Spark plugs and valve clearances
If you are facing problems only after crossing 60 then it can be related to fuel flow and carburettor issues
Also there are a couple of things you can check --
Check whether the air filter is dirty or the vent of the airbox below the seat is being clogged
Also examine the rubber hose connecting the carburettor to the engine
It should be properly fitted and all the screws must be tight.
Also check for over tightened chain and proper alignment of the rear wheel.
Hope this helps...



Comment