update...
clogged fuel filter...

Listen to Harley. Bad crankseals give exactly the issue you are dealing with. If one or both are leaking, the engine looses crank compression and the engine needs to rev up to work. The most common problem with old snowmobile engines. not that I don't believe him I just don't know much about them ....

engine runs good other than it being a little boggy at bottom..
why you say change the crank shaft seals?

oh keep me posted on this , I have a 80 trail-fire 340 and it bottoms out ,they have heavy duty ones?

Hello Everybody... I have a 340 trailfire got it for 200 bucks looks like the guy before me put a carb on it at some point in time because I have a extra one that came with it ... its a mikuni ... the only way I can get a decent idle is with the air screw all the way in and 1/2 turn out .. had it ou...