Motor is running backwards after shut off?
 

I went for a short cruise with the G/F last night and she wanted to stop to enjoy the sunset. So I back off plane and kick it into nuetral, I let it idle for about 10 seconds and turn the key off. The motor did the sputter cough, sputter cough thing a good five or six times before it quit. Whats going on here? The temp was around 60-65 outside and kinda damp. I turned the key on to check the temp and it was right at 150 degrees where it should be. The motor is a box stock hp500 non EFI. Whats the cause of this?

I'm thinking you may have some carbon buildup in the cylinders.

How old is your gas?

BAD FUEL?? Are you still on the winter tank?

No I am not on the winter tank, but maybe I just bought a bad batch. I guess it could be carbon build up too. Maybe I should go dump some 104 in it see if it clears up.

Be careful. Dieseling can cause the engine to suck water back into the cylinder when it kicks backward, leading to hydrolock. If it is hard to get it to turn over after this happens, best bet would be to pull the spark plugs and remove the lanyard before turning it over a few times to dispell any water that might be in the cylinders. Hydrolock can result in extensive engine damage.

Ive seen where 1 cylinder will get some pre-ignition or detonation on the shutdown, and if in the piston\crank are in the right (wrong) spot, the motor can start to diesel backwards. ive seen it happen in a friends (crappy) car, and it is hell to shut it down if it doesnt stall right away.

This is what I was afraid of, I'm going to make sure I have a plug wrench in with me next time I go out

Why do you think it's going backwards? Sounds like run on (dieseling). Usually due to low octane gas, high compression (carbon buildup?), to much ignition advance, or too high idle speed.
Check it all out. If it's dieseling at shutdown, it's probably detonating under load too.
Gary

You were definitely dieseling, we had that prob in one of our Apaches. Check your carb setting also

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.

even though you've run a bit this year, it does sound like a gas issue? change the fuel filters etc..??

I'd knock that idle down 100rpm and see what happens.;)

The best cure for carbon buildup is good old cold water. Start the motor and let it wam up then take a hose and spray water into the running motor through the carb. Spray a good mist but not so much it dies out. Its good to do this prior to a day your going to be boating alot as you may push a little water past your rings and a good day of running wi make sure it eveaporates off. I do min a least once a year. On my bike, you can see the intake valves and you wouldnt beleve how well water hitting hot carbon breaks it up.

Now THAT I've never heard of........

I'd get a second opinion on that procedure?

I'm sure someone will back it up. Do a search and youll see its a fairly comon procedure. You learn somthing every day.

i'm not knocking it.. it just seems you should know what your doing before trying something new of that nature! somebody could mess that one up a bit!

Hey! That's part of the problem! KNOCKING! :D :D :D

The water trick is a old as the hills...any old school mechanic will tell you that . If done correctly it WILL knock (pun intented)the carbon off the valves and pistons.

I would suggest a spray bottle of water instead of the garden hose. A lite mist is what you want. If you've ever seen the inside of an engine that has had a reversion or leaking exhaust problem you know this way works wonders, steam cleaned even. You want to be careful not to make the engine ingest a large amount of water at one because you can actually embrittle the valves and cause them to eventually shatter. That is extreme amounts of water though. If that engine is idleing 950 in gear it is too high. If it is 950 in nuetral, about right. I'd take a spark plug reading to make sure when working through the rich condtion a lean one was not created. Sounds lean to me.

BT :cool:

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