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Backfiring

Citrus1973

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Location
Palm Bay, Florida
My 'Doba is backfiring when i stab the throttle. In park or under load. I found that all three hoses from the vacuum canister were dry rotted and split. Replaced them. Changed plugs gapped to .035. New air filter. Upon starting cold did not backfire. Fter about a minute of running it started doing it again. I did notice that the old rear passenger side spark plug was full of deposits and almost burned off like at the tip. Have not changed egr valve or pcv yet. also have fuel filter to put on. Carb rebuild in order? BURNT VALVE? I hope not...
 
You didn't specify whether it was backfiring through the carb or thru the exhaust. For either, though, the causes are about the same. I'd start with the timing. It could also be a blocked heat passage or improperly operating choke, since you said it does it when it starts to warm up. As you mentioned, a sticky valve could cause it too. Popping through the exhaust could indicate a bad exhaust valve or even a faulty rocker. The only way to find out for sure is to remove a valve cover and take a look. Get (borrow if possible) a good dial-back timing light. They can be used to check not only the ignition timing, but also the operation of the valves. Something as vague as a backfire can only be tracked-down by eliminating things one at a time. Optimize your tune first, get the carb adjusted right, make sure the coil is sending a good spark and that its getting to the plugs. Kinda gotta get all Sherlock Holmes on it's ***. Eliminate the impossible and whatever is left over is your perp. Good luck, Jonny Y.
 
Through the carb. Appears to have cleared up. It has been sitting for some time, so it was probably just crudded up. A little fresh gas and sea foam seems to have helped.
 
What condition is the cap/rotor in? If there was moisture under the cap it can cause the same issue.
 
Through the carb. Appears to have cleared up. It has been sitting for some time, so it was probably just crudded up. A little fresh gas and sea foam seems to have helped.

When you started it,did it smell like stale fuel???It smells bad!!
 
What condition is the cap/rotor in? If there was moisture under the cap it can cause the same issue.

This is true, also: I found a Magnum sitting in front of a house for quite some
time so one day I knocked on the door to ask if it was for sale. The lady told
me that the engine was blown and I could have it for $100. I had to get my
friend and we came back with a car dolly. We started to load the car, that's
when the lady told me the when you tried to start it up it would backfire
really bad. Right after she said that we stopped and opened the hood. My
buddy tried to start the car and it did! It was backfiring and the first thing I
could see was that the distributor was moving! I grabbed it and turned it
until the engine smoothed out. :) Turned out to be a great car!!!
 
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