Occasional cyl fouling 1988 70hp 3 cyl Merc

[crappie43]

Generally at early morning startup, the top cylinder does not fire unless I put the motor in neutral and race the (about 3/4 full throttle) for about 15 seconds. Most of the time this gets all cylinders firing and the fouling or misfiring problem goes away until the next day. <br /><br />I've had the carbs cleaned at a repair shop. Following that work, not much difference was noted. <br /><br />I suspect a flooding problem. Could a float be misadjusted? Any help would be appreciated. Thx

 

[andy6374]

Re: Occasional cyl fouling 1988 70hp 3 cyl Merc

Your carbs would be leaking if the floats were set to high. Check'em anyways. What's your fuel/oil mixture? It should be 50:1. I've read that t-stats that are stuck open can cause excessive fouling.

 

[AMD Rules]

Re: Occasional cyl fouling 1988 70hp 3 cyl Merc

Running a 3cyl in neutral at elevated rpm is risky business, especially when it is the top cylinder that is questionable.<br /><br />Go back to square one and try to determine if it is spark, fuel , or air that is causing the misfire.<br /><br />Have you done a spark check yet? They should be able to show a strong spark, across a 3/8" gap.

 

[crappie43]

Re: Occasional cyl fouling 1988 70hp 3 cyl Merc

Originally posted by AMD Rules:<br /> Running a 3cyl in neutral at elevated rpm is risky business, especially when it is the top cylinder that is questionable.<br /><br />Go back to square one and try to determine if it is spark, fuel , or air that is causing the misfire.<br /><br />Have you done a spark check yet? They should be able to show a strong spark, across a 3/8" gap.

 

[Bob H.]

Re: Occasional cyl fouling 1988 70hp 3 cyl Merc

When was the last time you replaced the plugs? My 87, 80hp fouls the top plug but I've got the oil injection disconnected, and I've been doing a lot of deep water jigging at idle. Old & tired engines.

 

[crappie43]

Re: Occasional cyl fouling 1988 70hp 3 cyl Merc

I replace the plugs twice a year. It's a 1988 engine, but it hasn't been run more than 15 hrs a year and that is generally at 4000rpm or less (most of my fishing is with trolling motor). <br /><br />The dealer/repair shop says the compression and general condition is excellent. Also, the engine runs strong when it's hitting on all cyls.<br /><br />Andy6374 mentioned t-stats. What are they?

 

[andy6374]

Re: Occasional cyl fouling 1988 70hp 3 cyl Merc

Thermostats. I'm not sure if your engine has them, but it is larger enough that I suspect you do. They maintain the proper operating temp, 143 degrees or above. When they are open your engine cools and doesn't when they are closed, or not as much.<br /><br />I've read that (on larger models at least) that a tstat stuck open (constantly cooling) can cause excessive fouling.