Yamaha 50 TLR Surging and Starving for fuel

[Bigpontoon]

Hello all. I have a Yamaha 50 2 stroke, 50 TLR. On initial start after pumping primer ball, engine gets to WOT and runs for a couple of minutes then surges like its struggling for fuel and rpm drops down and up about 300 to 500 rpm and never regains WOT. If the bulb is pumped it quickly goes to WOT for a short time and repeats. I have found a sweet spot at about 75% throttle where the engine will run and not surge. I also noticed that it will run better when trimmed up a bit, lowering the load...then by lowering the trim, and putting more of a load, can cause the surging to resume. Also of note, from idle, jamming it to full throttle will cause the motor to die.

the carbs were rebuilt at the end of 2019 and ran fine. The gas is non ethanol. And i use stabil. The fuel intake screen and tube in the tank are pristine. There is no blockage in the tank vent. I have replaced the check valve at the tank, the fuel line and primer bulb, the quick connector to the engine, the fuel lines to and from the fuel filter, the fuel filter, and the fuel pump. I did not change anything past the fuel pump. I also used strong spring clamps to.secure hoses.

The surging was very bad before i changed all these items. Performance is better now But the surging and poor performance still continues. I am happy that i have become more knowledgeable about my engine, but i fear that i am reaching the end of ideas.

If anyone has any ideas or shared experiences that may save my summer, it will make me and especially my kids really happy. Thank you in advance.

[ausnoelm]

Carbs not cleaned properly, tank vent blocked, fuel line degrading internally, spark breaking down on one cylinder.

[99yam40]

sounds like the motor is running lean(carb jets/passages plugging) more fuel is needed for more load, and having trouble transitioning to higher RPM
does not take much to cause problems, and changing parts may introduce debris into the fuel system.
also check your fuel pump for leaking into #3 crankcase

[TownsendsFJR1300]

With priming the bulb and the RPMs increasing, sounds like lack of fuel to the fuel pump or the pump itself.

Might try testing/running off an auxiliary tank.

If no change, inspect/replace pump.

.

[Bigpontoon]

Thanks i may have to take to mechanic to break down carbs again. I changed everything else myself. It runs way better than before, will hold at 4k rpm all day, didnt do that before. but WOT was around 5300 for me, ill let you know what mechanic says. Thanks for the help.