Engine Kills

[micksee]

Twin Merc cruiser 5.7 star. engine kills while running in gear preceded by a backfire. Engine starts after a few minutes and runs for awhile and then backfires and kills. It will start and run fine for awhile and then kill. Star. Engine runs and then port engine kills the same way and then starts after a few minutes. Any Ideas?

 

[Texasmark]

Pre-ignition caused by carbon buildup on pistons. Buildup caused by lugging engine for a guess.

Any backfire has to come through the intake valve. The valve has to be open when the explosion occurs. If your ignition is as it should be the only thing that makes sense to me is that there is a glowing ember in the combustion chamber that sets off the explosion. The intake valve opens to let the fresh fuel air mixture into the combustion chamber. When the mix hits the glowing ember it ignites. Valve is still open so the explosion goes back out the intake valve. With both engines doing it seems to be a common problem and the fuel you use or the way you run your rig may be the source of your problem......I'll guess the way your run your boat......not enough WOT.

Makes sense that once you blow CO2 and lack of O2 back to the carbs, with nothing to burn, the engine will quit.

What about popping the plugs and having a look at the piston crowns. Are they clean or all caked up with carbon?

Let us know.

Mark