120 johnson wont rev past 3500rpm and backfiring

hey all i have a 120 johnson 1987 model. when i first bought it it ran great the slowly got worse.
what i have done to try and fix the problem.
new plugs and leads
new water impeller
new gear box fluid
new cdi
new staor (which helped it start better)
water seperator filter
new fuel line and new clamps
new primerbulb, then put new fuel and oil in at 50/1. the boat seems to have little power when taking off and only goes to 3500rpm at full throttle when it tries to go past that it jerks or pulses. i also cleaned 2 carbs but the were clean as. the compression was good and 3 of four plugs were brown and dry one was wet. i jusr cant seem to figure out the problem please help????

Re: 120 johnson wont rev past 3500rpm and backfiring

Clean ALL the carbs very very well....a clogged jet could cause a cyl(s) to run lean. The cyl that has the wet plug may not be firing.

What made you change the CDI and stator?

Re: 120 johnson wont rev past 3500rpm and backfiring

Compression 'was good', but what is it now?

Re: 120 johnson wont rev past 3500rpm and backfiring

ok ill try cleaning all the carbs the best i can. yes i dont think that cylinder is firing cause the plug is to wet, though it has really good spark when i tested it.
i changed the stator cause when the engine was hot and i would go to start and move to a new fishing spot it would struggle to start then after about 3 moves it wouldnt start till it cooled down. a local mechanic said thats what it would b and he was right starts awsome now when hot.
i changed the cdi first cause another mechanic told me thats what my original problem was but turned out to be wrong thats why i changed mechanics it was an expensive mistake.
i havent checked the compression yet ill do that this arvo.
could the timing be out on this motor at all or is it non adjustable?
its a 1987 model 120hp johnson

Re: 120 johnson wont rev past 3500rpm and backfiring

hey
just cleaned the carbs 1 of the floats was a little high so i bent the metsl tab slightly so that the float sits level
the only thing i noticed was that the jet sizes were different from 1 side of the engine to the other. on the side closest to the fuel pump the jets were 16 and 33. then on the other 2 carbs the jet sizes were 36 and 16. is this the normal sizes for them?
i then did a compression check this arvo and the results were 138 psi, 141, 130, 130.
not too sure if that is good or not though?