Have a 99 502 we re-built from the bottom up with all the good stuff and added a procharger as the engine was supposed to be built for it. Drained the tank and filled it with 93, topped it with 93 and lastly 91. Cruised for about 1/2 hour at about 3,500 rpm and ran it for about 30 seconds at WOT (5,700). Arrived at our destination and idled in, docked and shut down running perfect. 2 hours later engine is hard to turn and upon starting has a bad miss and barely keeps running. Any throttle is no improvement so we had a nice slow return back. Pulled the plug to check compression and found 5 plugs had the gaps closed to some extent and the one from #4 cylinder looked like it was dropped in a garbage disposal. Bad compression on all cylinders (2 and 8 made it to 110, the rest 90 or less, 0 on #4). Builder now says to change plugs as they used protruding tips and think the heat was wrong. I showed them the chewed up plug and they have now decided it's a fuel issue (water). The engine has 10 hours tops since re-build and I'll be pulling it again already, only been in since July. Any thoughts on causes or what to expect when I open it up? Also, would you hold the engine builder to make things right, especially since they admitted the plugs were wrong!? Thsnks in advance for aby advice!
-Nathan

Projected tip plugs + supercharger + garbage disposal looking plug= engine destroyed.

Find new builder.

Sounds like it dropped a valve seat and scattered it through the engine.

Sounds to me that the builder didn't check valve clearance or plug clearance.

Bummer!! Who was builder?

Id say it is time to start over. The only good thing is that it is only one motor.


Had similar deal a few years ago and once I started over and used a builder that knew what they were doing all I have done is put fuel in and change the oil for two seasons worth or trouble free boating.

Good luck!!

Chunks of the piston were flying off while it was detonating and closing up the plug gap. Sounds like you and the builder are going to need some adult supervision on the next build.

You will not know for sure until you pull the cylinder heads off. I wouldn't expect to find any good news. Most likely, you pushed a hole in #4 piston. If your intentions are to let the engine builder check out the damage, I would be there when that engine comes apart.

The plugs were definately all wrong for the build but at what point and time were the plugs pulled and checked to make sure it isn't lean, rich or detonating. The plugs should have been pulled out of that motor 10 maybe 15 times b4 it ever went WOT. I spent weeks on the water messing with my motor b4 I ever took it over 3000rpm. Believe me for a guy like me that's very difficult. I finally got the motor to where it doesn't detonate and it runs wide open all day long when I take it out.

as black baja said,detonation probibally severe during the wot ride.detonation can kill an engine in seconds in some cases.

Would using a wideband 02 sensor not eliminate some of the plug checking? Not saying to disreguard reading plugs.