Yz 250 crank seals

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10/4/2017 2:11 PM

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 Photo

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2011 Yamaha YZ 250 -MX
1996 Yamaha Banshee 350 -Trails
1982 Chevy S10
2005 Pontiac GTO -451rwhp

10/4/2017 2:44 PM

Both can be changed while the engine is in the frame. You have to drain oil and coolant. The timing plate can be market for position prior to removal. You will need a flywheel puller for that bike, gaskets and etc.
A service manual would also be helpful.

Paw Paw

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10/4/2017 3:22 PM

Paw Paw wrote:

Both can be changed while the engine is in the frame. You have to drain oil and coolant. The timing plate can be market for position prior to removal. You will need a flywheel puller for that bike, gaskets and etc.
A service manual would also be helpful.

Paw Paw

Thanks PawPaw. I have oil and coolant already drained. Flywheel puller is in the tool box along with a manual for my 2011 YZ. Never thought to mark the timing plate. Thanks again

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2011 Yamaha YZ 250 -MX
1996 Yamaha Banshee 350 -Trails
1982 Chevy S10
2005 Pontiac GTO -451rwhp

10/4/2017 3:36 PM
Edited Date/Time: 10/4/2017 3:40 PM

You should be good to go then. BUT, have you done a leak test and a vacuum test to see if they are bad? You may still have another air leak and the seals may be ok. I hate to see you change them only to find there is still another problem.
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

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10/4/2017 4:58 PM

I've heard a few issues with the v force reed blocks leaking air

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10/4/2017 11:50 PM
Edited Date/Time: 10/4/2017 11:54 PM

Paw Paw wrote:

You should be good to go then. BUT, have you done a leak test and a vacuum test to see if they are bad? You may still have another air leak and the seals may be ok. I hate to see you change them only to find there is still another problem.
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

Leakdown tester is a must have to do any work on two stroke IMO. You dont have to buy the fancy nearly $300 one from Motion Pro (though they are very nice to have.) I made mine for $35 from parts available at any big box hardware store like home depot in the plumbing dept. Lots of folks do, google search will provide you with instructions.

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.

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10/5/2017 1:12 PM

I've rode with him a few times at the track and he was still riding it with the issue and I didn't notice any abnormal smoke or more smoke then usual out of the exhaust. I'd say a normal amount of oil came out of the bike when I drained it also. Gotta love it when people run 10w30 in there sad So hopefully it's just the stator side. I have vforce3 reeds in my 2011 also so I'll do a reed & gasket swap tonight quick and see if it's any better or no help.

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2011 Yamaha YZ 250 -MX
1996 Yamaha Banshee 350 -Trails
1982 Chevy S10
2005 Pontiac GTO -451rwhp

10/6/2017 5:37 PM

When you say the carb was fine did you actually have the clean pilot jet in your hand and see through it?

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.

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10/7/2017 6:44 PM

speed_racer wrote:

When you say the carb was fine did you actually have the clean pilot jet in your hand and see through it?

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.

Very good ideas. I'll take the carb off my 2011 and put it on this 06 and see what happens first. Easy enough. My bike runs prefect how I have it set. Both bikes have vforce3 reeds & pipe/silencer mods. I'm going to work on it tomorrow and do some testing

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2011 Yamaha YZ 250 -MX
1996 Yamaha Banshee 350 -Trails
1982 Chevy S10
2005 Pontiac GTO -451rwhp