05 YZ250 Crank Seals

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#1
O.K Guy's I suspect I have a crank seal leak on my 05 yz250. I have a crank pressurize tool kit that I will pressurize the crank case with this week. My question is this, can the crank seals be replaced on this bike without opening up the cases? I replaced the crank seals on my kx500 and had to split the cases for that bike. A guy at work said that you can drill a small hole into the seal and thread in a machine screw and pull out the seal. I wouldn't mind opening up the cases but would like to get a few more rides out of the bottom end because if I'm going to open up the engine I'm going to completely rebuild the lower end.
 
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#2
What makes you suspect you have a crank seal failure? I doubt there's any bike where you can remove a crank seal without splitting the cases apart. Sure you could pull out the old one using that method but how the hell are you gonna get the new seal in without warping it?

As for rebuilding the lower end I would be tempted to leave everything else if they are within their tolerances. I might be tempted to change the crankshaft bearings but on a bike as new as yours I doubt they will be shot. Just change all of the usual gaskets and seals and put it back together.
 
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#3
I'm second owner. While riding the bike for the first time I noticed the bike didn't want to idle and was loading up. I tried tuning the mixture screw and got no repones from the bike/carb. I removed the carb and found the air pocket for the idle mixture screw was full of filter oil. Order new gaskets, float,needle and cleaned carb completely. Started bike up and now it runs away at idle speed(1500-1800 rpm's had tach on coil lead). O.K I'll check carb out one more time, check assembly,check float level install carb same issue. Next I removed the flywheel cover and spray carb cleaner behind the stator plate and engine's rpm raced even higher. So now it's time to pressure test the crank case.
 
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#4
Yeah, it's probably a good idea to pressure test the crank case just to make sure. Sounds daft, but you have checked the idle screw on the carb, made sure the throttle isn't sticking etc...? Also, check your exhaust valve actuator arm is moving in and out freely but with a nice springy return. If your exhaust valve is stuck open slightly the revs will build up more.
 
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#5
Throttle slide snaps closed and it had no plating wearing off of it. If the crank seal are leaking then I'm going to do complete lower and top rebuild, then I'll clean and inspect the power valve system. No crank seal / base gasket etc leaking after pressure test I'll check the power valve system. I never have had a stuck power valve on any of my bikes so am unsure of the issues it causes. My buddy has same bike we have talked about installing his carb on my bike just to see what happens. I'll pressure test the crank case this week.
 
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#6
On most if not all bikes you can change the crank seals with out splitting the cases.They are installed from the outside of the case, not the inside. Your bike was loading up because your jetting was off. Turning the air screw won't help much if you jets are too rich. A worn top end could also contribute to running to rich. As far as the idle running away, if it didn't do it before you took it apart, chances are you screwed something up putting it back together. A run away engine is usually a sign of an air leak. If the flywheel side crank seal was sucking air that would make the bike run lean, not richer. If the clutch side seal was leaking bikes usually smoke excessivly.
 
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#7
Big Bore Stroker said:
...I removed the flywheel cover and spray carb cleaner behind the stator plate and engine's rpm raced even higher...
I'd say that's a pretty clear sign that the seal needs to be replaced. The only question is what method you use to replace it.

Many folks here have done them by the method your coworker suggested, with the exception that you use a couple of sheet metal screws, not a machine screw - the threads are way too small and it will just pull right out. You'll need a couple to work the seal out without getting it cocked too badly. Be careful not to nick the bearing when it breaks thru. Clean everything thoroughly to remove any debris left from drilling. A deep well socket can be used to drive the new seal in square and without deformation. It's a quick, easy way of doing it.

On the other hand, you could go ahead and tear the engine down completely and split the cases. This method allows you to measure and inspect all critical components of the engine and gives an accurate assessment of condition. Everything gets thoroughly cleaned and eliminates any question of internal issues. The downside of course is a lot more time and money since you have to replace all the gaskets.
 
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#8
Red Pinger said:
On most if not all bikes you can change the crank seals with out splitting the cases.They are installed from the outside of the case, not the inside. Your bike was loading up because your jetting was off. Turning the air screw won't help much if you jets are too rich. A worn top end could also contribute to running to rich. As far as the idle running away, if it didn't do it before you took it apart, chances are you screwed something up putting it back together. A run away engine is usually a sign of an air leak. If the flywheel side crank seal was sucking air that would make the bike run lean, not richer. If the clutch side seal was leaking bikes usually smoke excessivly.
The idle mixture screw's air pocket was completely filled with filter oil. The carb was not mixing any air with the fuel at idle speed. The only air it was getting at idle speed was through an air leak (I suspect is the crank seal) that's why I got no response from the engine while adjusting the mixture screw. Now with clean carb I have air coming through the mixture screw pocket and from the suspected air leak causing the run away at idle speed. Top end is going to be replaced before the bike is rode again due to the fact that I have no history of service on the bike. We'll know more as soon as I pressurizer the crank case,hope to be able to that tonight. All jets are stock in this carb.
 
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#9
So if the screw's pocket was not letting any air mix with the fuel, and now it is, you should be able to turn the screw all the way in and that would stop any air from getting through and your run away idle should go away. Correct?
 
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#10
Red Pinger said:
So if the screw's pocket was not letting any air mix with the fuel, and now it is, you should be able to turn the screw all the way in and that would stop any air from getting through and your run away idle should go away. Correct?

Yes at this point I can turn the idle mixture screw all the way in and slow the idle but the engine rpm is still high 1200 rpm's. One other point I forgot to list was if I back out the idle speed screw and lower the slide the engine rpm runs away even higher. I am going to try my best tonight to pressure test the crank case and check main seals/base gaskets/intake etc. then we'll have more and better info then what we have at this point. Thanks. :cool:
 
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#11
Red Pinger said:
On most if not all bikes you can change the crank seals with out splitting the cases.They are installed from the outside of the case, not the inside.
Well unless my bike is different to yours my seals have to be installed from the inside. The crank bearings sit up behind the seals and there is a lip on the outer side of the case to hold the seal in place to stop it from coming out.
 
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#12
:bang: O.K pressure tested crank case holds 6 psi for at least a minute or longer. Can't explain what happen when I sprayed the carb cleaner behind the stator plate since that side isn't leaking. O.k does anybody got a 05 yz250 carb off their bike? I would like to see a photo of the carb with the bowl removed looking down at the pilot/main/float assembly. I have checked my carb against a parts list and everything is there. I know in my kx500 carb it has a part that isn't available nor listed in the part lay out. I did noticed that this carb has been apart before I got the bike.
 
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#15
Big Bore Stroker said:
...pressure tested crank case holds 6 psi for at least a minute or longer. Can't explain what happen when I sprayed the carb cleaner behind the stator plate since that side isn't leaking...
Repeat the carb cleaner test and see if you get the same result. As we've been discussing in another thread, the open side of lip seals will seal better with pressure on that side. Your crankcase goes thru both positive and negative pressure cycles therefore your seals need to maintain a seal in both conditions. The leakdown test only tests for positive crankcase pressure, not negative, and the added 6 psi you're using is helping the seals to seal. If spraying carb cleaner behind the stator causes any noticeable change in the engine, the seal is leaking with negative crankcase pressure and needs to be changed. Although it might be minor enough that you're able to compensate the lean condition it's causing thru carb adjustments, it's only going to deteriorate over time risking a lot of engine damage. :yikes: