Motorcycle Repair: Low compression 73 st90, honda st90, npr rings

Question
Had cylinder bored .5 over by local shop, used Vietnam piston with NPR rings,got only 75 psi, shop had advised not to hone cylinder which I thought was strange, honed cylinder reassembled got same pressure. Is there any way to determine whether valve timing is off enough to limit compression? Also how much is involved to replace timing chain if necessary, must the engine be removed? Thank you AJ

Answer
Al, I can't imagine why the shop wouldn't want to hone the cylinder, unless they overbored it to the piston size. A proper cross-hatch ensures a good ring wear-in procedure and end result. Clearance should be in the 1 to 1.5 thousandths of an inch range.

75psi after a bore job indicates either A. Leaking valves or B. incorrect cam timing. One tooth off throws the compression down about 30-50 psi. If you put the cam timing in more than a tooth off, there is a chance that the valves got bent and thus loss of compression. You can change the camchain without pulling the motor, but the head has to come back off, so you can remove the roller that is located in the cylinder up top to allow the chain to loop over it.

http://www.cmsnl.com/honda-st90-trailsport-90-k0-usa_model736/partslist/E++06.ht...

You will have to remove the stator and rotor (using special puller tool) to be able to get to the crankshaft sprocket area to remove and replace the camchain from the crankshaft sprocket.

Bill Silver