I have a 1991 champ that I believe is in need of a top end rebuild. The bike smokes like a 2 stroke and fuel keeps leaking past the carb and contaminating the oil. I can do the work myself but I don't know where to find to parts. Can I just buy any 100cc Yamaha piston? I think the stock one is 48mm and a big bore is considered to be 54mm. Can I have the head machined or should I just by a new one?

 

Sounds like you need a carb rebuild too. Have you done a compression check?
Might just need rings or valve stem seals.

 

Have not done a compression check as of yet. However, I was talking to a local mechanic this afternoon and I may be my own worst enemy. The float on this bike is either set to high or sticks so the carb would leak.

remember, these are my first quads so I make rookie mistakes, I am learning....


to stop the leak I put a bunch of JB-Weld over the hole that was leaking. I think this is the root of my problem and I need a new Petcock valve to start. I do believe the rings need to be replaced as well but I would like to put an aftermarket high compression piston in the bike as well anyhow.

 

I drained the fuel from the carb last night, put the drain screw back in and waited till this evening to pull it out again. One single drop of fuel came out, I guess that means my petcock is good. I am leaning towards bad rings now, so I guess I will pursue that. I will do a compression test this weekend and will also try to do a leak down test to see if I can hear the air coming out.

 

Do yourself a favor and got to tradebit.com and buy a repair manual for 8 or 10 bucks, unless you can find one somewhere free. It will save you $100's in time and even some parts.

Do a compression test, and compare it with your manual. if its like the yfm 80 they require 142lbs min
Often buy the time the piston and bore is worn it is time for new valves too.
If the bore is worn beyond standard max spec you will need to have a machinist bore and hone it to the next larger piston size.
Ken