I bought an 87 moto 4 with a yfm80. She told me it needed a key and battery that was it. So I did that and no luck. Replaced the starter, no luck, replaced the starter solenoid, no luck... replaced the starter clutch assembly, clutch, and still no luck. Decided to check the top end... ended up having a trashed piston and cylinder from water getting inside. So I bought a cylinder kit online that came with a new piston, rings, cylinder, air filter and fuel filter. So I replaced the old cylinder and piston and had the valves cleaned and replaced. I'm putting it back together and I cannot get the cam sprocket on with the chain. Either I put the sprocket on and can't work the chain onto it or I put the chain on the sprocket and can't get the sprocket on to the engine mount area. Anyone know if there are any tricks to doing this? Does it sound like the chain is bound? I'm pretty sure the 2 cylinders are identical so it's not too big for it... Please help

 

To be honest top sprockets can be pigs to get onto the cam, but you won't get them on without the chain on the lower one properly. If you can't see all of the lower sprocket, you may not have the chain on it right. Check that it is actually central on this sprocket, if it is, tension the chain lightly by pulling it upwards and rotating the crank through one turn, if the chain was not on the lower sprocket properly this should fix the problem.

 

I've tried to rotate the crank both forward and backwards to see if the chain is bound behind the lower sprocket but I cannot see it without removing the magneto. I am off by about an 1/8th of an inch. I didn't know if you are supposed to force it on the mount or if it should just go on without any problems. I figured since there is a cam chain tensioner it should be pretty loose when putting it together. The only other thing I keep thinking is that the cylinder I bought off ebay is off somewhere about an 1/8th inch and that's making the head back an 1/8th inch and causing it to not line up...

 

I can't say for certain, as I haven't rebuilt an 80 but on most bikes you can shine a light down the slot for the cam chain and see if the chain is central, you may have to take the head off again to do this so it could be as easy to remove the alternator case, but you need to be certain that the chain is on the sprocket properly. Some top sprockets have a cutout and will only slip onto the cam in one position, but this is fairly obvious.

 

The sprocket is where the problem is. The chain is on, the bolt is through but the sprocket needs to be about an eighth inch back further. I tried looking down into the lower sprocket and the chain is lined up and on, the only part I can't see is the back portion. The chain may be bound behind where I can't see but I am not sure. I guess I am going to remove the magneto/flywheel and see for sure and that will answer my question. Other than the cylinder being an eighth inch too big into the motor, I can't see why the chain isn't long enough to stretch and fit the cam sprocket.

 

Did you rotate the crank a full turn, if the chain has "looped out" on the lower sprocket, a full turn with a bit of tension on the chain will wind the looped bit off.

 

I did try that with no luck. I ended up pulling the flywheel and lower cover to look at the lower sprocket. The chain was not bound anywhere. I think I may have gotten a cylinder that is just a little too big. My consequences for shopping ebay instead of oem. I took the old cylinder to see if the atv shop can hone it...waiting to hear back.

 

well it turns out i had the lower chain guide in the wrong spot in the cylinder. there is a special groove cut out where the chain guide sits. this was putting a small amount of tension on the chain, and made it so there was not enough slack to put the gear on. so i put that in the correct spot, installed a new head gasket, and everything lines up perfectly with minimal force needed to put the sprocket on with the chain. We are now up and running, only have a few minor things to work out, and we'll be good as new.