Fouling top 3 plugs on 86 Mercury 90hp Tower of Power

tmfeaster

Petty Officer 3rd Class
Joined
Jun 22, 2011
Messages
94
If I idle. There are no problems. If I get it on plan and run 3/4 throttle it will eventually foul the top 3 plugs in cylinder 1, 2 and 3. When I pull them they have carbon on them.

If I run over 3/4 throttle it acts like its missing real bad and fouls the plugs much faster.

Timing or powepack? No issues with bottom cylinders. Good compression in all cylinders....no water in gas...carbs are clean...all coils deliver spark to all 6 plugs.
 

GA_Boater

Honorary Moderator Emeritus
Joined
May 24, 2011
Messages
49,038
I think you have two identical switch boxes. So switch them to see if the fouling moves to the bottom plugs.
 

tmfeaster

Petty Officer 3rd Class
Joined
Jun 22, 2011
Messages
94
I have switched the boxes. Seems to run better on the hose but I haven't put her in the water yet. Quick question. Should I be getting water from both exhaust ports on this engine? It only comes out of the starboard side and the tale tale hole. Good water flow out of both. However, no water is coming out of the port side exhaust port. When I pull the plugs and look into the pistons the port side of the pistons look dryer than the starboard side. Does this mean the left exhaust manifold gasket could be blown and getting water into the piston chambers?
 

CharlieB

Vice Admiral
Joined
Apr 10, 2007
Messages
5,617
Stacked 6? Is this one have two fuel pumps?

A faulty pump will leak fuel into the cyl crankcase that pulses the pump. The recirc system will spread that fuel until consumed. Pull the fuel pump(s) and see if the vacuum hose is wet inside, indicating a leaking diaphragm.
 
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