Engine misfires over 4000 RPM on 1989 Evinrude 150 XP

[Eddy]

I need some help troubleshooting engine misfire over 4000 RPM on 1989 Evinrude 150 XP. Engine starts right up. It's a little cold blooded as usual with electric choke but idles fine. Accelerates slow in gear. No problem with acceleration in neutral. When you get around 4000 RPM engine skips or misfires. Changed spark plugs with no results. Compression test is 95 to 100 on all 6 cylinders. Rebuilt all 3 two barrel carbs last year because of really slow acceleration. It ran great after carb rebuild last year. Now its back to really slow acceleration and now misfire over 4000 RPM this year. Would running engine tuner do any good or do I have multiple problems? Engine model number is E150STLCE, S/N 08189077. Any advice would be helpful.

[gearcaseman]

Eddy, A couple of things come to mind from my old crossflow days, Go to each bypass cover with engine running on water hose and spray soapy water around the gasket area of each one and watch for bubbles. This area is bad to leak! Secondly, check each reed box for possible broken reeds as they can cause a problem like your describing. Hope this helps.

[Eddy]

OK. I'll try that. This original problem of slow acceleration got a little worse each year. It would run great after I ran some tuner through it then later on that same day it would be back again. Thanks

[gearcaseman]

Maybe a sync & link would also be a good idea, also, take out your high speed jets and make sure they are clean.

[luvdmymerc]

The power packs could also be going out. They are notorious for working OK at lower
RPM's then screwing up at the higher RPM's.

[Eddy]

Verified sync & link with the shop manual and made small adjustment. Pulled new spark plugs with 2 hours on them and the left bank was starting to foul up while the right bank was nice and clean. Checked compression from last year still good. Ran outboard on garden hose with some old spark plugs and leak checked bypass covers with no leaks. Pulled carb cover and injected Mercury Tuner through each carb real good until engine stalled. Let it set for 30 min. then started it up and ran fogger fluid through primer valve with 2 second bursts and took it to the lake. Acceleration and top end was 4750 RPM @57MPH on speedo with old spark plugs.
The boat sat for 9 months and possibly gummed up the high speed jets.
Thanks For Every-Bodies Help!

[gearcaseman]

Glad it worked out!