Backfiring Through Exhaust
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Joined: Mar 2002
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From: Kent, WA
Backfiring Through Exhaust
I have a problem with my 65' F-250 4x4. When I take my foot off the gas and let it coast down a hill, it backfires through the exhaust. I can't figure out what could be causing this. Anyone got any ideas?
Tuned
Joined: Apr 2000
Posts: 354
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From: Itasca, IL USA
Backfiring Through Exhaust
Is it one loud bang? Or does it just kinda rumble, rumble, rumble? What kind of mufflers do you have? Glass packs maybe? Did you recently make any changes to your engine, exhaust, etc?Jerry (OldTrux) Biskup
Itasca, IL
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Joined: Apr 2002
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From: Coal country
Backfiring Through Exhaust
Several possibilities exist here:
1. Engine not quite tuned right (carb and/or timing) for a performance cam.
2. Too rich mixture caused by
a. Bad power valve in the carb, or not enough vacuume getting to it to keep it closed, caused by bad gaskets, o-rings and the like.
b. Improperly adjusted choke.
c. Bad needle/seat or float adjustment.
d. Idle mix too rich
3. Wrong port on carb used for vacuum advance, and/or, for the vacuum retard (on dual vacuum ported distributors).
4. Incorrect or messed up EPA stuff.
I put my money on a too rich mixture.
1. Engine not quite tuned right (carb and/or timing) for a performance cam.
2. Too rich mixture caused by
a. Bad power valve in the carb, or not enough vacuume getting to it to keep it closed, caused by bad gaskets, o-rings and the like.
b. Improperly adjusted choke.
c. Bad needle/seat or float adjustment.
d. Idle mix too rich
3. Wrong port on carb used for vacuum advance, and/or, for the vacuum retard (on dual vacuum ported distributors).
4. Incorrect or messed up EPA stuff.
I put my money on a too rich mixture.
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Senior User
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From: Kent, WA
Backfiring Through Exhaust
It's just a rumble with a couple of louder pops here and there. Yes I have glass packs, and I haven't changed my exhaust. I haven't really even touched the motor lately, it just started doing it last week.
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Joined: Apr 2002
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From: Coal country
Backfiring Through Exhaust
[updated:LAST EDITED ON 03-Jul-02 AT 11:41 AM (EST)]I would look for a loose or cracked vacuum hose somewhere, then, or a vacuum leak from somewhere else. Or, a rich condition, such as a sticky choke or a bad power valve. Check your timing; it slowly changes over time as the points wear.
Postmaster
Joined: Nov 2000
Posts: 2,557
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From: Texas
Backfiring Through Exhaust
John's right, check your pionts. Chevies are bad about this and usually its due to points bieng gapped too far. On deceleration, your cylinders are still pulling fuel into the cylinders and if the points aren't firing crisp, the un-burned fuel can be ignited in the exhaust system and cause backfiring and that "dually" crackle.
You can test this by running your truck about 40 mph in 2nd gear, turn off the ignition, coast with the trans engaged for about 500-1000 feet and turn the ignition back on. You'll either hear a load pop or your muffler will explode!
If it pops, your dumping un-burned feul into the exhaust because:
1. Incorrect ignition timing
2. Choke not fully open
3. Feul mixture too rich (presence of black smoke on acceleration)
4. Seating needle and seat worn
5. Point gap too large
Let us know what you find out.
You can test this by running your truck about 40 mph in 2nd gear, turn off the ignition, coast with the trans engaged for about 500-1000 feet and turn the ignition back on. You'll either hear a load pop or your muffler will explode!
If it pops, your dumping un-burned feul into the exhaust because:
1. Incorrect ignition timing
2. Choke not fully open
3. Feul mixture too rich (presence of black smoke on acceleration)
4. Seating needle and seat worn
5. Point gap too large
Let us know what you find out.
Tuned
Joined: Apr 2000
Posts: 354
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From: Itasca, IL USA
Backfiring Through Exhaust
Glass packs will really rumble when you let-up on the gas. Actually sounds kinda good!
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From: Kent, WA
Backfiring Through Exhaust
I know that it's not the choke, because I have it wired open (linkage busted). There's no black smoke when I accelerate but I'll go play around with the air/fuel mix. I'll check the point gap, that could be the problem. I think that the vacum line on my dizzy is cracked too. I'll replace that too, and see what happens.
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Senior User
Joined: Mar 2002
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From: Kent, WA
Backfiring Through Exhaust
Hey guys, I regapped the points, they were opening a little too far. And adjusted the carb a little. It don't bang as loud now. But it does still pop a little. I'm not worried about it, I think it's just my exhaust. I have nice exhaust leak before it gets to my glasspack so I don't think that is helping. Anyways, it seems to be running great now, except for the bad exhaust.
Backfiring Through Exhaust
