Chugs to stop when ignitions is shut turned off

[Joe12345]

I have a Mercruiser Alpha Force One 135HP I/O that was bought on my Bayliner last year. My question is that sometimes when I shut the motor down the engine chugs or backfires until it finally stops. What could be the cause of this?? Any help would be appreciated. There is only about 40 Hrs tops on the motor.

 

[Trent]

Re: Chugs to stop when ignitions is shut turned off

Common problem.<br /><br />Make sure timing is correct. Make sure the idle is 650-700 rpm. Let the engine idle for 2-3 min before shut down after a hard run. Switch to a higher grade of fuel. <br /><br />Do all above or one at a time til your problem goes away.<br /><br />Letting the engine cool down and your idle speed as stated above are the ones Id do first. Then a higher grade of fuel.

 

[olbuddyjack]

Re: Chugs to stop when ignitions is shut turned off

Chances are the idle is too fast. Carboned up cylinders could cause it too but not as likely with 40 hours. I remember something being said about a tsb and some "off road" carb tweaking in previous threads but i don't know if it concerns your setup. Verify your tune up specs and go from there.

 

[Trent]

Re: Chugs to stop when ignitions is shut turned off

This is Mercrusier service bullitin on this very subject...<br /><br />MCSB 97-17<br />Revised November 1997<br /><br />Engine 'Dieseling' or 'Running-On'<br /><br />MCM 4 Cylinder, V6 and V8 with Carburetors.<br /><br />Under certain conditions, carbureted engines may 'run-on' when the ignition key is turned off. Normally, it will take a combination of several of the following conditions to make the engine 'run on'. These conditions include but are not limited to:<br />1. Type and formulation of gasoline available in the area.<br />2. Type of boat and its engine compartment ventilation system.<br />3. Outside air, inside engine compartment air and engine coolant temperatures.<br />4. How quickly the engine is shut off after running at cruising or higher RPMs.<br />5. How the boat and engine are operated.<br />6. Engine idle RPM set too high.<br />7. Wrong heat range spark plugs or spark plugs with deposits on them.<br />8. Deposits in combustion chamber.<br /><br />Carbureted engines, unlike EFI, do not shut off the fuel supply to the engine when the ignition key is turned off. As the engine is coasting to a stop, air is still flowing through the carburetor into the manifold. This air flow can still draw fuel from the carburetor, through the intake manifold and into the combustion chamber. A hot spot in the combustion chamber will cause this fuel/air mixture to burn. The engine now 'diesels' or 'runs on' although the ignition system is not firing. EFI engines do not have this problem because the injectors do not inject fuel once the ignition key is turned off.<br /><br />Suggestion to Minimize 'Run On'<br />Items number 1 and 2 can correct or reduce most of these 'run on' problems. Letting any engine idle a minute or two before shutting it off is desirable. This helps the engine to normalize at idle RPM before being shut down.<br />1. Idle engine 1 to 1-1/2 minutes before turning key off after running at cruise or higher engine RPM.<br />2. Be sure engine idle mixture, idle RPM and ignition timing are correct. On 4 cylinder and V6 or V8 engines with Thunderbolt IV ignition, setting idle RPM as low as possible can help stop 'run on'.<br />3. Try a different brand of regular fuel or try some premium fuel.<br />4. Increase idle mixture by 1/8 turn 'richer'.<br />5. Go to next colder heat range spark plugs.<br />6. V6 and V8 only: Install a 140o F. (60o C) thermostat.<br /><br />Available Colder Spark Plugs<br />Original Spark Plugs in the Engine: AC-MR43T AC-MR43LTS Colder Replacement Spark Plugs: AC-MR41T AC-None <br /> NGK-None NGK-TR6 Champion-RV12YC OR Champion-RV91MC Champion-RS9YC

 

[O'Jim]

Re: Chugs to stop when ignitions is shut turned off

Had this very problem yesterday, with our 94 Sunbird OMC 4.3L. Ambient temp was mid 90's, lake water 80*. Engine temp 160* idling for 1-2 min did not help. Dropped idle speed from 800 to 650 RPM, helped a bit. Fuel was crappy 87 octane (only thing the local marina had). Motored clear across this fairly large reservoir to another marina, which sells only 89 octane. Pumped 10 gallons in on top of the 15-20 remaining, problem solved! Even had to do a couple fairly quick shutdowns to retrieve fallen skiers, and no more dieseling. I would think an automotive engine with very low hours would run on 87 octane. And it has, in cooler weather... From now on, it will get minimum 88 octane fuel.

 

[18andup]

Re: Chugs to stop when ignitions is shut turned off

My 4Cyl 130hp Merc does this also, but noticed it goes away when I idle for awhile.

 

[maphillips3]

Re: Chugs to stop when ignitions is shut turned off

I have had the same problem with my 86 volvo, after a minute of idle it will shut down ok without the sputter, and this is running it on the 87 octane.