Surging

[jamesleon]

1988 Yamaha 40 hp outboard. Model 40ETLH. Code 6H4. Serial 554267. Replaced all three plugs and coils, engine has 115 lbs compression on each cyl. Motor runs excellent at full throtle. Mid throtle motor surges, feels like losing a cyl. Number 2 plug fowls out (wetter than 1 and 3). Thanks for any help.

[mitch]

check your fuel pump diaphram for a leak into crank case