The engine keeps running for a couple secs after shutoff

[dlogvine]

When I shut off the engine my Ebbtide, 1998 5.0 V8 engine with a two barrel mercarb, the engine does not stop immediately but turns and fires several more times, for about 20-30 secs. I wonder if it could be caused by high idle setting on the carb, so it does overflow the carb and keeps running. Any ideas? Thx

 

[StarTed]

Often that could be carbon buildup in the cylinders/pistons/valves. Also the carburetors sometimes use dash-pots to drop off the idle when the key is shut off.

Too high an idle doesn't help.

I don't know about your engine but I'm sure some others will chime in with more detailed suggestions.

 

[Scott Danforth]

your motor is dieseling. that can be extremely bad as the last little bit of rattle before it stops is the motor running backwards, which could suck water in thru the exhaust.

something is causing a hot spot in the cylinders to be a source of ignition. my guess is either too far advanced timing or carbon build up. if the motor runs hotter than normal, this will also cause dieseling.

turn it off in gear as short term, find problem and fix for long term.

 

[Alumarine]

Another temporary solution that might work is to let it idle for a minute before turning it off.

 

[bruceb58]

I agree with letting the engine idle for awhile before turning it off. Make sure idle is low enough. Turn on blower before a few minutes before turning off.

There are no dashpots on marine engines.

 

[dlogvine]

So besides the idle running before shutting off, should i do anything else short of rebuilding the engine and cleaning the cylinders from deposit? Is there some deposit removal compound i can add to the fuel?

 

[bruceb58]

What is your idle set at now?

 

[Alumarine]

As others have said, Adjust Idle and Adjust Timing and see what happens.

 

[Fishermark]

I agree with the others - check idle and timing. If you decide to decarbonize the engine, you can simply use water. There are many videos that show how to do it. By simply adding a little water to the carb you are in essence steam cleaning the cylinders.

 

[Scott Danforth]

Bring rpm's up to 1500, pour 8 oz of water into carb slowly

 

[dlogvine]

the idle is currently running at about 1200-1300rpms

 

[Fishermark]

That's way too high.

 

[bspeth]

I think it should be in between 650-750. Set idle 750 on muffs,adjust the timing to spec,then readjust the idle.When you get to the lake adjust the idle in gear on open water.

 

[Scott Danforth]

the idle is currently running at about 1200-1300rpms

Way too high. Will take out your dog clutches in a very short time. Fix this issue ASAP.

Could be throttle cable not returning to idle, could be crud in carb and someone turned idle up to keep it running.

Should be idling at 600-650 rpm in gear.

 

[dlogvine]

I got some marvel oil to clean up the cylinders and pistons from carbon deposits. will adjust the timing next time I get the boat from the storage. I thought it was running a bit high

 

[bruceb58]

Get the idle set correctly. Don't bother with the snake oil.

 

[dlogvine]

Did set the idle, looks like tachometer was not showing the right rpms, was a little too high. Will check out the boat tomorrow on the lake