Posts
2404
Joined
7/9/2015
Location
Saint Marys, OH
US
Edited Date/Time
10/4/2017 3:40pm
Hey guys I've got a yz250 I offered to fix for my cousin who doesn't know much about bikes other then how to ride them. He had some issues with the bike and after listening to it I for sure knew it had an air leak. High idle, rev hang, hard to start. I first started with the carb and it was fine. Took the reed cage out (vforce3) and there was I only one gasket. The gasket was on in between the cage and air boot. I suspected it was sucking air from that spot since the intake was super dirty. I've had a lot of other people tell me the crank seals are bad and need replaced. Whats the best way to replace these? Pull the engine, split the cases and go about it that way? Or can it be done with the engine still in the frame? I'm also wondering about the timing plate. Does it only go one way or can you move it and mess with timing. I don't want to screw that up. Any help is appreciated 
A service manual would also be helpful.
Paw Paw
If the hanging idle is the only issue and no excess smoke or trans oil usage is noted, then the seal behind the flywheel may be all that needs to be changed if no other problem is found.
Paw Paw
The Shop
As Paw Paw very wisely suggested, its the only way to properly diagnose your problem with out just throwing parts, time, and labor at it (and potential frustration). Your lean air leak could be the dry side crank seal, base gasket, intake boot reed cage, or even the case halves themselves (rare but possible) among other things.. Just about all can repaired with the engine in the frame.
Even if they look good visually, the only real way to tell is to do a proper leak down test. The cheap 4 strokes ones dont work on two strokes. You need to either make one or buy one specifically for 2 strokes. They are just as important as a flywheel puller and torque wrench IMO and will save you time and money diagnosing problems. Also gives me a piece of mind after building a fresh motor on my bench before installing it in the frame.
I'm an old guy who has rebuilt a lot two strokes, so take it with a grain of salt. Paw Paw is giving very wise advice. Your experience may vary.
Doesn't this sound like a plug pilot and the rider has turned the idle screw way up to make it idle?
You'd get a over run and fast idle.
Hard to start because no Venturi at kick and no fuel from jet? Hard to start because when you turn idle up it raises the slide to much, causing air and no fuel. Then if the throttle cable becomes tight because you fix the slide you'd have to adjust that also.
Just asking, I have no idea what is wrong with the YZ.
If crank seal was leaking on the left the thing would rev to the moon and scare the pants off you, if the crank seal was leaking on the right you'd know it, you'd have burning man smoke coming out.
If the reed was leaking you could just spray contact cleaner at it and see if it changes anything.
Another test would be to lean bike over until fuel comes out over flow, this would spill fuel into cylinder, if it started fast then you'd know you are missing fuel at kick start, flooding it over comes the loss of fuel and bike starts fast.
All you have is a fast idle and over run, sings carb, carb, carb.
I've been wrong many times before, take it with a grain of salt, internet cycle repair is difficult.
Post a reply to: Yz 250 crank seals