View Full Version : Motors run on

the Captai's Quarters
06-09-2015, 07:37 PM
I have a 1996 Lakeview with twin 5.7 GM motors with Mercury Bravo 1 out drives. The starboard motor has at times " run on " after shutting off the ignition. After de-winterizing this year the port motor has done it once also.
What I have been told so far is anything from carbon on the valves, to leaking carburetors, to fuel octane could cause this. Just wondering if anyone has had this problem or could give me an idea what could be causing this?

06-10-2015, 08:02 AM
Does it last for more than just a few seconds? If it does, then something is keeping the butterflies in the carb from completely closing. My 5.0 Mercs do it sometimes and I have remanufactured carbs on them. If mine do it, I turn the key back on, put it in gear, and shut it off in gear. It being in gear normally creates enough drag to kill the motor.

06-10-2015, 08:59 AM
Even with the butterflies open the engine SHOULD stop when you turn off the ignition. My first thought would be gas, or possibly tuning.

Putting the engine in gear has another advantage. Most engines won't start in gear, accidentally or otherwise. I leave mine in gear after shutting it down.

06-10-2015, 09:53 AM
Gas engine run-on is also called dieseling. It is so called because it is similar to a diesel engine where compression causes sufficient heat to ignite a fuel-air mixture without the need for a spark. It isn't exactly the same as compression-caused heat because dieseling in a gas engine comes from hot spots in the combustion cylinder. You don't hear much about dieseling in cars these days because modern fuel injection does a better job of shutting off fuel than carburetors ever did.

Possible causes for hot spots are carbon build-up on the bottom of the head or the top of the cylinder. High operating temperatures contribute to dieseling since that can help hot spots to form. Bad carbs can contribute by dribbling gas, especially if your idle mixture is too rich.

I would get a read on engine temperature other than your dash gauge. That is more easily done than it sounds, You just aim a non-contact thermometer at various spots on the motor and pull the trigger to get a temperature readout. If you don't have a non-contact thermometer, they're pretty cheap. You don't need a $100 Fluke. You can just Google "non-contact infrared thermometer" and come up with a $20 thermometer that should serve your needs.

If your carbs are otherwise behaving badly, It might be a good time to have them rebuilt.

I would not pull heads to check for carbon build-up on an otherwise healthy engine. If your engine temp and your carbs are okay, I would just put your engines in gear when you shut them down like east tn and Stmbtwle said.

the Captai's Quarters
06-11-2015, 10:02 AM
Thank you for the reply's. The motors do not run hot according to the gauges but I have been looking for a reason to but an IR thermometer, so I'll check that. Both motors idle in the 700 to 800 RPM range but I will also check the linkages to make sure they are free.

I have been told that to get rid of carbon deposits I could pour a small ammount of water or transmission fluid down the throat of the carb while the motor is running. Not sure I want to try either of those options.

06-12-2015, 06:50 AM
I would say 700-800 rpm for idle is a tad high. Is it in or out of gear?

06-12-2015, 03:13 PM
Verify you're not running hot with the IR temp gauge 1st. If temp isn't the problem, then you may have carbon build up in the engine. What you can do is get a 5 gallon gas can and mix it with Sea Foam carb cleaner. Bypass the fuel tanks by leading a fuel line into the gas can and take the boat out for a good fast ride. If that doesn't do the trick, get the carbs serviced.