Help- Exhaust Backfire
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Senior User
Joined: Sep 2002
Posts: 142
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From: Cary NC USA
Help- Exhaust Backfire
Help please,
Just put a "remanufactured" 2bbl carb from Champion Parts on my '69 F-100. (Actually 2 , the first one had a broken screw in choke attach hole, second also had some linkage problems.)
Now it backfires through the tail pipe. Did it twice on 3 mile drive.
My son drove to a fastfood restaurant and some busybody was going to call 911 because she thought she heard a gunshot.
Any Suggestions?
I also posted in carb and 360 conferences.
Richard
Just put a "remanufactured" 2bbl carb from Champion Parts on my '69 F-100. (Actually 2 , the first one had a broken screw in choke attach hole, second also had some linkage problems.)
Now it backfires through the tail pipe. Did it twice on 3 mile drive.
My son drove to a fastfood restaurant and some busybody was going to call 911 because she thought she heard a gunshot.
Any Suggestions?
I also posted in carb and 360 conferences.
Richard
Thread Starter
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Senior User
Joined: Sep 2002
Posts: 142
Likes: 0
From: Cary NC USA
Help- Exhaust Backfire
Correction-
I have a correction! after I put the third rebuilt carb on.( still had to rob a couple clips from #2) This is slightly different than the old one that was on when purchased 2 months ago..
The engine is backfiring through the carburetor not the exhaust.
It did not back fire with the old carb. just hung up and would not idle down. Why would changing the carb cause backfire??
I tried to check the timing and can NOT find any mark on the harmonic balancer. There is an indicator mounted overhanging it.
Thanks,
Richard
I have a correction! after I put the third rebuilt carb on.( still had to rob a couple clips from #2) This is slightly different than the old one that was on when purchased 2 months ago..
The engine is backfiring through the carburetor not the exhaust.
It did not back fire with the old carb. just hung up and would not idle down. Why would changing the carb cause backfire??
I tried to check the timing and can NOT find any mark on the harmonic balancer. There is an indicator mounted overhanging it.
Thanks,
Richard
Laughing Gas
Joined: Jul 1999
Posts: 1,048
Likes: 22
From: Lacey, WA
Help- Exhaust Backfire
Richard,
Has the engine ever ran correctly or is it new to you? I ask because it sounds as if you have a timing problem due to not finding any marks with the timing light. If you turn the engine over by hand can you then see the marks on the crank?
A carb back firing usually means a timing problem or an over lean condition due to a vacumm leak or something.
I currently have a 600cfm Holley that I cannot stop from back firing. I have re-built it 3 times. I have checked everything on this carb I can think of but cannot fix it. I just put my 750 Holley back on and it operates fine. Just drinks to much gas.
Good luck,
Lee
Has the engine ever ran correctly or is it new to you? I ask because it sounds as if you have a timing problem due to not finding any marks with the timing light. If you turn the engine over by hand can you then see the marks on the crank?
A carb back firing usually means a timing problem or an over lean condition due to a vacumm leak or something.
I currently have a 600cfm Holley that I cannot stop from back firing. I have re-built it 3 times. I have checked everything on this carb I can think of but cannot fix it. I just put my 750 Holley back on and it operates fine. Just drinks to much gas.
Good luck,
Lee
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Senior User
Joined: Sep 2002
Posts: 142
Likes: 0
From: Cary NC USA
Help- Exhaust Backfire
Need more carburetor help!
Thanks for the insight Lee,
The truck only on INITIAL ACCELERATION, hesitates, stalls and or backfires through the carb.
The truck ran ok, not great, for two months since purchased with only an occasional hesitation upon inital acceleration. The carb that was on it hung up on deceleration until the gas pedal was tapped. Then it would idle down.
So I replaced it with "remanufactured" autolite. The last two had slightly different configuration on the horizontal plate behind the carb opening- one is smooth and the other, as the one we ran for two months, has an indention about 1 1/2 inches in diameter.
Both cause the same symptoms.
I found the very faint timing marks after a thorough cleaning. Engine is now set at 8or9BTDC and advances to about 30BTDC upon acceleration when it doesn't stall out.
Hesitation/Backfire occurs only on the initial accelerator pump. There is at least some gas coming from the two pump jets.
I'm open to any suggestions.
Thanks for the insight Lee,
The truck only on INITIAL ACCELERATION, hesitates, stalls and or backfires through the carb.
The truck ran ok, not great, for two months since purchased with only an occasional hesitation upon inital acceleration. The carb that was on it hung up on deceleration until the gas pedal was tapped. Then it would idle down.
So I replaced it with "remanufactured" autolite. The last two had slightly different configuration on the horizontal plate behind the carb opening- one is smooth and the other, as the one we ran for two months, has an indention about 1 1/2 inches in diameter.
Both cause the same symptoms.
I found the very faint timing marks after a thorough cleaning. Engine is now set at 8or9BTDC and advances to about 30BTDC upon acceleration when it doesn't stall out.
Hesitation/Backfire occurs only on the initial accelerator pump. There is at least some gas coming from the two pump jets.
I'm open to any suggestions.
Laughing Gas
Joined: Jul 1999
Posts: 1,048
Likes: 22
From: Lacey, WA
Help- Exhaust Backfire
Richard,
Your description sounds like the Accl pump linkage needs an adjustment. I am not familiar with your exact carb as I can only relate to a Holley since this is all I have really ever worked on. Does the pump start to discharge immediatly when you move the gas pedal? There should be no delay. The pump should start to discharge as the gas pedal is pushed from idle. Holley's have an adjusting screw on the Accl pump linkage just for this. The only caution is you don't want the linkage adjusted to far in that it tries to drive the pump past it's limit when the gas pedal is all the way down.
There is another item to consider if the above doesnt work. On a Holley you can change the Accl pump nozzle size. This will increase or decrease the duration of the pump shot. Decreasing the nozzle size increases the shot duration and has been known to stop "stumbles" and lean condition back fires as you describe. Not sure if your carb has this feature or not.
Any more questions just fire away.
Good luck,
Lee
Your description sounds like the Accl pump linkage needs an adjustment. I am not familiar with your exact carb as I can only relate to a Holley since this is all I have really ever worked on. Does the pump start to discharge immediatly when you move the gas pedal? There should be no delay. The pump should start to discharge as the gas pedal is pushed from idle. Holley's have an adjusting screw on the Accl pump linkage just for this. The only caution is you don't want the linkage adjusted to far in that it tries to drive the pump past it's limit when the gas pedal is all the way down.
There is another item to consider if the above doesnt work. On a Holley you can change the Accl pump nozzle size. This will increase or decrease the duration of the pump shot. Decreasing the nozzle size increases the shot duration and has been known to stop "stumbles" and lean condition back fires as you describe. Not sure if your carb has this feature or not.
Any more questions just fire away.
Good luck,
Lee
Thread Starter
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Senior User
Joined: Sep 2002
Posts: 142
Likes: 0
From: Cary NC USA
Help- Exhaust Backfire
Thanks Lee,
Carb is a motorcraft " remanufactured". It actually has two holes on the pump lever arm and the pump rod is in the outermost hole now.
I will have to look at the timing and the volume of spray from the accelerator pump tonight.(maybe drive home at lunch) I will have to put new batteries in the flashlight.
Called the rebuilder techline. They thought it might be a pump problem. I have a second call in to see if there is any adjustment about volume or timing.
I looked at Holley website and saw a 0-80457S w 2bbl at 600 CFM listed as the number for '69 with 360. Listings that I found all had automatic choke and the price tag of $295 is more than I can stand.
I will investigate your suggestions.
If volume of carb was wrong I should have had problem before the switch out. All discussion I have heard leans toward the accelerator pump now that I am sure timing is reasonable close. And the fact that it runs well at idle and cruise.
Richard
Carb is a motorcraft " remanufactured". It actually has two holes on the pump lever arm and the pump rod is in the outermost hole now.
I will have to look at the timing and the volume of spray from the accelerator pump tonight.(maybe drive home at lunch) I will have to put new batteries in the flashlight.
Called the rebuilder techline. They thought it might be a pump problem. I have a second call in to see if there is any adjustment about volume or timing.
I looked at Holley website and saw a 0-80457S w 2bbl at 600 CFM listed as the number for '69 with 360. Listings that I found all had automatic choke and the price tag of $295 is more than I can stand.
I will investigate your suggestions.
If volume of carb was wrong I should have had problem before the switch out. All discussion I have heard leans toward the accelerator pump now that I am sure timing is reasonable close. And the fact that it runs well at idle and cruise.
Richard
Thread Starter
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Senior User
Joined: Sep 2002
Posts: 142
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From: Cary NC USA
Help- Exhaust Backfire
Lee et al.
I tried accelerating with the choke partially closed and it did not backfire. It backfired more as I opened the choke plate. Thus agreement that it was getting more air than fuel.
Investigated and found there is some adjustment on the acceleration pump rod. The end pin was in the 2nd of 5 small holes in the connection arm at the carburetor. I moved it out to the 5th or furthest from the pivot point. That I believe increased the stroke distance.
At the pump lever arm I moved the pin to the innermost of two holes and I think that started the pump earlier.
Now it does not seem to hesitate.
Will verify it on the road once I get the gears changing.
Thanks to all,
Richard
I tried accelerating with the choke partially closed and it did not backfire. It backfired more as I opened the choke plate. Thus agreement that it was getting more air than fuel.
Investigated and found there is some adjustment on the acceleration pump rod. The end pin was in the 2nd of 5 small holes in the connection arm at the carburetor. I moved it out to the 5th or furthest from the pivot point. That I believe increased the stroke distance.
At the pump lever arm I moved the pin to the innermost of two holes and I think that started the pump earlier.
Now it does not seem to hesitate.
Will verify it on the road once I get the gears changing.
Thanks to all,
Richard
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