MOTOR RUN ON

------------------------------

--------------------------------------------------------------------------------
This a 1989 merc 3.7 4 cyl.
Starts great but when running from place to place and shut the kee off the motor wants to keep sputtering and run on for a few seconds.

"You need to fix it quickly or

"You need to fix it quickly or it will run backwards and injest water into the cyliners via open exhaust valves; cause major engine damage.

Pull the spark plugs; if carbon fouled replace them. If you have been using octane boosters; stop. All they do is foul the plugs and pistons w/red carbon.

Check the timing; 6 deg. BTDC. After the engine shuts off look inside the carb throat to see if fuel is leaking into the intake; carb float needs adjusted or there is an internal leak from the bowl.

Decarbonize the engine; w/the engine warm very slowly pour into the carb throat, while maintaining the RPMs at 1500-2000, 1/2 can of SeaFoam--keep it running while doing it. Shut it off w/the last drop of SeaFoam. Wait 30 mins. and start the engine and take it out on the water and run it @ 3000-3500 RPMs for 15-30 mins. Shut it off after idling for 5-10 mins.

If it still deisels then repeat the process except stall the engine w/the last 2 ounces of SeaFoam and let it sit overnight. When you start it and run it the engine will smoke alot as it burns off the carbon and SeaFoam.

Run Chevron or Shell gas in your tank a few fillups."

An idle speed that is set too

An idle speed that is set too high is often a cause of "run on" or "dieseling". Sometimes a choke which malfunctions and doesn't open fully will cause it to run on the fast idle cam and exhibit the same symptoms.

"Just curious,
Without a sou

"Just curious,
Without a source of ignition, whether from the ignition system, a glowing piece of carbon, the glowing tip of an over heated sparkplug etc. etc., how would a too high idle speed or a malfunctioning choke cause “run on” or “dieseling”?"

"Allan... all 4 cycle engines

"Allan... all 4 cycle engines actually have two compression ratios...a "mechanical compression ratio" which is the ratio of the cylinder volume at bottom dead center compared to the cylinder volume at top dead center. If the cylinder volume at BDC is 50 cubic inches and the volume at TDC is 5 cubic inches, we say the mechanical compression ratio is 10 to 1. The "effective compression ratio" of an engine with a "mechanical" compression ratio of 10 to 1 compression ratio can be as low as 5 to 1 under idle conditions. With an almost closed throttle(Idle speed) there is a very high intake manifold vacuum and the cylinder is actually under negative pressure when the intake valve closes. The cylinder may move up as much as an inch before it starts to build any positive pressure. At idle speeds when the throttle is closed, cylinder pressures are much lower than they are at Wide open throttle. The only time effective compression ratios approach mechanical compression ratios is at WOT. Even if you had an engine with a 13 to 1 compression ratio, it would run well all day on regular gas because at idle, because the effective compression ratio is actually only about 8 to 1. If an engine is going to knock or "ping", it usually does so under acceleration when the throttle is suddenly opened and cylinder pressures increase and the effective compression ratio almost matches the mechanical compression ratio. When a carburated engine is at idle and the ignition is turned off, the effective compression ratio is low and "run on" rarely occurs. If the idle speed is too high,the partially open throttle will result in a higher effective compression ratio and the engine will often "diesel" or "run on". There does not have to be a "hot spot" or piece of carbon for this to occur...the heat of the combustion chamber will do it. Almost any carbureted engine will "diesel" if you leave your foot on the accelerator when the engines ignition is turned off. Electronic fuel injection has largely put an end to this problem because the fuel is shut off when the ignition is turned off."

Bob:

Good explanation.

Bob:

Good explanation.

"yes i agree with Guy well sai

"yes i agree with Guy well said bob and with that said robert you might want to consider the pertronics ignition system"