Motorcycle Repair: back fires at high reves, critical adjustments, combustion chamber

Question
hi michael, i am trying to get a jawa manet 98cc 1965 back on the road, it is hard to start unless i spray easey start into the carby, it runs ok at low reves but when it is opened up it backfires and smokes like hell. points, coil. ht lead are ok . it would be great if you can help regards tony

Answer
Tony,
# 1 reson for backfiring is incorrect timing.  Be absolutly sure you understand the procedure for setting the points, there are two critical adjustments. One, the gap between the points at full open AND the EXACT time or angle they start to separate.  The second reason for backfiring is liquid fuel being drawn into the exhaust pipes.  This can happen when the fuel level is too high from improperly set floats and too much fuel gets through the barrel and out the exhaust port before it has time to catch fire. Third reason, preignition from overheating or a glowing ember of residual carbon in the combustion chamber.  This can cause the mixture to ignite at the wrong time, for instance when the exhaust port is exposed.

Hope this helps,
Mike