My engine ran smooth until it got hot. Then it would make a random metallic sound like something being flung around inside the engine. I completely disassembled the engine and cleaned out a lot of black burnt oil (I think) including some big chunks of what appears to be carbon. I don't think the prior owner of this bike ever changed the oil. Anyway I did not find anything mechanically wrong inside the engine. I expected to find the gear rotation limiter broken but it was not. After about four hours of cleaning I reassembled the engine/transmission, installed a new clutch, cylinder and piston & rings. I adjusted the valves while the head was off and easy to get to and set the cam timing alined with the crankshaft/flywheel key. Much to my supprise the bike started on the second kick and idled smoothly. It has a lot more power now, gets my 250 lb rearend moving much easier now, and the metallic sound is gone. I don't know exactly what fixed it but it is gone. Could have been piston slap maybe. Bike runs good but I am taking it easy till the rings get seated in. I do need to fix a little oil leak from the head gasket and fiddle with the ignition timing some but overall I am happy with the results.
1970-73 C70 Honda Service Manual: http://tinyurl.com/6ebwtw
1970-73 C70 Troubleshooting Guide:
http://tinyurl.com/6ebwtw (scroll to sec.7.1 page 101.jpg)
1980-83 C70 Honda Service Manual: http://tinyurl.com/hu42c
1980-81 C70 Troubleshooting Guide:
http://tinyurl.com/hu42c (scroll to sec.18-1 page 170.jpg)
1982-83 C70 Troubleshooting Guide:
http://tinyurl.com/hu42c (scroll to sec. 20-28 page 205.jpg)
1980-81 C70 Maintenance Schedule: http://tinyurl.com/z4zn6
1982-83 C70 Maintenance Schedule: http://tinyurl.com/hw35c
More resources for C70 Passports and Cubs:
http://www.shlaes.com/Vehicles/Scooter.htm
http://www.velodrome.com/HondaC70/HondaC70.html
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