Motorcycle Repair: z50, vacuum leaks, compression stroke

Question
I have a Honda Z50J1 motor - rebuilt top end (rebore, 1st oversize piston and rings, new valves - reseated) also new carby, engine starts 1st kick and idles ok (when cold), it seems to lose power and not idle as it gets hot. When it stalls (when hot), it will start again 1st kick. Also if one blips throttle, it idles high for a few seconds before dropping to correct idle.

Answer
Clive,  Go back over your engine settings first.  Recheck the valve clearances to .002" (.05mm) on the compression stroke, engine cold. Valves can sink in a bit on a fresh valve job and then the clearances go away when the engine heats up.

Check your compression, if possible 171psi (12kg-cm2)

Now, clean the ignition points and check to see that they are just OPENING at the F mark alignment on the flywheel and engine case marks.

Check for vacuum leaks at the intake manifold by squirting some carby spray around the gasket/o-rings.

If the "new carby" was an aftermarket, Chinese copy, then the jetting may be off from where it is supposed to be. Compare with the original unit, if possible.

Bill Silver