Hmmm. I'm not experienced with US emissions stuff, but I have a few thoughts.
Backfires in the exhaust are unburnt fuel igniting off the hot exhaust surface. So for some reason, your cylinders (on both sides) are throwing unburnt fuel into the exhaust. Does the exhaust have a link pipe between the two sides - this could cause confusion....? How did the symptoms start? Suddenly....or did they gradually get worse over time?
So unburnt fuel is in the exhaust. It is either:
- no or weak spark, so no ignition in some cylinders
- leaking valves, so mixture leaks out of the cylinders as they compress
- head gasket failure, so mixture is leaking between cylinders. Is it losing water?
- retarded timing, so the spark arrives some time after the exhaust valve is open.
- fuel blockage - so the engine is running massively lean - but that would make it miss, and not chuck smoke
My plan of attack?
0) Check that the plug leads are going to the right plugs. Easy mistake to make.
1) Check the timing. Find TDC, not by looking at the marks on the pulley, but by dialling in the top of no 1 piston though the plug hole. Mark up the pulley from the dialling check, then put the timing light on it. Check against spec. Don't worry too much about fast idle advance at this stage, you're looking for massive retardation at idle.
2) Compression test. Would pick up bad valves and leaking gaskets. If you have a leakdown tester available, that would be good also (similar principle, but better at pinpointing the problem)
Beyond that...well, I'd put a scope on the ignition and see what the heck was
going on - you'd easily see a flaky pick up on the screen. Might also be worth checking the carb for blockages.
Backfires in the exhaust are unburnt fuel igniting off the hot exhaust surface. So for some reason, your cylinders (on both sides) are throwing unburnt fuel into the exhaust. Does the exhaust have a link pipe between the two sides - this could cause confusion....? How did the symptoms start? Suddenly....or did they gradually get worse over time?
So unburnt fuel is in the exhaust. It is either:
- no or weak spark, so no ignition in some cylinders
- leaking valves, so mixture leaks out of the cylinders as they compress
- head gasket failure, so mixture is leaking between cylinders. Is it losing water?
- retarded timing, so the spark arrives some time after the exhaust valve is open.
- fuel blockage - so the engine is running massively lean - but that would make it miss, and not chuck smoke
My plan of attack?
0) Check that the plug leads are going to the right plugs. Easy mistake to make.
1) Check the timing. Find TDC, not by looking at the marks on the pulley, but by dialling in the top of no 1 piston though the plug hole. Mark up the pulley from the dialling check, then put the timing light on it. Check against spec. Don't worry too much about fast idle advance at this stage, you're looking for massive retardation at idle.
2) Compression test. Would pick up bad valves and leaking gaskets. If you have a leakdown tester available, that would be good also (similar principle, but better at pinpointing the problem)
Beyond that...well, I'd put a scope on the ignition and see what the heck was
going on - you'd easily see a flaky pick up on the screen. Might also be worth checking the carb for blockages.
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