these engines (rotax 122) suffer from the powervalve going into the barrel too far and hitting the piston causing the rings to jam in and get low on compression i would say take the exhaust off and have a look up the port with a torch to see if you can see any damage on the piston you may eed to turn the engine over a few times to re position the pston to see it properly also the big ends go on them i have had to rebulid lots of them, the rotax 122 is easier than the rotax 123 as the clutch come off easier and the gear selecting mech is alot simpler.it will tell you what model it is on top of the engine behind the barrel.
adam bike doctor
July 2009
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