Why do the Yamahas pick on me? I need some YT125 help!

[thefox]

OK, I need a few ideas on my bro's YT125. It gets very little compression, like under 20psi. what should I check out? Here is the background info. This is the fugly "quad" that my bro (theolderfox) got about 2 years ago and changed it back into the yt125 it was originally. The engine had a cracked case and piston so he got another lower end that was supposed to be good, and a new stock bore piston, the cyl was also honed. He put some gas in the cyl and pulled the engine over and it started and ran for almost a second. After that it would pull over very easy. Since then it sat outside for a few months. I took it apart over thanksgiving break to see if one of the rings broke or something, they looked fine so I slapped the thing back together and back outside it went. I will have some extra time to work on it in the next few weeks so what should I look for? I am not a 2 stroke guy.

[2XTREME]

Take the cylinder and piston to a motorcycle shop and have them measure them to make sure there isn't too much clearance.It sounds to me like you need to have the cylinder bored and get a oversize piston for it.Just a thought.
X

[holmstrom]

Did you measure the bore? Hows the bore taper or any out of round conditions? Check the gaskets, torque everything to specs.

[thefox]

I am not sure on how close the bore size is as it was my brothers project. It did get good compression after he put the new piston in, it was just after it tried running that it lost compression.

[Unclediezel]

Check yor reeds---You can only squash what you can take in---if the reed is restricted-it wont let any charge in----and it wont have anything to compress

[Howdy]

I am not sure on how close the bore size is as it was my brothers project. It did get good compression after he put the new piston in, it was just after it tried running that it lost compression.

Unclediezel may have hit the nail on the head here. It's very possible a reed broke. This would explain the sudden loss of compression. It could be other things as well, but it sounds more like a broken reed.
Howdy

[thefox]

Good idea, What should I check with them though? Do they just sit flat on the cage? I think they are new reeds on a used cage but sice this is our first 2-stroke trike they could have been put in wrong or something.

[350x'inNY]

Good idea, What should I check with them though? Do they just sit flat on the cage? I think they are new reeds on a used cage but sice this is our first 2-stroke trike they could have been put in wrong or something.

Hey... brother fox'es... You want a NOS HL shroud for the 125? I certainly owe ya 2 for all the wenching on my yellow banana's!~ My 200ERN still lays apart... we got to fire up the furnace at the shop and do some winter wrenchin and riding!

Bruce

[thefox]

Well Bruce if I do get this thing going it will be a beater for friends to ride so I won't need any nice parts on it but thats for the offer. As for riding and wrenching I will probably be up for heading out that way over christmas break.

[350x'inNY]

Well Bruce if I do get this thing going it will be a beater for friends to ride so I won't need any nice parts on it but thats for the offer. As for riding and wrenching I will probably be up for heading out that way over christmas break.

[Unclediezel]

I cant think of a way to put them in wrong?---If you need some scans , Yell back

[thefox]

I wasn't there when he put them in so I don't know how they were installed. I have the Haynes manual for it I just don't really know where to start. Would bad crank seals cause this? Maybe a leaking gasket? I just find it weird that it gets almost no compression even though the piston moves up and down.

[Unclediezel]

I had the same problem switching from V8's to two strokes FOX. It seems nothing makes sense...Try This---

http://science.howstuffworks.com/two-stroke.htm