You're getting a lot of advise without knowing what the problem is. If it diesels just once, dont worry about it. If it does it regularly you're probably having detonation too. That will kill a marine engine.
Dieseling and detonation is caused by high cylinder temps igniting the fuel instead of the spark plug. This can be due to too much spark advance or it running lean. Do the simple stuff first.
Check your initial and total advance.
Check your plugs for a lean condition.
Set your idle speed to specs.
If the plugs look lean, change your filter, fuel water separator. Check for vacuum leaks. Rebuild the carb if needed. Checking fuel pressure may be a good idea too.
Carbon buildup will cause increased compression and hot spots in the cylinder. The carbon should be on the plugs too. If this is the case there are products at auto supply stores that are designed to remove it. Buy it and follow directions.
If there is that much carbon, then you've got another problem that caused that. You'll need to check out the carb, ignition secondary, etc.
My $0.02
Gary

The water trick works.
A spray bottle is what I use also.

I didn't think a HP500 would get
carboned up either, til I was working
on a friends boat, I was dealing with a starting
issue, I had the coil wire off, cranked it over,
and it "dieseled", ( this boat is regulary flogged,
so, you wouldn't think it would have any carbon
build up) this was with the engine running on
the "hose" too, ( not that much engine heat )
throttle closed.

I doubt the timing is going to cause any
issues,as far as the "dieseling"/run on
when shutting it off,
since the ignition is off, there should
be no spark.

jt

jt29olhp500s
Incresed timing causes incresed cylinder pressure=incresed heat. Heat builds up in the cylinder and is not dissapated immediately. Dieseling is caused by too much heat in the cylinder. A hot spot will ignite the compressed air/fuel insted of the spark plug. The ignition is not on during this.
Gary

It happened on three different occasions on that trip. If it can be caused by a lean out I'm scared. It hasn't been loading up after I adjust the carbs. It was only leaning out in the #1&2 cylinder's. I had better check my plugs. I'm going to fire it up on the hose tomarow to check my timing and if I have time I'll take it out and check the plugs. I haven't changed the fuel filter since I've bought it but that was only last July.

Well, I changed the cap and rotor, richened the idle a quarter turn, and ran 92 octane and the dieseling stopped. Now I'm just waiting to see if I start fouling plugs again.

Actually Shane the higher the octane the less combustible the fuel is. Lower octane fuel(bad fuel) will cause this condition because it is so volatile.Otto

Now I'm getting all kinds of soot on the transome, I can't win.

The idle mixture will have little to do with the dieseling. Readjust it to factory specs or use a vacuum guage.
Gary

the dieseling has gone away since doing this and it didn't do it before I leaned out the idle to fix a plug fouling problem.

How do I find out the factory setting?

Dieseling was probably cured by higher octane. A lean condition in the primary or secondaries will cause dieseling. The idle screw adjustment will not affect this much.
If you have fouled plugs after extended idling, it's from your idle screws. There are a couple ways to set them on holleys. Check out their website, their instructions are very good.
BTW, I said earlier that if your engine suddenly starts dieseling, you may have another problem that could cost big $ if not checked. You may still want to check that out.
Gary