I have a 250x, .060 over 12.5 piston. Recently it developed a sluggish condition from about 3/4 to full throtle. I really notice it it 3 to 5 th gears when it nail it while going at an average pace.
I don't have an airbox lid and I run a 128 main in the stock carb, that I have already cleaned.
Is it a lean main jet, or maybe a vaccum leak of some sort. This thing has about killed me twice now by bogging out on hills in third gear. What do I do???

 

First of all, what does your spark plug look like? It sounds to me like your main jet is off. If the plug is reading black and oily the main is probably too large. If the plug is whitish then it is lean. Hope this helps.

 

YOU HAVE TO DO YOUR PLUG READINGS AT FULL THROTTLE. GO THROUGH ALL THE GEARS, WHEN YOU GET TO 5th GEAR FULL THROTTLE SHUT THE MOTOR DOWN, DON'T LET IT COME TO AN IDLE. JUST SHUT IT DOWN, THEN TAKE YOUR PLUG READINGS. LIKE QRDUDE SAID WHITE COLORED TO LEAN INCREASE JET SIZE, BLACK OR VERY DARK COLORED TO RICH, SMALLER JET. YOU WANT THE PLUG TO BE LIGHT BROWN ABOUT CARDBOARD COLOR. HOPE THIS HELPS.

 

If you're not running race fuel, I would suspect a seizure due to overheating -- causing piston collapse and ring failure. This would cause the symptoms you have.

Your jetting sounds about right.