'97 Yamaha 50 4stroke runs at 3/4 power

[14JOHN74]

I have a problem with the motor when it's cold. I take off and it runs at 3/4 throttle for about 4-5 football fields, then it kicks in and away I go full throttle. Once it's warm it'll run all day, until you stop and fish for 45 to an hour. Then it's 3/4 until it warms up again.
DID: change plugs
replace thermostat
replaced fuel pickup and line
replaced fuel filter and bulb

Any ideas are welcome.

[99yam40]

my guess is too much or not enough fuel getting to a cylinder. On some motors the fuel pump can leak fuel into the crank case if it goes bad, but not sure that would affect that 4 stroke.

May need to clean carbs

Yamaha does recommend warming up the motor at idle before loading up the motor. Do you do this? For how long do you let it idle?

Did you mean it will not run at normal full WOT RPMs when you open the throttle wide open

[TownsendsFJR1300]

If the fuel pump is indeed leaking, the oil level in the crankcase will rise and smell of fuel if bad enough..

[14JOHN74]

I idle up the channel to the lake. Should be warm. Engine runs smooth just 3/4 RPMs. Then away you go, RPMs kick in. Thought not enough fuel or vapor lock. That's why I changed fuel lines and filter.

[14JOHN74]

If the fuel pump is indeed leaking, the oil level in the crankcase will rise and smell of fuel if bad enough..

No gas in the oil.

[14JOHN74]

????????????