he was laughing so hard he couldnt write the ticket out. (Apparantly he had been getting reports of someone firing a firearm off while driving and he had been searching all day for the culprit, I guess he had a sense of humor)
Thanks Scott
Two; I purchased a new gas cap, (because I lost the old one, oops). The reason I suspect the gas cap might have somthing to do with the problem is because it seals really well, and when I take it off after driving around for a while I get a whoosh of air being sucked into the fuel tank.
The backfire occurs when decellerating, and occasionally when lugging. It is an exhaust backfire. The truck accellerates really nice after adjusting the idle screws, so I dont know what to figure.
Any help would be great, I dont want a noise violation ticket, I have already been pulled over by the cops once but he was laughing so hard he couldnt write the ticket out. (Apparantly he had been getting reports of someone firing a firearm off while driving and he had been searching all day for the culprit, I guess he had a sense of humor)
Thanks Scott
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In my mind to have an exhuast backfire you are really loading up the system with unburnt fuel, then BANG! With the cap to tight you would not be able to idle well and may 'run out' of power when passing due to not enough fuel delivery. When you decelerate and it backfires this is because the throttle plates close to tight. I would check the dashpot that keeps the throttle from closing to tight. this is a monentary unit but if not set right can cause backfires or to high an idle. Your having to adjust up the idle circuit is because the vent on your tank is plugged or kinked shut. You will need to correct it or install a vented cap.
An old trick with sticks was to go about 30mph, shut off the key, pull the choke or stand on the gas. Release choke or gas and turn the key on. KAWHOOM. Got me pulled over once.
An old trick with sticks was to go about 30mph, shut off the key, pull the choke or stand on the gas. Release choke or gas and turn the key on. KAWHOOM. Got me pulled over once.
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From: Calgary Alberta
Thanks for the replies, I am going to check out the exhaust leak possibility, I have headers and a dual exhaust system- could be a bad collector gasket?
I am also noticing poor mileage, so the possibility of running too rich and flooding the system with fuel is another possibility.
The adjustment screws near the base of the carb- are they for adjusting idle or mixture and do you turn them in or out to make the mixture leaner?
I am running a 2bbl carb torn off a 1979 LTD that I had rebuilt.
I am also noticing poor mileage, so the possibility of running too rich and flooding the system with fuel is another possibility.
The adjustment screws near the base of the carb- are they for adjusting idle or mixture and do you turn them in or out to make the mixture leaner?
I am running a 2bbl carb torn off a 1979 LTD that I had rebuilt.
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I had this problem on my truck. Just like 68Torino said, when your engine continues to spin and the plugs don't ignite the fuel charge, you pump explosive vapors into the exhaust system. It doesn't take much to make it explode. My problem was that one of the connectors to my coil had worn loose. It was one of the stock type that kind of snaps on. Anyway, it would get loose enough without coming off to break the conductivity for a second or two, then it would vibrate and make contact and--KABAM. It would scare the crap out of me. Long story short, check to see how tight your connectors are, maybe that's the problem. I put on new round-ended connectors so I could bolt them tight.
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