1997 yz250 top end rebuild

[javalen]

we are considering rebuilding the top end on my sons bike because its putting out a lot of white smoke. 3 questions:

1) will this cure the problem

2) Is this a difficult task?

3) Can anyone point me to a site with a howto?

Thanks in advance

[daBIGstuff]

the white smoke could be three things; rich fuel/oil mix, coolant leak into combustion chamber (not to sure if the coolant does put out white smoke), or band crank seal(trans oil leaking into crank.

top end rebuild would fix if the coolant is the prob.

I would check to see if your trans fluid is low or coolant is low. and go from there

[stk563]

If its leaking anti freeze or not leaking any fluids at all start with this.

well its not hard to do a top end if you have any mechanical abilities at all. they are pretty easy.

Need the following:
First pull off the head
take off the 6 bolts that hold the head on top of cylinder this will be the part that has the radiator fluid in it.
Next take the cylinder out of the bike
take piston off rod replace rings
Take the cylinder to local shop to have it mic'd out see if it is still in specs may only need rings may need bored and new piston


Reassembly
put Piston back on rod with bearing and new wristpin with 2 c-clips 1 in each end
place the rings on the piston and set them to the 2 metal stoppers
make sure when u put the piston in u put in with arrow toward the exhaust side.
place cylinder on top of piston more than likely u will have to set piston at lowest point to do this
dump a little oil on the piston and down the newly redone cylinder this will cause a bit of smoking for a couple mins but when u do break it by end of that it will all be out of the exhaust.
place new gasket for base down on it.
It is wise when doing this to use honda bond or suzuki gasket sealer to make sure this gasket seats as this could be the cause of an airleak.
take a knife and slowly push the top ring to where it fits inside the cylinder do the same for the bottom
torque down the cylinder bolts to the motor. They dont have a specific torque but get them tight enough that the gasket is tighly protected.
Next put head gasket on the bike dont use gasket sealer with this as this seal is metal most of the time and gasket sealer will only make it leak badly.
change the oring in the head which will be around the dome or the spot that spark plug screws into. there should be an oring up there.
finally
put the head back on and torque those 6 bolts back down

Testing for leaks
after all is torqued down and tight you need to be sure to do a leak down test there are testers that screw into the spark plug spot that allow you to see whether or not your rebuild is going to suck air and blow your sons bike back up. dont fear its normally rather obvious but if you dont do one and start it and it is about 15 mins before u realize you have an air leak chances are ur going to need new gasket set as the ones u put on may not come back off
If you do leak down test and u can hold at least 6psi for more than 6 mins then your bike isnt going to leak air put spark plug back in change the oil add antifreeze reconnect all hoses fire it up.
After the rebuild
Breaking in a fresh top end.
Start the bike let it it warm up drive it long enough to get the pipe hot to touch.
Take it to garage and let it cool down to room temp.
Do this 3 times never go above half throttle in ur heat cycle. This is a time that you need to be listening for any weird sounds the motor may be making.
Two strokes are really easy to work on just pay attention to how it comes apart put it back just how it was replacing all seals u will have no problem.
In the last 2 weeks i have done about 40 rebuilds on 2 strokes they dont take much time about 50 mins to do top ends if u have all parts in hand before u start.

If its leaking oil into case
My suggestion here is dont try this for the first time by urself. If you have any friends that have kids that also have 2 strokes that have done one before get them to help you here.

All of the first has to be taken out just like if it is leaking anti freeze or no fluids at all.
Now the bad part
Splitting a case
Tools needed
Flywheel puller
clutch basket holder
Sockets and wrenches.
Impact wrench
New case seals
Honda bond

Starting point
take motor out of bike
Take both side covers off the motor
take the kick starter gear and spring out
Remove the clutch all plates Leave the basket for now.
Go to opposite side use flywheel puller remove flywheel.
u will thread this in counter clockwise then attach to an impact wrench and shock it a few times be very careful these tools are rather expensive and flywheels arent cheap if u strip this off when u go to take it off ur in for about 80 bucks more on the bottom end rebuild
Remove stator pay attention to its timing if adjustable.
Go back to clutch side and remove clutch basket using basket holder tool.
take all remaining gears off this side now that all should be free pay attention to where they came from because they will all go back right where they came from.
Now you are ready to split the case on any particular bike some are horizontal other are vertical splits.
Vertical splits tend to be much easier.
start by removing the bottom bolts of the motor as they are normally tight and need both cases together to get them out.
Take all side bolts out
when it comes time to pull case apart take many pictures of what the inside looks like where everything is at.
Split case and start snapping pictures
Take all the insides of the bottom end out.
Take motor to the car wash or if u have power washer use it.
power wash this motor so it is very clean.
take a razor blade and remove all the honda bond or other sealant as there is no bottom case gasket on most bikes.

get this area very clean
remove all old oil seals from the crank shifter sprocket and water pump
keep these parts covered in an area that isnt going to get them dirty.

Reassembly
Just the opposite of assembly
when u do the oil seals put honda bond around the seals and around the case.
all over make sure to clip the bottom end clips for crank and tranny back into the spot this would be a disaster. if u do all this work and destroy motor because u forgot to reset the clips.
tighten all the bolts around it and reinstall clutch side spots use ur pictures to keep u straight.
If u get lost at any time doing a rebuild just post a couple pictures or send them to me. i can tell you what you are looking at and how it is supposed to go in or come out.
Hope this helps
Bottom end for you would be better to send motor off. and have mechanic do for u. Top end do yourself.