15 Johnson poor running at high speed

[islander9]

Hello folks, I have a 1988, 15 hp Johnson model CE15RCCS with a high speed running problem. I have a fair bit of experience working on two strokes, but this one has me scratching my head.

Engine runs very good at idle and low midrange, but won't accelerate to full throttle. It doesn't seem to have an ignition miss, but you can see a trail of smoke behind you at WOT. To me it seems like the power loss starts as the fuel is transferring from the low speed jet to the high speed jet??

Following are my actions/observations:
- cleaned (soaked) carb, installed carb kit
- replaced head gasket & thermostat
- combustion chambers look consistent using boreascope
- primer ball stays hard
- compression is 120 each cyl
- repl impeller
- tried another power pack
- top plug seems lean (slightly white), lower plug is much darker.
- runs the same with either plug wire removed
- replaced spark plugs
- at idle, spark is consistent using two inductive timing lights to monitor
- removed carb to verify assembly and adjustments- ok
- hooked up known good fuel tank & line

At this point I am unable to determine if I have a spark or fuel problem. I was hoping someone might be able to help me with diagnosis tips to determine the source of the problem.

Appreciate your thoughts....

 

[5150abf]

Re: 15 Johnson poor running at high speed

Probably your high speed jets are a dirty, try squirting a small amount of fuel into each cylinder, if one lights up when you do you have a fuel problem.

You will have to run it on the water or a test tank though, some times it is hard to seperate ignition and fuel problems.

 

[islander9]

Re: 15 Johnson poor running at high speed

Thanks for the tip, I have cleaned the carb by soaking it and removing the weltch plugs, then installed carb kit. I also removed it a second time to check it. Seems to be fine...but?

I tried slightly pulling the choke out at WOT which reduced the rpm, leading me to believe it might be running rich if anything, although I do have a slight difference in plug colors?