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Help!!! 1987 YZ 261 Top End

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I NEED KNOWLEGDABLE TOP END HELP. READ ON.

1987 YZ250T with apparently a 2mm-over kit. And as a start, I might have the "exhaust/intake/LH/RH" PISTON pictures mixed up. Sorry, lost track, and no mark.

Geared up, kicked once, popped into gear... Blasted off down trail...

One mile later, I was un-intentionally bogging up a hill. Desperately wailing the engine and slipping the clutch to keep the revs in the powerband, she would die anyways. No power whatsoever. Finally died out while I was checking the petcock. "Oh well," I said out loud, "crap boat."

Leaned her against a tree and had some water.

I let the engine cool down from barely warm, and single kick started it (no choke).

"Okay! Checklist!" I caught myself speaking out loud to no one, again.

1. Fuel? - Check

2. Revs freely? - Check

3. First gear? - Check

4. Let the clutch out slow and feel for slight grab? - Engine speed "slows" slightly :thumbsup:

5. Wind-up rocketship/dump clutch? - Dies/single kick restart

6. Red-line motor and slip clutch ever so slowly (I'm a self proclaimed "clutch feel" master) - Dies ANYWAY!!! WHAT???? Single kick restart.

It acts like it has absolutely no power. I couldn't even walk the bike back. It had NO POWER. Within one mile of mostly level trail it went from "wheelie magic" to nothing...

After replacing the clutch, (because my toyota feels the same way when the clutch gets overheated), It has the same symptoms, even from dead cold. First kick start and NO POWER, NO FORWARD MOVEMENT. It rolls forward and backward in neutral and in gear with the clutch pulled, all smoothly.

TEAR-DOWN

This picture shows how the HUGE top ring gap was letting the blowby through to the second ring and almost out to the skirt.

1_Exhaust.jpg

2_LH.jpg

Intake side of the piston has light scoring and has evidence of rubbing near the face of the piston.

3_Intake.jpg

4_RH.jpg

Help me out here! Someone said this was good to see "even" blackening of the piston face. What do you think???

5_Top.jpg

Bottom of piston shows no discoloration or hot spots or cracks or holes.

6_Bottom.jpg

Head looks clean and ready for new o-rings.

7_Head.jpg

Also this side of the cylinder looks good.

8_Intake.jpg

9_Exh.jpg

These rings are worn 0.5 mm more where the piston shows wear. I caught it at the right time. Just before cylinder damage.

Obviously the gaps are WAY TOO LARGE. So I'm hoping this is the power loss source.

10_Rings.jpg

Yes, the crank is covered at all times with that bag and weatherproof tape. But I uncovered it for the picture.

11_Crank.jpg

NOW FOR MY QUESTIONS!!! Thanks for hanging in there...

1. Could this be the cause of my power loss?

2. Why would it have come on so suddenly?

3. If my piston measures 69.75 MM on average (skirt and face) and my cylinder on average is 69.85 MM (below and above ports), Should I get my cylinder bored to 70MM and get a 70 MM piston kit?

4. Or should I stick with a 69 MM piston kit and leave the cylinder bore alone?

Stock is 68mm...

PLEASE HELP. I have never had to deal with a 2MM-over bike before. Any help is greatly appreciated. -BIG DAN:thumbsup:

EDIT: This thread plays in nicely to my signature... at least we know what's on my mind....

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1. Could this be the cause of my power loss?

Yes

2. Why would it have come on so suddenly?

Scoring like that can do it, there is also quite a bit of blow by too, judging by the carbon.

Scoring like that on the intake side 9 out of 10 times is caused by particulate (dirt) getting into the chamber.

3. If my piston measures 69.75 MM on average (skirt and face) and my cylinder on average is 69.85 MM (below and above ports), Should I get my cylinder bored to 70MM and get a 70 MM piston kit?

4. Or should I stick with a 69 MM piston kit and leave the cylinder bore alone?

Stock is 68mm...

Try just getting new rings, piston and air filter and make sure your jetting is correct.

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Try just getting new rings, piston and air filter and make sure your jetting is correct.

So, which piston? A 69mm would be too small I think, and a 70mm wouldn't fit without bore modification. Is there a 69.5mm??? Help. I know I need new stuff but, WHICH new stuff? -BIG DAN:thumbsup:

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Just to start fresh, I'd go with a 70mm. If you were to use the 69.75, I'm afraid of piston slap.

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So, I need a local shop to mic it and find out for sure because the bore ranges from 69.65mm-69.85mm with my micrometer and t-gauges. The piston measures 69.4mm at the ring grooves and 69.6-69.8mm at the skirt. What size piston is this?

The piston fits in nicely to the cylinder with a nice tight smooth fit. Slides in and out both ends with very little play and NO binding. Also, I can spin the piston 360 degrees in any part of the cylinder and it doesn't bind at all. Very smooth. The cylinder is very smooth where the piston is worn/scored lightly.

How much space do the rings take up? Is this an okay tolerance? 0.05mm? What size piston should I buy being that the cylinder is good and is about 69.65-69.85mm.

My thinking is that a 69.5 would leave a .25mm gap on each side of the piston. Will rings take up a half mm from piston to cylinder?

Should I take the jug to a machinist? I need a local san bernardino/riverside CA shop to go to.

-BIG DAN:thumbsup:

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The old piston looks like a Wiseco. There should be numbers stamped into the top of the piston. You may need to clean some of the carbon to see them. This will tell you for shure what size you have. It sounds like you will need to order the 70mm piston and take it to a machine shop to bore and hone to size. The new piston will have all of the specs needed for the machinist to set the piston to wall clearances. Make shure the machinist is familiar with two strokes and chamfers the horizontals on all of the ports.

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Can anyone else verify this is a Wiseco? I thought they were all dot printed and not stamped. These markings are all stamped. -BIG DAN:thumbsup:

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it's a wiseco See the "w" on the underside of the piston where the wrist pin goes. Clean the carbon and the size should be on top. I have a 250r with a 310 kit on it. The cylinder mics at 73mm and the top of the piston says 72.95mm There shouldalso be a number probably 4 digits on top. If you call wiseco they will hook you up. they are good to deal with andvery knowledgeable.:thumbsup:

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Found a few people with the right piston. Mine says 2.00 on it. So it's already 70mm. I have found ALL replacement pistons are EXPENSIVE. I remember when a top end was $120. Now I'm lookin' at $230 with bore/hone. So much for the cheap two stroke. Anyways, parts are on on order with 25% employee discount! Still $230 DOH! I could have pieced the whole thing together off ebay for like $225 delivered. If only wiseco had PRO X piston kits in stock. Apparently, lot's of parts are available in my size but only at full retail price! I'd rather just ride the KToom...

Soon this will be over and I'll be big boring down the trail. Thanks for all your help. -BIG DAN:thumbsup:

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Completed the top end job... Found parts on ebay/chaparral for $99!!! Top end parts for under $100. Got it honed to 70mm. Installed 70MM wiseco piston with new rings, pin, and clips. Installed piston with arrow to exhaust. Filled up all fluids. Kicked it 3 times and it started really nicely.

STILL NO FORWARD MOVEMENT!!!!! What am I doing wrong? :banana:

I changed the clutch fibres/steels.

Top end job.

Starts and idles perfectly.

Revs up just fine.

Trys to roll forward but bogs the engine. Even if "RED-LINED" and slipping clutch ever so slightly. (Video coming.) Acts like it has no power. HELP ANYONE HELP!!!

-BIG DAN:thumbsup:

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With the bike off, try and move the bike forward about 10 feet in first. You want to be able to here the piston moving and the rear tire rotating, if it's locked up then yea you got some tranny problems.

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In first and second it turns the engine freely, rolling it forward. Neutral acts completely normal. I'm pretty sure 1 down from neutral is first. It turns the motor more freely then 2nd. And 2nd turns the motor faster per foot. It even tried to start on me while pushing it forward.

This bike is second hand to me.

Since it is already bored all the way out, could it have some tight compression ratio that 91 wont run? The piston doesn't look like detonation to me... The video has ok sound maybe someone with a little experience can take a listen.

http://www.youtube.com/watch?v=fdHOI96xgL4

http://www.youtube.com/watch?v=http://www.youtube.com/watch?v=fdHOI96xgL4

-BIG DAN:thumbsup:

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someone hit me with some love... please... I"M STUMPED -BIG DAN:thumbsup:

PS: acts like it's in 5th gear but is definitly in first. Engine pulses power every second or so. No movement though. It tries to roll forward but it dies.

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You sure your clutch is adjusted properly? Even if it grabs when the engine is off, it can still be way out of adjustment.

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sounds like the power valve is not opening, powervalve problems? seems like it is choking-out or is binding, but assume it rolls fine in neutral was it broke in properly?

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Haven't managed to break it in properly because it won't go for me...

I installed the power valve on the linkage with the flat side down, so that as the engine revs, it rolls the valve forward and makes the exhaust port taller as revs increase. I can see it moving just fine, and the spring pulls it back down nicely. Powervalve has been modified to accept the 70mm Piston. NO TOUCHING is happening. -BIG DAN:thumbsup:

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do a compression check, after reading your first post more closely, I think you stated u could push the bike in first or second without the clutch, if u have good compression the bike should not be able to turn over the motor in low gears as the back tire will just skid.

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