YZ250 top end rebuild

[2strokeYardSale]

My 2006 YZ250V, purchased new in November 2007, accumulated approximately 50 hours of use. It was time to rebuild the engine with a new top end.


It has been trail ridden winters only in sandy high desert, 5000 ft base elevation, mods as follows:

- milled the head to correct the squish after the first ride
- FMF Gnarly pipe
- leaned the carburetion progressively over time
- No-Toil oil on stock air filter or No-Toil filter, swapped after every ride except when riding alone in perfect conditions
- FMF Q silencer for most rides, stock recently
- BR8EG spark plug for most rides, BR8EIX recently
- Motul 800 oil at 50:1 for most rides, Motul 710 at 50:1 recently
- various fuel, from 91 pump to 93 StreetBlaze to 100 StreetBlaze to VP 110 to VP 113 to VP C12 to a mixture of these

This engine saw a lot of low RPM chugging. It also saw several high-load charges up long, steep, soft hills, a few of which caused it to spit coolant, not steam, from the overflow.

Let's see what the top end looks like after 50 hours...

Start with a clean motorcycle. Use compressed air to blow away any loose dirt around the engine.

First, remove seat, right side panel, shrouds, fuel tank. Next, remove silencer and exhaust pipe. Then drain coolant. (The skid plate interferes with the coolant drain.) Pour distilled water through system to clean it of glycol. Disconnect hose from cylinder head, carburetor from intake, lead from spark plug, power valve drain hose from frame or cylinder.

Next, remove the brackets that connect the frame to the cylinder head. I previously marked both to orient the brackets for re-installation but it's not needed. Then remove the spark plug. Next, break loose the cylinder head nuts in a criss-cross pattern before removing them and the washers and head.

The head shows no peen marks that would indicate detonation. Look for them around the outer circumference of the combustion area, the squish band.

Remove the o-rings and head locator pins.

Remove the power valve actuator cover on the right side.

The manual says to use the special tool the dealer included with the purchase of this fine racing machine. I received no such tool from my dealer but this steel punch works fine.

It should be obvious, but you slide it through the U-shape of the actuator linkage part into the hole on the cylinder. This prevents the actuator from moving as you apply (a little) torque to remove the bolt.

Remove said bolt and fold away the linkage. Push the the kickstarter slowly to move the piston near bottom. Break loose the cylinder nuts in a criss-cross pattern and remove them. You can't get a socket on the forward nuts due to the cylinder design.

Give the cylinder a wiggle or jiggle to break its seal from the gasket and remove it. Clearance is very tight near the frame and radiator, or in my case, radiator guards. Remove the base gasket. Cover the crankcase opening with something clean and lint-free that will prevent foreign objects from entering.

To remove the piston circlip, place your thumb over it so it doesn't bounce off your eye before dropping into your crankcase. Insert a small screwdriver or needle-pry-thing in the detent and lever it off. Use something like a T-handle socket to push the wristpin out from the other end toward this opened end.

Remove the piston, wristpin, and bearing. Examine each for signs of trouble.

Just right in the center but dark around it. (Picture came out darker than actual color.) No peening from detonation.

No shiny wear at the bottom of the skirt to indicate back-and-forth rocking of a loose bore. Blow-by past the rings says the rings could have been replaced well before the piston.

No burn mark on the bottom of the piston, so it probably never got too hot.
No discoloration on the wrist pin.


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[Phaedrus]

What's the proper quench on an engine like this?

 

[2strokeYardSale]

Disassemble and remove the reed block parts from the cylinder.

Remove the power valve cover. Remove the power valve drain cover. (I didn't do this because the Phillips head screws were not leaving without a fight.)

Pre-spooge in the power valve assembly

Clean the cylinder. The vast majority of my time spent rebuilding engines is spent cleaning parts. If you are hurried, at the very minimum, the gasket surfaces need to be spic and span.

My cylinder has a beautiful bore with excellent cross-hatching. After cleaning it and the cylinder, I realized the PV itself was so clean there was no reason to disassemble it. Thanks, Motul!

Coat the cylinder with oil and put some oil on the power valve dealio.

Inspect the reeds. RTFM and look for chips and frays.

No chipped corners off the carbon reed petals, which happened with my Honda CR250. I put in new fiber reeds anyway.

Reed block re-assembled on cylinder

Check the piston-to-cylinder clearance. Lacking a bore measuring instrument, I did it with a long piece of Starrett gauge stock.

The Wiseco piston instructions did not include clearance data, so I started at .003" which wasn't even close, and finally ended at .0015". At that point I discovered printed on the piston box label: .0015" clearance. Nice!

The YZ250 cylinder does not have an exhaust bridge like my previous CR250 and I did not have to drill the piston. Woot!


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[PantyBuncher]

have you learned your lesson, will you be disassembling this again after 50 hours ?
what do you suppose the actual MTBF really is ?

 

[2strokeYardSale]

Check the ring end gap. This is absolutely neccessary on a Wiseco kit. Put a ring in the cylinder and push it in with the piston to square it up.

Use a feeler gauge to measure the gap between the ring ends. This is a bitch to do and to photograph.

The ends are stepped so make sure you are measuring the smaller gap by keeping the gauge squaure against the cylinder wall.

Wiseco includes some piss-poor data, suggestions really, on the ring end gap measurements. I used the data from the Yamaha manual. The Wiseco rings will likely have not enough gap and you will need to file them. I filed too much.

Actually I filed just enough to go to the maximum ring end gap. Lacking a comprehensive understanding of how the gap affects the performance and reliability of the engine over time, I would prefer just to have it in the middle of the specification. For shits and giggles, I set the gap on the second ring at minimum. Stone the filed edges so there are no burrs.

As a side note, I use stock Yamaha pistons and rings for my TZ250 and the ring end gaps are dead on every time right out of the package.

These Wiseco rings don't have an up/down orientation but the gaps do have to be located correctly on the piston.

Install one piston circlip. If you manage to shoot it across the garage and lose it, and your Yamaha dealer isn't nearby, Honda CR250 circlips are the same.

I am a huge fan of the C-Clipper tool to make this impossible task a breeze. However, my tool fits my little 56mm TZ250 piston and its smaller circlips, not the YZ250 66.4mm piston and its 18mm circlips.

Once you get the clip in the groove, orient the clip gap away from the access gap in the piston.

Clean the crankcase cylinder base surface, being careful not to allow debris into the crankcase.

Oil up the new wristpin and bearing and put the bearing in the connecting rod. Orient the piston with the arrow pointing toward the exhaust, or the front wheel. Attach the piston and slide the wristpin through. Insert the circlip and re-orient its gap. Make sure you didn't get any pieces of your latex or nitrile glove torn off and stuck in the grooves.

Install a new cylinder base gasket. Consider lightly greasing the crankcase side to make removal and clean-up easier at the next rebuild. Pictured is a plain fiber aftermarket gasket, which I greased.

Considering how easily the stock gasket came off after 50 hours of run time, I would not grease a Yamaha gasket, which has goop on the part of the surface. I will probably use a Yamaha gasket next time, as I didn't replace any other gaskets besides the base and the head o-rings.

Remove your gloves to install the cylinder because you are likely to tear off a piece of glove while compressing the piston rings. Orient the ring end gaps over their respective locators on the piston and compress them while sliding the cylinder squarely on the piston. This is difficult considering the limited space and limited angle that imposes. If you feel resistance, back off and try over, checking the end gap posititions.

When it slides on freely, with only a little more resistance than when you checked your new piston in your cylinder, you know you've got it. Install the cylinder nuts hand tight then criss-cross to torque spec. You can't fit a torque wrench on the forward nuts but you can use an adapter and do the math.

Lightly grease the new o-rings and install them in the cylinder grooves. They might appear too big but they will fit. Don't let them pop out. Install the pins.

This is your last chance to check that the arrow on the piston points in the direction of your exhaust. If not, there will a nuclear holocaust when you start the motorcycle. You might as well slowly work the kick starter to see that you haven't dropped a beer bottle cap in there and ruined things.

Install new (ahem) copper washers and the cylinder head nuts, hand tight. RTFM and torque in a criss-cross pattern. Install a new (ahem) spark plug. Install the frame brackets. I used anti-seize on the large bolt that corroded and thread locker on the smaller bolts that didn't.

Attach spark plug lead.

I guesstimated my 50 hours of run time but from now on I'll know for sure with this hour meter.

Wrap the wire around the spark plug lead and tie off as pictured.

Run the wire up to the meter location. Press the wire into the groove on the back of the meter.

(Whoa, made in the U.S.?)

Install carburetor. Add anti-freeze/distilled water mixture. Install radiator cap. Install exhaust chamber and silencer. Install fuel tank, radiator shrouds, side panel, and seat. Attach carb to fuel tank.

Mix oil and fuel. Add premix to fuel tank. Turn on petcock. Turn on choke. Slowly push through kickstarter several times. Notice increased compression. Left foot on short man's stand. Ready... kick! DOH! Something was messed up. Oh yeah, shifter hit the stand and put it in gear. Neutral... ready... kick!

Ring ding ding!

 

[2strokeYardSale]

What's the proper quench on an engine like this?

You mean squish clearance? I guess it's up to the tuner but I relied on community consensus and had very good results:

http://www.thumpertalk.com/forum/showthread.php?t=573021

have you learned your lesson, will you be disassembling this again after 50 hours ?
what do you suppose the actual MTBF really is ?

What lesson? I'm afraid the stock cast piston will disintegrate at some point well before it reaches the limit of piston-to-cylinder clearance. That's why I got a Wiseco, for the forged piston. The MTBF is random. Some people put hundreds of hours on their stock pistons.

I will probably disassemble this before 50 hours and replace the rings, checking clearance at that time.

Notice I didn't even bother putting my dial gauge on and checking the crank play. Or checking the squish clearance with the new piston. (Umm, maybe I shoulda done that.)

 

[PantyBuncher]

<<What lesson? >>

i guess that answers that.
my point was that 50 hours is 5-10 days...and that stock piston looked great.
you really going to disassemble your top end every 5-10 rides ? LOL.
i didn't realize hollister was that competitive. but whatever makes you happy.
don't forget to put the weisco stickers on.

 

[87cr250r]

I'm jealous of 50 hours. I only get 20ish and the skirts are gone on my pistons. The no wear area in the middle of the piston is from the intake port. This bike has huge intake ports so much of the piston never sees cylinder wall.

 

[explorin]

<<What lesson? >>

i guess that answers that.
my point was that 50 hours is 5-10 days...and that stock piston looked great.
you really going to disassemble your top end every 5-10 rides ? LOL.
i didn't realize hollister was that competitive. but whatever makes you happy.
don't forget to put the weisco stickers on.

im not sure about the YZ,but my kx250 only cost 180 bucks and 2 hrs of work to rebuild the top end..i probably went a little more than 50hrs before each rebuild, but still..its not like its some ridiculously expensive maintenince to do to keep the bike running in top shape...remember, were talking about dirt bikes..when you ride them 20 miles into the middle of the mountains and you blow up the top end cause you were being cheap, you cant call AAA..and theres nothing like trying to walk a 240lb bike out..

 

[2strokeYardSale]

I went for a break-in ride today and it revved to the moon upon startup, dieseling, so that the stop button wouldn't kill it and pulling the spark plug lead wouldn't kill it. I had to click into a high gear and stall it.

Nothing has changed since it first started and ran perfectly, other than adding more of the same premix and adding the seat and side cover.

 

[VP1]

Airleak. I'd bet somewhere on the intake side of things

 

[2strokeYardSale]

Yeah, suspect #1. Airbox boot is secure to carb, carb is secure to intake boot, intake boot/reed block is in excellent condition. Slide bottoms out and operates freely.

I checked the squish clearance and it has .004" more than it did before. So unless the piston crown is shaped differently and taller, it's a larger combustion chamber for less compression. Nonetheless, my fuel is old and suspect.

I wish I had a leak test for this engine.

 

[87cr250r]

Check to make sure the slide in the carb bottoms out with the handle bars in all positions. I've seen improperly routed throttle cables pull when the bars are turned.

 

[Maddevill]

Or you could have a serious air leak at the left crank seal, although if it was good before.....I'd look at the throttle cable being hung up on something first.

 

[Maddevill]

BTW, you can sometimes stop a runaway 2 smoke motor by pulling full choke.

 

[2strokeYardSale]

With a four day weekend ahead of me I tackled the problem today.

I removed the carb and checked the float height, which was a pain in the ass. Holding it down always bottoms the floats. After a long time I figured out I could hold it sideways and watch the tab hit the plunger and measure the float at that point. The 170 main jet seemed too lean for these cold temperatures so I pulled it and the pilot, which turned out to be a 45 which was OK.

Speaking of cold temperatures, it was 37 F in my garage with a little space heater going and constantly tripping my circuit breaker. I wore glove liners under my nitrile gloves and working with little parts was a bitch.

Just because I neglected it during the rebuild, I used my spark plug hole dial gauge to check for up/down crank play.

The needle should immediately reverse direction as the crank moves past bottom and top while you move the flywheel by hand.

If there's any discrepancy between flywheel movement and dial movement, there's trouble.

This turned out to be a waste of time as I remembered I checked this recently when I checked my ignition timing.

I got my leak test kit from my TZ250 spares and tools bin. The intake plug was too small but might be able to be clamped to seal. It didn't work, as the engine wouldn't accept air and I had to assume it was leaking there because it was such a huge leak.

I wrapped the intake plug with electrical tape and duct tape for a perfect fit in the intake boot.

The exhaust is a spigot and won't accept the internal rubber plugs I use for my TZ250 but an external 1.5" Qwik Cap from Home Depot did the trick.

With my spark plug in place I pumped and the engine wouldn't accept air. Nooooo! I tried to listen for the leak as I pumped but street noise overwhelmed it. Then I saw the powervalve vent hose dangling. DOH! I plugged it with a Torx driver.

It accepted 6 psi and held it. Woot! But that only contributed to the head scratching.

I put the carb and airbox back on and attached everything else after dumping all the premix I used previously. I replaced it with VP 110 mixed 40:1 with Motul 800.

Now I had to start it and stop it immediately if it revved and ran away again. I pointed it at a wall and planned to shift up into third or fourth. Then I decided why not start in fifth and just dump the clutch if it runs away. Genius plan except the clutch dragged so bad kicking the starter with the lever pulled tried to turn the rear wheel. Neutral it was.

Ring ding ding, no runaway. How about that?

Cliff Notes:

1. Ran OK at home with pump gas
2. Ran away at riding area with same pump gas
3. Checked squish clearance, air integrity, throttle, carb, etc.
4. Ran OK at home with race gas

 

[CockyRooster]

My guess would be the slide needle in the carb somehow jammed/pulled out/stuck. Sometimes when putting the carb back together the needle doesn't sit down all the way in it's seat and can flop a little sideways and stick on top of the main jet.

 

[2strokeYardSale]

Except I didn't have the carb apart. The only difference between starting the bike in the garage after rebuilding it and later when it revved was the fuel. Which is now in my truck.

 

[CockyRooster]

Evil spirits then. Although dirty fuel will make a slide stick. But I still bet on the evil spirits of the 4stroke conspiracy.

 

[2strokeYardSale]

I chalked it up to "Two Stroke Voodoo." I mean, it ran without the spark plug connected for dogsake.