Engine kills and a Locked Rear wheel

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May 24, 2007
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#1
Hey everybody. i have already done searches and couldnt find my answer
i just got my bike fixed. its a yz85 with big bore and other things. i got a new crankshaft asembly and new top end. now after breaking it in, when i am going really fast in 6th gear and let off the gas it suddenly kills the engine and locks up the rear wheel. i have to stop completely and put it in neutral to kickstart the engine. and it does this occasionaly but only when i give it alot of gas in 5th or 6th gear and let off the gas quickly. can anyone tell me what is causing this. oh yea and i already pulled off the cylinder head and checked the piston for seizure or damage and there isnt any.
 
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#3
thats what i was thinkin like when you let off the gas it gets really lean. but would that cause the rear wheel to lock up and i checked the plug and its half tan and half black like its rich. when i ride it leaks oil out of the silencer like its rich.and what would a clogged crankcase breather hose do.
 
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#4
I am not sure on the breather hose but check the air filter and clean it. Also, the jetting could be rich on top but lean on the bottom. Try to go a size up on the pilot jet.
 
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#5
i really doubt the compression is severe enough to lock the rear wheel when the motor stalls. If you can kick it over after you stop it's obviously not seized so it's kind of weird. i would check your drivetrain, as in chain.
 
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#6
It sounds to me like the piston clearance is too tight. A hard run puts a lot of heat into it and it expands, becoming basically a press fit to the cylinder. But the engine makes enough juice to keep it moving. When you let off the gas it sticks, and also locks the wheel. By the time you stop to restart the piston has cooled and contracted enough to be free again.

You are sure there are no signs of seizing? This really sounds like a case of it.
 
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#7
im pretty sure there are no signs of seizing. i checked the rings on it and for any gouges in the sides or on top. is there any other way to check for seizure? how about the timing could that set it off and do anything. air filter is just cleaned and ill have to get the bigger pj. i also just got a new chain. i will have to ride it soon due to all the new stuff i just did to it and see how it works.
 
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#10
Sounds like a siezure to me. I had the same thing happen on a 125 when hauling fast through a sandy straight. Main jet is probably fine, but you're maybe too lean on the needle (backing off of wide open is when it locks up?) Try going one clip richer on the needle.
 
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#11
Hey thanks everybody. yea it locks up when i back off of full throttle. It was a seizure and i cleaned off my piston and shaved down the high spots. it was rubbing against my cylinder alot.What should I clean my cylinder with? I have tried muriatic acid and most came off but still have some scuffs on the cylinder. and are stock oem circlips the same size for oversize pistons or would i have to buy them from wiseco. and i know you shouldnt use the same clips twice but if its not bent and it came out fine wil i still be able to use it.
 
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#12
Don't let off the gas quickly from running wide open. Your fuel flow is your oil flow, which lubricates your piston. By suddenly letting off the throttle, you are going from wide open oil intake to the pilot-jet-only intake, which does not give you enough oil to lubricate that very very hot and expanded piston. You are seizing the piston from lack of lubrication when you go from wide open to closed too quickly.
 
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#13
ok but do you know if OEM circlips will work on an oversize piston setup and how else to clean black scuffs on the cylinder besides muriatic acid
 
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#14
2strokerfun said:
Don't let off the gas quickly from running wide open. Your fuel flow is your oil flow, which lubricates your piston. By suddenly letting off the throttle, you are going from wide open oil intake to the pilot-jet-only intake, which does not give you enough oil to lubricate that very very hot and expanded piston. You are seizing the piston from lack of lubrication when you go from wide open to closed too quickly.
+1 My son just grenaded a perfectly good KX100 engine this way! And I have the bad piston, trashed cylinder and empty wallet to prove it. :nod:

But this was 100% my fault because I never told him how to properly slow down from that kind of speed, so dad is to blame for this one! :bang: