Hi guys, quick question on behalf of a friend.

He has a Chinese bike, which has what seems to be a copy of the YBR 125 engine, we had to srip the engine down to replace a bent valve, because it's a copy and not an actual Yamaha engine, it was pants. The bolt and locking nut had rattled out of the top of the intake valve and got lodged in the exhaust valve. Nightmare!

Now that's sorted, the problem is with the timing, we put everything back together, timed the engine lining up the notches in the cam sprocket with the head and the notch on the fltwheel eith the point in the timing viewhole thing (I'm not sure of the actual term for that thing) and turned the engine with a socket wrench. Now, when the enginge has gone through all 4 strokes (We watched the valves) the timing is out again!? by a good 6/7 teeth. The only way to match the marks back up is take off the tensioner and set the timing by hand again. Then, the same thing happens!

Any idea what this could be? He just wants to get it running again so he can get rid of the thing!

thanks in advance for any help, guys!

cam chain tensioner is not holding pressure on the chain,slipper guied is worn out,chain is streched beyond the tensioner ability to hold pressure

If you guys have acess to a welder you can make a manual tensioner out of a bolt and nut and a plate of steel, make a plate the same as the tensioner and drill a hole in the middle weld the nut to it and then use a bolt and another nut to set the tension and lock the nut dow to keep it from loosing,option 2 is to us a 1/2 thick piece of steel and tap threads into it.

I just checked, my friend hadnt tightened both bolts on the tensioner, one side was out a bit, I'm guessing that's the problem! :')

i'm going to tighten them in now and see if that helps!

thanks for the reply, though, i wouldn't have thought to look, otherwise!

Now that's sorted, but when I try to electric start the bike, it just sounds like a food processor! Have compression and a spark, full tank, oil is new, charged the battery, the lot!

Tried push starting it, too, but it just thuds a few times then does nothing?

try a squirt of oil down the spark plug hole, it will restore compression to help start the bike

Did you check the valve clearance , after regrinding them in,,, as you said a nut fell in and jammed valve , i take it its rocker arm , and not OHC..?

And people say 2 strokes are a pain in the ars

How much compression do you have with the cam out of time you could have bent a valve.Do a real compression test and report back