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5.7 Mercruiser Thunderbolt Ing. Backfires thru carb at 3000 rpm Help??

J

JOwen

New member
Have a 1999 Bayliner 3055 w/twin 5.7 Mercruiser. One engine backfires thru carb at 3000 rpm. I had carb rebuilt, replaced fuel filter, put on new dist cap and rotor, new coil, new platinum plugs, still the same. I had a friend in the engine room while running and making it backfire, he removed plug wires one a time untill it stopped backfire. Number 3 cylinder plug wire off and it ran on up and did ok. We thought maybe a valve... Others say NO WAY. Help
 
Compression test #3 cylinder show no compression at all... Tested #1 cylinder that gauge was working it reads good on it.
 
Sounds like you know where the problem lives....just gotta figure out if you are gonna fix it or get somebody else to.
 
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