Question about 50cc 4-Stroke Shortcase Engine - QMB139

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50cc qmb139 64mm valves to 69mm now motor really tight wont start with starter burnt up 2 starters will kickstart with a fight

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When reinstalling the head you most likely damaged the timing chain or it's bound up. another possible issue is it's put together wrong, i.e. cam gear, timing chain guides. without the spark plug in that engine should roll over by hand... Changing valve size isn't a part of the problem. If you swapped a different cylinderhead to do this then is it part of the problem ???

Posted on Oct 20, 2013

Testimonial: "yes I got a rebuild kit and if it starts it runs great but cant use the elec start bec it wont turn the motor over but can kick start it with a fight"

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4. Bank angle sensor needs a reset or is faulty.
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