Timing and Backfire
Thread Starter
Intermediate
Joined: Jun 2011
Posts: 31
Likes: 1
From: Calgary AB
Timing and Backfire
I am fairly new to working on my car ('75 L48) but I managed to follow Lars' great instructions to reset my timing to close to his specs. I have an issue now with backfiring. The backfire will occur when I release the accelerator after a period (short or long) of high rpm (like passing or full throttle acceleration). Have I advanced the timing too much? Perhaps also of note is that I live at a high altitude (3600'). Thanks for any ideas - I was considering new wires, could that help? Distributor worn? These are ideas I got from other threads but I wasn't sure if those situations were similar to mine with the backfire only happening after high throttle application.
Thread Starter
Intermediate
Joined: Jun 2011
Posts: 31
Likes: 1
From: Calgary AB
Thread Starter
Intermediate
Joined: Jun 2011
Posts: 31
Likes: 1
From: Calgary AB
So I've adjusted the idle mixture screws and the car still pops and/or backfires when releasing the accelerator after depressing it sharply. Thanks for the suggestion, I'll do some more digging.
Tech Contributor
Joined: Aug 1999
Posts: 14,385
Likes: 6,413
From: At my Bar drinking and wrenching in Lafayette Colorado
"Backfiring" occurs when the engine pops up through the carburetor. "Afterfiring" occurs when there is popping in the exhaust. I assume you are experiencing "afterfiring" and not "backfiring"..(?)
If timing is grossly out of spec, backfiring can occur. Once timing is so far off that the engine is backfiring, the vehicle is usually not drivable.
Afterfiring is normally a sign of a lean condition - not a timing issue. If the problem occurs with the throttle closed, you have a lean idle and transition circuit. An idle mixture screw adjustment will usually not cure this - you usually need to check float level, APT height adjustment, and IFR sizing. If you have a commercially rebuilt Q-Jet, you may also want to check to see if they removed your low speed air bleed restrictors.
Lars
If timing is grossly out of spec, backfiring can occur. Once timing is so far off that the engine is backfiring, the vehicle is usually not drivable.
Afterfiring is normally a sign of a lean condition - not a timing issue. If the problem occurs with the throttle closed, you have a lean idle and transition circuit. An idle mixture screw adjustment will usually not cure this - you usually need to check float level, APT height adjustment, and IFR sizing. If you have a commercially rebuilt Q-Jet, you may also want to check to see if they removed your low speed air bleed restrictors.
Lars
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Thread Starter
Intermediate
Joined: Jun 2011
Posts: 31
Likes: 1
From: Calgary AB
I set the timing last year a few weeks before I parked for the winter - I went right up to Lars' 36* but had to back off to around 32* because of much more frequent backfires... I guess maybe I have to back off more. Total timing was in the 50* range.
The backfire is at the tailpipe.
Cheers
The backfire is at the tailpipe.
Cheers
Thread Starter
Intermediate
Joined: Jun 2011
Posts: 31
Likes: 1
From: Calgary AB
"Backfiring" occurs when the engine pops up through the carburetor. "Afterfiring" occurs when there is popping in the exhaust. I assume you are experiencing "afterfiring" and not "backfiring"..(?)
If timing is grossly out of spec, backfiring can occur. Once timing is so far off that the engine is backfiring, the vehicle is usually not drivable.
Afterfiring is normally a sign of a lean condition - not a timing issue. If the problem occurs with the throttle closed, you have a lean idle and transition circuit. An idle mixture screw adjustment will usually not cure this - you usually need to check float level, APT height adjustment, and IFR sizing. If you have a commercially rebuilt Q-Jet, you may also want to check to see if they removed your low speed air bleed restrictors.
Lars
If timing is grossly out of spec, backfiring can occur. Once timing is so far off that the engine is backfiring, the vehicle is usually not drivable.
Afterfiring is normally a sign of a lean condition - not a timing issue. If the problem occurs with the throttle closed, you have a lean idle and transition circuit. An idle mixture screw adjustment will usually not cure this - you usually need to check float level, APT height adjustment, and IFR sizing. If you have a commercially rebuilt Q-Jet, you may also want to check to see if they removed your low speed air bleed restrictors.
Lars
Racer
Joined: Sep 2011
Posts: 414
Likes: 55
From: Humboldt county Ca
Oh *****! All those years ago when we ran no mufflers and backfired through the pipes (usually at the BACK of the car) you mean we were actually afterfiring? I'm gonna point one pipe toward the front and one out the back so I can fore-fire, backfire and after-fire, all at the same time.
COOL!
COOL!
