Motorcycle Repair: 1970 honda qa50, manifold vacuum, cam timing

Question
hi bill,i,ve got a 1970 honda qa50 that i re-built,it was running great until i tore it back down to fix a slipping kick starter! now it will start briefly(runs about 5 to 10 seconds)then dies? it only starts with the choke off! will not start with choke on! and when it starts it feels like its at about 25% power(just sort of putting)then dies? i figured it was a fuel delivery problem so i cleaned carb and still the same!! i then took a carb off a running qa-50 i figured it would start right up....but no,same problem...runs about 10 seconds then dies out! the points are set dead on,i,ve got a really strong spark(new coil and condenser)but cant get it to run longer than a few seconds??? could this have anything to do with valve settings?im starting to wonder if i have the timing chain off by a tooth??? would the bike run like im describing it if the timing chain is off??any input would be greatly appreciated!! thanks,rick

Answer
Rick, check the compression readings after you have rechecked the valve clearances. You should be reading about 150-187 psi according to the Honda books. If you are down 20-30% from that and you know that the valves are seating properly, then the cam timing is probably wrong

The engine will probably run (poorly) with the camchain off a tooth. More than that, then they won't start at all. You have ruled out all the other possibilities, it appears to me. 10 seconds isn't enough to run the fuel bowl out of gasoline, so the problem must lie somewhere else.  The lack of power and early shut down all seem like cam timing to me. When the cam timing is off, it affects the carburetion directly, due to changes in manifold vacuum signals to the metering circuits.

Bill Silver