That's another possibility.
Back fire through exhaust
Thread Starter
Advanced
Joined: May 2008
Posts: 50
Likes: 0
From: Port Washington Wi
Back fire through exhaust
Hi all,
I have a '79 L48 and experience backfireing thru the exhaust when I start it cold and when the engine is under load ( e.g. accelerating from stop) in the 1400 - 1800 rpm range. When it's warmed up it does not pop at idle. When I posted about this last year the suggestion was I was lean at idle or the timing wasn't right. Following Lar's paper I have set the timing as advanced as I can get it with out jerking at low rpm and dieseling when I shut it down and have tried turning the idle mixture screws out a full 7 - 8 turns. Since it happens when then engine is cold, is this a choke problem and I'm really too rich? As far as the pop when under load, I noticed I have fairly low manifold pressure at idle ( 10" which is something else to look at!) and I'm wondering if the power piston spring is too stiff and leaning it out during the transition to the primary circuit.
Thanks.
I have a '79 L48 and experience backfireing thru the exhaust when I start it cold and when the engine is under load ( e.g. accelerating from stop) in the 1400 - 1800 rpm range. When it's warmed up it does not pop at idle. When I posted about this last year the suggestion was I was lean at idle or the timing wasn't right. Following Lar's paper I have set the timing as advanced as I can get it with out jerking at low rpm and dieseling when I shut it down and have tried turning the idle mixture screws out a full 7 - 8 turns. Since it happens when then engine is cold, is this a choke problem and I'm really too rich? As far as the pop when under load, I noticed I have fairly low manifold pressure at idle ( 10" which is something else to look at!) and I'm wondering if the power piston spring is too stiff and leaning it out during the transition to the primary circuit.
Thanks.
Burning Brakes
Joined: Dec 2008
Posts: 971
Likes: 45
From: Middlesex NJ
Ive always understood popping through the exhaust to be a rich condition and popping through the carb to be a lean condition (with all other components working properly).
Being a 79 your car will have egr and a temperature controlled vacuum dash pot in the head I believe that controls the egr function or timing advance when the engine is cold (i simply don't recall).
10"s of vacuum is very low for your car. If you don't have leaky intake or carb gaskets or broken vacuum lines then maybe your egr valve is hung open.
I would check the compression on the motor and also with the engine running spray some carb cleaner around the intake gaskets and carb gaskets to see if you have leaky intake gaskets.
Try plugging the hose that goes to the brake booster and see how it runs.
Just some things I can think of. I also wonder if your secondary carb plates maybe hung open slightly.
Being a 79 your car will have egr and a temperature controlled vacuum dash pot in the head I believe that controls the egr function or timing advance when the engine is cold (i simply don't recall).
10"s of vacuum is very low for your car. If you don't have leaky intake or carb gaskets or broken vacuum lines then maybe your egr valve is hung open.
I would check the compression on the motor and also with the engine running spray some carb cleaner around the intake gaskets and carb gaskets to see if you have leaky intake gaskets.
Try plugging the hose that goes to the brake booster and see how it runs.
Just some things I can think of. I also wonder if your secondary carb plates maybe hung open slightly.
Race Director
Joined: Oct 2002
Posts: 12,065
Likes: 2
From: Smalltownville Michigan
Cruise-In 5, 6 & 10 Veteran
St. Jude Donor '09
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
I think you may have adjusted the valve lifters incorrectly and you have valves staying open a bit. If you did adjust the lifters, I suggest you recheck them first.
If you haven't messed with the lifters, your electronic ignition module may be 'shot'. It could be screwing up the timing on you. Also, since you have an HEI ignition, you need to follow the procedure for setting the initial timing up on an HEI system, since that module takes care of the rest.
If you haven't messed with the lifters, your electronic ignition module may be 'shot'. It could be screwing up the timing on you. Also, since you have an HEI ignition, you need to follow the procedure for setting the initial timing up on an HEI system, since that module takes care of the rest.
Thread Starter
Advanced
Joined: May 2008
Posts: 50
Likes: 0
From: Port Washington Wi
Thanks for the suggestions.
The A.I.R. system has been removed, I have not messed with the lifters. I just replaced the intake manifold gasket over the winter to try and stop my excessive oil consumption. I reset the initial timing per the paper on the timing sticky. When I checked the vacuum I plugged the guage into the PCV line. I'll check the compression and start checking the vacuum lines this weekend. I'll give the egr a thump for good measure.
Thanks again
The A.I.R. system has been removed, I have not messed with the lifters. I just replaced the intake manifold gasket over the winter to try and stop my excessive oil consumption. I reset the initial timing per the paper on the timing sticky. When I checked the vacuum I plugged the guage into the PCV line. I'll check the compression and start checking the vacuum lines this weekend. I'll give the egr a thump for good measure.
Thanks again
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