Compression

Spine_Thrust · Oct 14, 2005

I tested the compression the other day on my 04 YZ250 and got 250 PSI with no leakage. It has over 100 hours on top end and runs like it did on day one.

Is this normal? Another thread I responded to got me thinking about this.

SirHilton17 · Oct 14, 2005

250 psi sounds a little high but im used to 125's 250's are probly different....
Thats awsome that it runs as good as day one after 100 hours!!!!
my bike on a new top end only runs really really good for like the first 5-10 hours... i i do ride my bikes really really hard and it is a 125 sooo....

WillyM · Oct 14, 2005

The compression will last along time, the thing you need to worry about is the skirt on the piston.
250 seems high to me also.

adrenaline420 · Oct 15, 2005

I too have a 2004 YZ250, still riding on the original top end,and I just tested the compression, and I got 230PSI. I ride mostly trails, dont race yet. My bikes probly got around 80hrs. spin thrust what premix are you using, i'm just curious?

darringer · Oct 15, 2005

A compression reading is a poor indicator of top end wear. Compression can remain almost constant with good ring seal even though parts are worn out. Tear down the top end and measure the piston, ring(s), ring end gap, and cylinder. Pay close attention to the piston skirt measurement and the ring end gap, as these are the main wear points. I was regurlarly getting over 210psi out of my CR, but at teardown it was definitely time to replace parts. While the top end is off, check for play in the rod at the big end bearing. Side-to-side play is ok, but it should have ZERO up-and-down.

Spine_Thrust · Oct 15, 2005

OK, thanks for the information darringer. I think it's time for a new top end eh?

I run 32:1 gas to oil ratio.

Spine_Thrust · Oct 16, 2005

I took the top end apart today and I have some revelations for those reading this.

After many hours of riding, the piston diameter is within spec and the rings are just barely out. The piston looks great from what I can tell, but the top was severely carbonated and was heart-shaped. I thought that was weird. The underside of the piston had some oily deposit but was on the right side only (weird again). The reason I mention is because Eric Gorr said if the underside was deposited with a large black spot you need a bigger main jet (richer). I don't believe a larger main is needed, it was not a large black spot.

The piston pin was at its limit so I will be replacing it and the small end bearing together along with the gaskets between the cylinder and head plus circlips. Copper washers get replaced too.

The only traumatic part of this whole experience was that the two cylinder bolts on the left side (magneto side) were rusted in the case. The only way to explain it is water from pressure washing got down through the nut...??? We inspected everything and there was no way water seeped past the gasket underneath, it had to come from the top. Is there a way to stop this? I am thinking a silicone glob after the nut is installed will be sufficient but I would like to hear others' remedies.

I was able to get the cylinder bolts out by using penetrating oil (WD-40), using two cylinder nuts butted up against one another, and vice-grips just below the nuts. I made sure not to touch the case with the vice-grips and just hoped it would break
free. It did! I will need replace the bolts though.

The rod had no up and down free play which I was very happy about. But the crank was interesting. Where the big end bearing goes, you could see heat discoloration to the crank. Not burnt but iridescent and you could say that it was hotter than the rest of the crank. Is this normal?

I'm glad I tore it down, just hope it goes back together as easily. This is my first top end and don't know what to do to the cylinder if anything...it looks good. I need to read that sticky cylinder prep thread again.

BigRedAF · Oct 17, 2005

250psi??? sounds like a bag gauge to me. I had a 2000 and a 2002 YZ250. Both bikes had work done including a milled head and they only had 235psi and required race gas. Stock on those bikes was 210psi.

Be carefull when reconnecting the powervalve linkage. Somepeople that haven't done this before over tighten the nut and warp the pull rod causing the powervalve to stick.

A black spot under the piston is not a lean condition!

I would be willing to bet that you have some black areas below your bottom ring too. Even with your measurements saying your within spec it is an indication of poor sealing by the rings.

Specs are a wide range. Yamaha even rates their cylinders and pistons A,B,C,D. Be sure if you have a C or D cylinder that you use the C or D piston from Yamaha. The aftermarket piston kits are sized for B cylinders.

If you do some math with numbers provided in your manual you will quickly see that a D piston in an A cylinder is going to be way to tight. And an A in a D is going to rattle and maybe even break the skirt off over time.

You can find the A,B,C,D rating marked in black ink right above the reed cage area on the outside of the cylinder. I always tried to buy a bike with an A or B so I had more piston options. Putting a B piston in an A cylinder will tighten up the bore and give more life to your bike.

Spine_Thrust · Oct 17, 2005

Could have been a bad gauge but I doubt it.

The PV linkage has a small metal dowel to use when taking the linkage bolt off and putting it back on. As long as I follow the torque specs and use the dowel, I will be fine.

From Eric Gorr's article on two-stroke technology:

"Look at the underside of the piston crown for a large black spot. The spot is burnt oil deposits that adhered to the piston because the piston crown temperature was too hot. This is an indication that the carb's main jet needs to be richer."

This bike is an A cylinder and piston. So you are saying put a B piston in the A cylinder? What is the trade off?

skipn8r · Oct 17, 2005

Spine_Thrust said:
The piston looks great from what I can tell, but the top was severely carbonated and was heart-shaped.

Did it seem that there was enough carbon to increase the dome height and therefore the measured cranking compression?

Spine_Thrust · Oct 17, 2005

skipn8r said:
Did it seem that there was enough carbon to increase the dome height and therefore the measured cranking compression?

Yes that could be it. So my next question is, why so much carbon build up? I used Yamalube 2R for the first 8 months, then switched to Maxima 927, then back to Yamalube...I know, they say use one oil and stick with it and I plan on sticking with Yamalube.

I have the JD jetting kit and the float height has been adjusted from when I got it brand new because it was fouling every other ride. Is my air adjustment out maybe? I still have trouble with fouling if I am not careful.

SirHilton17 · Oct 17, 2005

the 927 is a really nice oil so is the yamalube... ive herd that the 927 gives alot of carbon... if i were you i would give maxima fourmula K2 a try my bikes top end has lasted quite a bit long it seems like with this oil and i mix it at 28:1.... but mines a 125 and in my oppinion a 125 will require a higher quality lube and more of it than a 250, be cause the high rpms that a 125 does..

BigRedAF · Oct 17, 2005

Spine Thrust:

I'm saying that if you spec your cylinder and need to tighten it up a bit a B piston will be fine from Yamaha or any after market one is spec'd to B standards.

I beleive in Eric he is a wealth of knowledge. However, not everything is black and white. If you have a dark spot under your piston but your piston crown isn't gray as in burnt carbon and your plug is good as in light to medium brown with no aluminum specs on it then your not to lean.

An A cylinder is great. My last bike was a D and my only piston choice was OEM. If I recall the normal piston to cylinder clearance on that motor is .002, if you can run a B piston and not get tighter than Yamaha recommends then you get a more stable piston that doesn't rock around in the bore at high RPM.

You need to use a dail bore indicator NOT a feeler guage. If you don't have one then just use an A piston and press on.

Your correct about the dowl pin, since you said this was your first top end I assumed you might lack the knowledge, sorry.

Rust on the left side? are you talking about the studs? If so I used to put a dap of grease on them before I put the top end back on. I replaced the top end more often than ever 100 hours. I don't even think my bike had that much total time. Practice and two motos at most tracks is around 14-16 laps at 1.5-2.0 minutes per lap.

BigRedAF · Oct 17, 2005

What is your city, I'm wondering about your altitude and temp. 927 is a castor oil, good lube but way messy. Yammalube R is great but I ran AmsOil 40:1 and do so in my current bike as well. Very little carbon and no smoke.

I would like to know your jet specs. You mentioned a JD kit. Both my 2000 and 2002 YZ's were very close. Only a main jet and air screw adjustment required. The only differences in my YZ's and yours was the tranny as far as the engine is concerned.

RM_guy · Oct 18, 2005

Even with no wear on the piston I'd replace it with that many hours on it. The stock piston is cast and while dimensionally it is OK, it may fail catastrophically due to cyclic loading. 100 hours is way to long on that piston and you risk major failure if you reuse it.

If you find that you can get more life with the rings and piston then I'd highly recommend going to a forged piston (Weisco) to avoid any problems. I also go 80 to 100 hours on a top end because I rarely scream the engine but I'd never attempt it without a forged piston.

Spine_Thrust · Oct 18, 2005

I thought the Yamaha and Suzuki OEM pistons were Wiseco???

RM_guy · Oct 18, 2005

Spine_Thrust said:
I thought the Yamaha and Suzuki OEM pistons were Wiseco???

Not in my bike.

Spine_Thrust · Oct 18, 2005

RM_guy:

Someone on here said the newer RM and YZ pistons were made by Wiseco. A forged object should have cleaner lines, should it not? So I should be able to tell if this stock piston is forged or cast...maybe.

BigRedAF:

Here in Columbia, it's hot, hotter, and hottest and the altitude is ~300 feet. My jetting specs are as follows: JD's red needle, clip position 2, 172 main, 45 pilot, and a/s out 1 1/2. The red needle is leaner than the blue and the kit said to use the 170 main but I chose the 172 fearing too lean on the main.

This bike spooges like a beotch, and I wish I could get rid of it. I'm hoping that when I change the piston out, it will not spooge again for a while. Does anyone think the carbon build-up on top of the piston was causing the bike to spooge so much?

RM_guy · Oct 19, 2005

Spine_Thrust said:
RM_guy:

Someone on here said the newer RM and YZ pistons were made by Wiseco. A forged object should have cleaner lines, should it not? So I should be able to tell if this stock piston is forged or cast...maybe.

I guess I'm not sure about the new bikes. I'll have to check. Usually a forged part will have a smoother shape but I wouldn't go by that alone.

jmossbarger · Oct 19, 2005

The OEM pistons are not Wiseco. They are cast pistons made in Japan (look for "ART" on the inside). Wiseco was the OEM supplier for GAS GAS for a while, but that has since changed.

The discoloration on the crank is normal (around the crank pin). That is part of the manufacturing process.

Good Luck, it sounds like you are on the right track....

Joe

BigRedAF · Oct 19, 2005

Yep,

OEM pistons are ART and have been for a long time. The RM motor is a copy of the YZ and uses the same reed cage and piston if you choose to do so.

Hot, Hotter, Hottest and your moist air means you need to lean that baby out. The spooge is the 927 bean oil. Great lube but messy as hell! Yamalube R2 is a better choice for your needs if you stick with 32:1.

I ran one size smaller on the pilot, two sizes smaller on the main and the leaner optional OEM needle in the middle location with a 40:1 fuel:oil ratio. In Phoenix it is very hot in the summer and that combo netted great results with no carbon or plug fowling. In the winter when the temps cooled down I put the stock pilot back in and one size leaner than stock on the main jet, I left the optional needle in the above setting. Air screw was adjusted for each event for best rusults. Allways in the 1.0 - 2.5 turns out range or a new pilot was selected. We get 30-40 degree temp swings out here in the desert daily.

Good luck, it sounds like you have a handle on the situation.

Spine_Thrust · Oct 29, 2005

Ok, the bike's back together and everything is fine. And it still has 250 pounds of compression. Someone mentioned that the gauge could be bad, and I agreed it's a possibility, but after the new top-end it was the same compression as before. If the gauge is bad that's fine, but I suppose the compression must not have changed since day one. Is this a valid argument?

Two issues I had were:

1. The rear shock bearing at the bottom was rusted to hell and back. When I began trying to salvage it, the needles fell out and they looked like rat turds sitting there. So I cleaned it best I could and put 'em back in with loads of grease. Hope it lasts a few months before I have to press new ones in.

2. I accidentally sprayed water in the crank when cleaning the outside because the rag covering the intake wasn't very tight. It couldn't have been more than a 1/4 cup. I knew it might rust so I made it a point to rotate it nightly with lubrication. This is very important if you don't have the parts on hand. If you are like me and can't replace the parts right then (I waited 11 days on parts) make sure you message the crank with WD-40 so it doesn't start to rust. Hopefully the crank and rod bearings will last a couple more years from my little snaffu and when that time comes, the motor's off to Eric Gorr for a 265 kit!

Micahdawg · Nov 22, 2005

FYI, I've got a 01 RM250 with maybe 10 hours on a rebuild and it gets 235psi compression. The head has been milled though. And I'm still getting detonation on 99 octane gas.

Micah

Compression

Spine_Thrust

SirHilton17

WillyM

adrenaline420

darringer

Spine_Thrust

Spine_Thrust

BigRedAF

Spine_Thrust

skipn8r

Spine_Thrust

SirHilton17

BigRedAF

BigRedAF

RM_guy

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Spine_Thrust

RM_guy

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Spine_Thrust

RM_guy

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jmossbarger

BigRedAF

Spine_Thrust

Micahdawg

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