My son and I got two complete Motobecane Mobys from a guy and are trying to make on runner out of them.
We took both motors apart and the motors are pretty shot. We bought a new stock crank, crank bearings, seals, piston, cylinder and gaskets from Treatland (pretty much everything but the head and the cases).
We put it all together and all was well until we got to putting the head on. With the head NOT in place, the piston goes up and down in the cylinder fine. However, when you put the head on and begin to tighten the 10mm nuts on top, the piston gets bound up in the cylinder before TDC. So we're not hitting the head or anything and the sparkplug is not in. If you push the cylinder forward with force, you can get the piston to move.
My conclusion is that something is making the cylinder and piston "out of true" when the head is bolted on.
Things I've looked at/considered and my conclusions...
(a) Crank (which is new) isn't centered in the case. (This wouldn't create the current behavior as the piston can move side to side on the connecting rod)
(b) Connecting rod (which is new) is bent. I checked this fairly carefully (by eye, just watching the connecting rod go up and down with no piston/cylinder and using the seam of the cases as my "micrometer" to look for movement)
(c) One or more cylinder studs (which are old) are bent and torquing the cylinder out of position. Nope, I pulled them all out and they're dead straight.
I took the motor to my friend who is the head mechanic at the local Yamaha/Kawasaki dealership ("What a cute little motor," he said when I brought it over to his house. 😂) He's the one who found that pushing on the cylinder can "free up" the piston. Pulling on the studs can also free up the piston which is what made him think a stud might be bent. He did say that he was shocked there weren't locating pins at the bottom of the cylinder/top of the case and said it was weird to him that there was so much free movement in the cylinder before the head is installed.
So I come to all you hoping someone has had this experience or can offer a troubleshooting idea I haven't thought of. To my mind, the only thing I haven't checked are the sides of the cases themselves.
I'm just completely stumped....