Mercury 150 HP(1974 model) high speed miss

[waynob]

My Merc 1500 has a high speed miss that is worse on damp days. The compression is good. Spark plugs 1 thru 3 had brown oily ceramic at the electrode and 4 thru 6 had white ceramic at the electrode. It seems like it is an electrical problem. Any idea what the problem is?

 

[Boatwizard]

Re: Mercury 150 HP(1974 model) high speed miss

Possible causes,
Dirty distributor cap/ broken carbon brush
Incorrect float setting/ trash in carb/ strainer screens dirty
Low fuel pressure at high speed
Exhaust plate leaking water into cylinders
Worn out carb housing around throttle shaft
Plastic ventury missing
Bad plug wires
Trigger bad-check timing running down the lake at WO
A lot of stuck rings
Timing wrong
There's more, but this will get you started. Good luck dude.

 

[tashasdaddy]

Re: Mercury 150 HP(1974 model) high speed miss

have you ever change the plug wires, damp days, leads me to believe that they are arcing to the block or something grounded. check all your wiring, i have a '71 1350 and had to put a new wiring harness on due to cracking insulation. my plug wires had been replaced.