1998 gs acceleration problem

[Portwol]

Ski starts and runs great for 15 minutes. Then it begins to bog down on acceleration and will not speed up. Will continue to idle fine and will run at low speed. Carbs rebuilt,new fuel lines,fuel selector,fuel filter. Please help!

 

[mikidymac]

Try opening the gas cap and see if it runs, could be a bad vent check valve.

 

[Portwol]

Try opening the gas cap and see if it runs, could be a bad vent check valve.

Ok , I tried opening the gas cap and the same thing happened. After 15 min the engine sputters when accelerating, bogs down and speed is slowed till it will only idle. I really thought I had a fuel / vent blockage- may still. Any thoughts?