90s Yamaha 85hp 2 stroke misfiring

[Mrfinch12]

After about 5 minutes of running on plane my Yamaha will rev down and start shaking. Will idle just fine and go slow but when trying to accelerate it shakes almost as if spark is being cut or timing is off. What I've tried
-cleaned carbs thoroughly to make sure wasn't fuel issue
-verified timing advance marks on control box are in correct position (1 line mark when idle, 2 line mark at wide open)
-alarm does not work but ruled out limp mode from overeating by unplugging temp sensor

Need ideas, would love to check coils, stator, trigger, control box, etc but have no idea of the specs .

Also and probably the strangest part is, while issue is occuring, if you go to neutral, turn engine off and fire it back up issue will be gone for about 10 seconds and engine will start to accelerate without shaking before issue returns

 

[mike_i]

Have you checked the fuel filter, 2 stroke oil level? What condition is the fuel primer bulb when the motor acts up? Is it collapsed or normal? Try pumping the bulb when the motor starts to run bad and opening the gas cap. Can it be the motor is starving for gas as the bow rises as it gets on plane? Is the fuel pickup clear? If your tank has a reserve position try switching the tank to reserve and run it. Are you able to temporarily run the motor on a separate gas tank?

 

[Mrfinch12]

Have you checked the fuel filter, 2 stroke oil level? What condition is the fuel primer bulb when the motor acts up? Is it collapsed or normal? Try pumping the bulb when the motor starts to run bad and opening the gas cap. Can it be the motor is starving for gas as the bow rises as it gets on plane? Is the fuel pickup clear? If your tank has a reserve position try switching the tank to reserve and run it. Are you able to temporarily run the motor on a separate gan

Fuel filter is new. It's not oil injected it's premix. I've tried pumping the ball while the issue is occuring. I'm 99 percent sur it's not a fuel issue. I have a 90 Yamaha 2 stroke and whenever that's had fuel delivery issue it has bogged down, this engine is not bogging down it's more like the RPM is being limited or the timing is messed up

 

[99yam40]

a timing light hooked to the plug wires will show if timing is out of spec and also show if spark is not getting to the plugs when this is happening

 

[brodmann]

You mentioned timing marks on the control box. I don't know what you're describing. Timing marks are on the flywheel. Explain how you are checking the timing. If the timing is incorrect, this could very well be your problem and could cause serious problems is you have over-advanced timing and spark knock. Here's how the Yamaha manual says to set timing without a timing light. Turn the flywheel to whatever the idle timing should be. (On your motor I believe it's 6 degrees ATDC (After Top Dead Center).) Be sure you're using the numbers on the flywheel for cylinder #1. Under the flywheel is the timing plate. It has a mark on it too. It should also line up with the mark and the pointer that pointing to that mark. This should be very close without making any adjustments. Then, without letting the flywheel turn, move your control into get and move all the way up to full throttle. The flywheel should still be where it was, but the timing plate under the flywheel should now have it's mark pointing to the timing mark on the flywheel that is your WOT (Wide Open Throttle) position. On your motor the WOT timing is either 16 or 18 degrees BTDC (Before Top Dead Center). If this mark is not where it should be, you need to make an adjustment on the length of the arm that connects between the timing plate and the throttle arm. You'll see when you throttle up and down the parts that I'm talking about.