Engine kills and a Locked Rear wheel


TCYFudge

Member
May 24, 2007
11
0
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.
 

TCYFudge

Member
May 24, 2007
11
0
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.
 
Jan 3, 2007
1,860
0
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.
 

rosscopeeko

Member
Feb 25, 2005
156
0
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.
 

adam728

Member
Aug 16, 2004
1,011
0
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.
 

TCYFudge

Member
May 24, 2007
11
0
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.
 

2-Strokes 4-ever

~SPONSOR~
Feb 9, 2005
1,842
4
Missouri
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.
 

TCYFudge

Member
May 24, 2007
11
0
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.
 
Last edited:

2strokerfun

Member
May 19, 2006
1,500
1
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.
 

TCYFudge

Member
May 24, 2007
11
0
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
 
Last edited:

griffbones

Member
Sep 12, 2006
329
1
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:
 

adam728

Member
Aug 16, 2004
1,011
0
Bigger pilot jet. That's one of the tricks desert guys use. Since they usually never run on the low end they can get away with an overly rich pilot. Then when you lift from a high speed run the fire is basically put out and the fuel cools the piston.

Still, I think you have a clearance issue more than jetting. You "shaved down the high spots"??????? Not the right way to go!!!! You need to get a new piston and match the bore to it's diameter. Depending whether it's cast or fordged will determine the clearnace needed.
 

griffbones

Member
Sep 12, 2006
329
1
The correct way is to either leave the bike pinned and just hit the kill switch once in a while or pull the clutch in, but coasting (decelerating) from a high rate of speed with the clutch out and throttle closed is a killer of 2 strokes!
 

TCYFudge

Member
May 24, 2007
11
0
no i didn't mean i shaved down a bigger piston. it was only rubbing against the cylinder in a couple spots. its the right size piston. and thanks for your help everyone. and i normally do pull in the clutch and after it was seizing on me i would pull in the clutch and give it some gas when decelerating and it never did it.
 

adam728

Member
Aug 16, 2004
1,011
0
TCYFudge said:
no i didn't mean i shaved down a bigger piston. it was only rubbing against the cylinder in a couple spots. its the right size piston. and thanks for your help everyone. and i normally do pull in the clutch and after it was seizing on me i would pull in the clutch and give it some gas when decelerating and it never did it.

I understand you only shaved the spots where it was seizing. This is still incorrect! A piston does not expand evenly, the shape, and different thicknesses around the piston cause it to expand at different rates. Usually when you seize it will leave a pattern.
http://www.smellofdeath.com/lloydy/piston_diag_guide.htm

You need to get a piston and then measure the clearance when installed. If it is not correct, the cylinder needs to be taken out further. If you're big bore is a steel liner it can be hined to size.
 

Top Bottom