Motorcycle Repair: 1990 Honda NX-250 22,000 mi., honda nx 250, cam lobes

Question
QUESTION: I replaced the sprockets and chain not too long ago and noticed the drive sprocket was somewhat loose on the splined trans. output shaft.  I used red Loctite on the splines but now it's loose again.  Is this normal?  If not, any fix other than replacing the output shaft?  BTW the lock plate teeth are now pretty worn too and I'll be replacing it.

thanks, Tom

ANSWER: Tom the sprocket will be a little loose and the locking plate holds it in place but its not very tight either. If its worn replace it any way it will keep the sprocket on the shaft. As far as locktite on the shaft,its not going to work as you may have noticed. Even a new sprcket will be a bit more snug but wears an the splines and wobbles. They have used this design for years and it works but I personally think its a poor design.

---------- FOLLOW-UP ----------

QUESTION: Hi Gary, same bike (4 valve), new problem.  I'm an ASE master auto tech  but this one's got me stumped!  The other morning the bike started running ratty; won't idle, back fire on decel. Runs OK at higher rpm. Acts like a vacuum leak, i.e. will rev up when spray carb cleaner into intake but doesn't respond to when spraying carb cleaner at carb mount. (no external vac. leak)  R&R and opened carb but it was pristine inside, idle jet clean, idle circuit flowing carb cleaner when blasting with the straw.

I also get some popping back sounds at intake.  Broken valve spring?  At 22,000 miles?

Answer
Tom,This is a long shot but try checking the valve clearance. This bike uses a shim under the bucket set up and may be losing the valve lash. Set the piston at TDC check the flywheel to see if the timing mark is in the timing plug hole.Also pull the plug to see if the piston is at TDC. You can use a piesce if wire or zip tie  or small screwdriver and poke down the plug hole and feel the top of the piston. The cam lobes should be pointed away from the valves,now check the clearance between the cam and the bucket.There should be no less than .004" on the intake and .006" on the exhaust.Any tighter and it can cause backfiring and erratic running. Also the shims may have a number etched into them for there thickness. Its possible that there is a valve spring broken or weak but not likely.