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Pit bikes engines and most motorcycle engine will smoke a little at times
But when a constant and large amount of smoke is present the issue needs to be addressed
1) New Pit Bikes and Engines MUST be broken in with Petroleum base oil (non synthetic)
2) Used or Worn engines that smoke may be suffering from:
a) worn piston rings or broken piston rings
b)worn out Piston (yes a piston is a wear item and needs replaced every other ring change)
c)worn valve seals
3)Valve sealing surface
a)The valves may have excessive carbon build up allowing a leak
this can fixes by removing the carbon
b)The valves can be slightly bent , Replacing bent valves may cure the issue , But if the valve sealing surface on the head is damaged a valve job will be required and that is a shop job
c)A simple leak test can be performed by filling the intake or exhaust port with light oil or kerosene and observing if the liquid can leak past the valve seal
4)Make sure you have 110 in lbs torque on CRF50 Type engines Min on the Top head nuts