Backfire! (long)
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Joined: Mar 2013
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From: Lake Norman NC
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I may have found something; while tighting the manifold bolts to 30-35 ft pds (they were a little loose) I noticed the drivers exhaust "doughnut" is brittle, crumbling and can freely rotate. Passenger side crumbled a bit to the touch on the outer edges, but feels "rubber-like" and tight.
The drivers "doughnut" feels like a loose, broken plastic toy tire.
Will head in to local shop if possible tomorrow to remedy; hope that was the culprit.
Thanks to everyone here again!
The drivers "doughnut" feels like a loose, broken plastic toy tire.
Will head in to local shop if possible tomorrow to remedy; hope that was the culprit.
Thanks to everyone here again!
Drifting
Joined: Feb 2011
Posts: 1,259
Likes: 18
From: Dixon California
Back fire
good deduction on the donut....
ok ....there are IMHO 2 kinds of donuts....the fiber/asbestos......and the ones that are all steel...
I have the steel ones and dont seem to break down....I assume all the major vendors have 'em and possibly a local shop Beveled on both sides...... just like the fiber ones...
been there with the exhaust leak/air introduction in the system....
ok ....there are IMHO 2 kinds of donuts....the fiber/asbestos......and the ones that are all steel...
I have the steel ones and dont seem to break down....I assume all the major vendors have 'em and possibly a local shop Beveled on both sides...... just like the fiber ones...
been there with the exhaust leak/air introduction in the system....
I may have found something; while tighting the manifold bolts to 30-35 ft pds (they were a little loose) I noticed the drivers exhaust "doughnut" is brittle, crumbling and can freely rotate. Passenger side crumbled a bit to the touch on the outer edges, but feels "rubber-like" and tight.
The drivers "doughnut" feels like a loose, broken plastic toy tire.
Will head in to local shop if possible tomorrow to remedy; hope that was the culprit.
Thanks to everyone here again!
The drivers "doughnut" feels like a loose, broken plastic toy tire.
Will head in to local shop if possible tomorrow to remedy; hope that was the culprit.
Thanks to everyone here again!
Race Director
Joined: May 2000
Posts: 12,712
Likes: 2,270
Backfire is when the carb spits back. Afterfire are pops in the exhaust system.
If the latter it could be caused by a too lean idle mixture. If you lift off at high revs the mixture may be too lean to ignite, but it ignites in the exhaust system causing the pop. Back when Sprint Cup cars ran carbs that's why you saw flame out the exhaust when they lifted off.
Go through the idle speed mixture adjustment procedure. Start with the mixture screws out one turn from the seat and set idle speed at 500. Turn the screws in (leaner) and out (richer) in quarter turn increments until you get the highest idle speed gain with a smooth idle.
Reset idle speed to 500 and do it again. Once you get no speed increase and the idle is smooth, take it for a test drive. If it still afterfires try a quarter turn richer (out) on each screw and go from there.
The final adjustment should have the mixture screws out the same amount from the seat. I've seen plenty that are up to one turn different.
If there was a leak in the exhaust system you would hear it all the time.
Duke
If the latter it could be caused by a too lean idle mixture. If you lift off at high revs the mixture may be too lean to ignite, but it ignites in the exhaust system causing the pop. Back when Sprint Cup cars ran carbs that's why you saw flame out the exhaust when they lifted off.
Go through the idle speed mixture adjustment procedure. Start with the mixture screws out one turn from the seat and set idle speed at 500. Turn the screws in (leaner) and out (richer) in quarter turn increments until you get the highest idle speed gain with a smooth idle.
Reset idle speed to 500 and do it again. Once you get no speed increase and the idle is smooth, take it for a test drive. If it still afterfires try a quarter turn richer (out) on each screw and go from there.
The final adjustment should have the mixture screws out the same amount from the seat. I've seen plenty that are up to one turn different.
If there was a leak in the exhaust system you would hear it all the time.
Duke
Last edited by SWCDuke; Oct 14, 2016 at 12:07 PM.
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Joined: Mar 2013
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From: Lake Norman NC
2020 C1 of the Year Finalist - Unmodified
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Backfiring Solved!
It turned out that the broken donut was not the cause, but a symptom!
The drivers donut had blown out; a symptom of the backfiring along with muffler deformation (both of them). Once we had the car up on a rack, it was plain both sides were experiencing backfiring. It's likely I just didn't hear the passenger side as well when both of them backfired
The culprit turned out to be a cracked restrictor/cover plate for the passenger side metering rod on the AFB. Apparently under full throttle, the spring loaded rod would rise too high, allowing too much fuel into the carb. Under normal conditions, vacuum would pull the rod down far enough that both sides would function normally. Only under hard acceleration would the rod surpass the cover plates correct height.
Backfire is altogether gone.
Many thanks again to all who helped!
The drivers donut had blown out; a symptom of the backfiring along with muffler deformation (both of them). Once we had the car up on a rack, it was plain both sides were experiencing backfiring. It's likely I just didn't hear the passenger side as well when both of them backfired
The culprit turned out to be a cracked restrictor/cover plate for the passenger side metering rod on the AFB. Apparently under full throttle, the spring loaded rod would rise too high, allowing too much fuel into the carb. Under normal conditions, vacuum would pull the rod down far enough that both sides would function normally. Only under hard acceleration would the rod surpass the cover plates correct height.
Backfire is altogether gone.
Many thanks again to all who helped!
Last edited by 61 Roadster; Oct 14, 2016 at 02:25 PM.
Team Owner
Joined: Aug 2007
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Army
It turned out that the broken donut was not the cause, but a symptom!
The drivers donut had blown out; a symptom of the backfiring along with muffler deformation (both of them). Once we had the car up on a rack, it was plain both sides were experiencing backfiring. It's likely I just didn't hear the passenger side as well when both of them backfired
The culprit turned out to be a cracked restrictor/cover plate for the passenger side metering rod on the AFB. Apparently under full throttle, the spring loaded rod would rise too high, allowing too much fuel into the carb. Under normal conditions, vacuum would pull the rod down far enough that both sides would function normally. Only under hard acceleration would the rod surpass the cover plates correct height.
Backfire is altogether gone.
Many thanks again to all who helped!
The drivers donut had blown out; a symptom of the backfiring along with muffler deformation (both of them). Once we had the car up on a rack, it was plain both sides were experiencing backfiring. It's likely I just didn't hear the passenger side as well when both of them backfired
The culprit turned out to be a cracked restrictor/cover plate for the passenger side metering rod on the AFB. Apparently under full throttle, the spring loaded rod would rise too high, allowing too much fuel into the carb. Under normal conditions, vacuum would pull the rod down far enough that both sides would function normally. Only under hard acceleration would the rod surpass the cover plates correct height.
Backfire is altogether gone.
Many thanks again to all who helped!
If that's the cause of the backfiring then that's one for the memory banks
Race Director
Joined: Aug 2004
Posts: 15,306
Likes: 2,263
From: NorCal
It turned out that the broken donut was not the cause, but a symptom!
The drivers donut had blown out; a symptom of the backfiring along with muffler deformation (both of them). Once we had the car up on a rack, it was plain both sides were experiencing backfiring. It's likely I just didn't hear the passenger side as well when both of them backfired
The culprit turned out to be a cracked restrictor/cover plate for the passenger side metering rod on the AFB. Apparently under full throttle, the spring loaded rod would rise too high, allowing too much fuel into the carb. Under normal conditions, vacuum would pull the rod down far enough that both sides would function normally. Only under hard acceleration would the rod surpass the cover plates correct height.
Backfire is altogether gone.
Many thanks again to all who helped!
The drivers donut had blown out; a symptom of the backfiring along with muffler deformation (both of them). Once we had the car up on a rack, it was plain both sides were experiencing backfiring. It's likely I just didn't hear the passenger side as well when both of them backfired
The culprit turned out to be a cracked restrictor/cover plate for the passenger side metering rod on the AFB. Apparently under full throttle, the spring loaded rod would rise too high, allowing too much fuel into the carb. Under normal conditions, vacuum would pull the rod down far enough that both sides would function normally. Only under hard acceleration would the rod surpass the cover plates correct height.
Backfire is altogether gone.
Many thanks again to all who helped!
Glad you got it fixed!
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From: Lake Norman NC
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Thanks again to everyone!
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Glad it's fixed... but I think we should adhere to SWCDuke's suggestion of calling this "Afterfire".. as this is very different from backfire.
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Joined: Mar 2013
Posts: 605
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From: Lake Norman NC
2020 C1 of the Year Finalist - Unmodified
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So when I heard a loud noise similar to a 30-06 round firing off, it was afterfire? My mufflers will never look the same again 
Perhaps I've been using the wrong terminology.
In any case the neighbors weren't pleased!
Thanks to all.
Perhaps I've been using the wrong terminology.In any case the neighbors weren't pleased!
Thanks to all.
Last edited by 61 Roadster; Oct 14, 2016 at 04:11 PM.
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From: NorCal
That's a great idea among us gearheads, and I'm sure the term is technically and semantically correct for all combustion engines. Backfire would have to go backwards through the combustion process, and afterfire would have to be after the combustion takes place.
"After" all
in a jet its called an afterburner!I think its way too late to change the use of backfire in the common lexicon though. How many of us former troublemakers remember intentionally turning off the key (in the days before steering wheel/ignition interlocks) while driving at speed and then back on again to intentionally create a "backfire"? I blew a muffler off of my Mom's 390 4bbl Ford Country Squire wagon that way............
Heck I remember the really old guys talking about accidentally causing a "backfire" by bumping the timing lever on the steering wheel on their Model T!Maybe us gearheads and engineers can at least use it correctly when trying to diagnose a problem.
Last edited by tuxnharley; Oct 14, 2016 at 04:49 PM.
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Never heard the term 'afterfire' before, but it is an accurate description and I like it. Will use it in the future. I too have never run into this problem with an AFB, and have owned, tuned and driven many of them. Another one for the memory banks. Glad the car is fixed, and even more glad the solution was posted in detail.