As many others have said, it definately sounds like run-on or dieseling.

Bad fuel, with a degraded lower octane can cause this; low octane will cause the fuel to light off(detonate) prematurely, often times just the temp of the combustion chamber along with high compression will be enough.

Also, carbon build-ups will leave large enough deposits on occasion, and will glow red hot within a combustion chamber; this can mimick a glow-plug and will continue to light off the fuel after the spark is eliminated.

My thoughts exactly. Check your plugs, timing and fuel. Remember, running super high octane fuel is not beneficial if the compression is not high enough. In fact it is worse. Run what is recommended. GOod luck and keep us posted.

If it starts to diesel, just give it some gas and it should quit. The gas will help cool the hot spot.

Go to the marine store and buy Carbon Buster for 8$ a bottle.

check you idle

if its idling to high it can cause this problem

also low octane gas or carbon build up




cardoctor

Yuck, I hope not. The idle when not in gear is at 900rpms, the spark timing hasn't been touched since I've owned it (wouldn't be a bad idea to check), and the carbon build up is a possibility cuase I was having problems with loading up and fouling out plugs for a while. I think I got that cleared up though.

My 1999 HP500 will diesel every once in a great while. I run 87 octane, idle is @ 750-800 RPMS and I don't have a plug fouling problem either? Spark advance is 32 Degs. I wonder what is causing this also?
MD

Fuel might be 87 when you bought it, but if it sits long, it can degrade.

Heat Soak could also be a culprit...The engine's simply hot enough to detonate volitile fuel. This can occurr after hard runs, and shutting off the motor too quickly.

I would check idle speed first (to High), then Ignition timing. i find it hard to beleive there would be any carbon build up, unless you don't run her!!! HP500.........I'd run her!!!!!!;) :D

I wasn't running hard at all, and I was running 89 octane. I've never run less than 89 and I've run at least a tank of gas through since the winter tank. Water temp was good. I hoping I just bought some bad gas. I'm gonna check the timing anyway though. I think the cap and rotor are original (1995) as far as I can tell, if that would make a difference. My idle is at 900 to 950rpm's.