Motorcycle Repair: sticking valve, honda z50, residual contamination

Question
bill,
         74 honda z50- wont start, ocassinal
backfires through exhaust. recently installed new
int and exh valves-lapped in and seating good.
so i pulled the head and when i turn cam the exh
valve seems to stick open slightly. with springs removed, valve[s] slide and rotate freely in guide
with no binding. whats causing the valve to ocassionaly stick? thank you for any advice!
     -gary

Answer
Gary, kind of hard to pinpoint this w/o being there to look and feel the parts together. You could have a burr in the valve guide from taking the old valves out. Often, when the valves and keepers have been working together for a long time, the keepers raise a sharp edge on the keeper groove and that can drag down the guide causing a little irregularity. Either get a guide sized drill bit or a reamer and just touch up the guide bore inside, also ensuring that there is no leftover carbon or varnish in the guide bore. If you used anything other than OEM valves, they may be slightly warped/bent or out of true from the manufacturing process. Chuck them in a drill motor or lathe and check them for being true and having no runout on the stems or the faces. Check also the stem diameter from the head to the tip for any variances. I am assuming that you have nice tight guide clearances. If you can wobble the valve around in the guide with your fingertips it may be worn excessively and/or the guide could be loose in the head!

Check and clean every part surface for residual contamination. Put only a drop of oil on the valve stem for reassembly. DO NOT put any kind of grease on the stems, especially the exhaust side, as it will coke up and gum up the guide, causing the valve to sieze in the guide.

Did you get the cam timing in okay when it went back together? If you were off a tooth, the piston and valve may have contacted each other and bent the stem.

Bill Silver