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Blown spark plug

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no1important

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Help!!! I have a 1991 Thompson fisherman with a Mercruiser 4.3 pre vortec with the thunderbolt ignition. I have 2 questions if anyone can help me diagnose a problem. What causes this motor to run rough under power but start and idle fine. And the 2nd most important question. What causes a spark plug to literally blow out of the cylinder?
 
Spark plug threads stripped out. This is probably related to the rough running. Offhand, I'd say that side cylinder head needs to come off and go to a shop for repair. Probably both while you are at it.
 
I doubt that "start and idle(s) fine" reflects reality....where "run(s) rough under power" does. Many potential causes for rough running. I'd suggest getting the factory manual and doing the troubleshooting in the sequence they offer.

2X on the threads stripped, for the plug blow outs...
 
no1important said:
Help!!! I have a 1991 Thompson fisherman with a Mercruiser 4.3 pre vortec with the thunderbolt ignition. I have 2 questions if anyone can help me diagnose a problem. What causes this motor to run rough under power but start and idle fine. And the 2nd most important question. What causes a spark plug to literally blow out of the cylinder?
Click to expand...
Ayuh,..... My guess is,..... It runs rough at speed due to restricted fuel flow,.....
'n, if the porcelain blew outa the spark plug, that's from detonation, caused by the lean condition,.....
 
The threads and nut for the plug are still in the cylinder which makes me believe your right about the detonation. How do I solve the lean condition? Is it a timing issue, carb or both? I'm going to try a solid tune up and carb rebuild before taking it in to our boat guy. Appreciate your help.
 
makomark said:
I doubt that "start and idle(s) fine" reflects reality....where "run(s) rough under power" does. Many potential causes for rough running. I'd suggest getting the factory manual and doing the troubleshooting in the sequence they offer.

2X on the threads stripped, for the plug blow outs...
Click to expand...
Had it out prior to the plug blowing, ran rough on plane but trolled and trolled without issue. Idk. Where can I find a service manual besides a Seloc manual?
 
you can usually find used ones on E b a y...or your local dealer can get you one for the going retail rate...somethings you can find a pdf download searching the www...you'll want Mercruiser Manual #7...
 
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