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?

 

[Chris1956]

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

Waynob, That's a tough one. The motor has a distributor, so the electrical system is the same for all cylinders. Cylinders 3 and four share a carb, so that shouldn't be the problem. I don't like the "white ceramic" stuff. How about the cooling system? Does it have a steady stream of cooling water out the tell tale? If so, it sounds like carbs two and three are running lean. You may want to clean up the main jets. Sometimes 2 cycle motors "sneeze" when they run too lean.

 

[timmathis]

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

Give it a good dose of seafoam. Change plugs. Adjust carb's. Check timing, Plug wires and the distributor cap.
Tim