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first rebuild-pulled the motor apart... hows this look

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its an 02 yz250. as far as i kown the only thing done to it is a re-usable air filter. not sure about the reeds or anything like that. guy had it since it was a year or so old and left it stock. the bike was used only in dunes and it was his 2nd extra bike. it was only run a few times over the last few years. i picked the bike up last week and after about 5 hrs of riding (ran really good) it just died and lost compression. pulled the head off and found it just like this. i assume it pushed the o ring out and thats why it lost compression.

229517_10150597192870083_549810082_18613567_2828014_n.jpg

this is the first 2 stroke motor i am doin and i was wondering how the piston looks ( color and such). to much oil/ not enough oil/ wrong size jetting? i was under the impression it should be darker and have a more distinct patter around where the ports are. i also noticed the piston seems to be rounding off like some is missing going into the power valve. what are your thoughts on what i need to do. looking down in the cylender you can still see the cross hatches in the walls and all the ports look fairly clean.

250906_10150597203050083_549810082_18613914_231806_n.jpg

thanks jeff

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Running lean or too low of octane fuel.

In the second pic you can see some damage to the piston on the exhaust side, where it's slightly erroded away. The whole outer circumfrence looks slightly sand blasted, which would be from detonation. Not much for piston wash, so I'm guessing it's a touch lean.

Wish I could find my dang camera cable. Pulled my engine apart this morning and took a bunch of pics, but now can't upload them. Grr! My piston wash was very distinct and quite good, would be a great example. Stupid cord......

These bikes are also known for having too much squish band clearance, which leads itself to detonation as well. HAving the head milled for proper squish, then opening the bowl up to get back to stock compression will make it run better everywhere, and be less prone to detonation.

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im gonna check and see how much squash clearance there is. any idea on whats good for a stock bike? i was running 93 octane with yamaha oil

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Something around 0.045-0.050 is pretty decent for most people. Stockers seem to measure out 0.070-0.090 on this forum. In theory you could get away with as little as 0.030" (1% of stroke), but that'd be a "tear it down and check everything each race" type of engine.

Depending on piston & gasket tolerances mine is 0.044 - 0.048" after each rebuild.

What jets are in your bike?

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Something around 0.045-0.050 is pretty decent for most people. Stockers seem to measure out 0.070-0.090 on this forum. In theory you could get away with as little as 0.030" (1% of stroke), but that'd be a "tear it down and check everything each race" type of engine.

Depending on piston & gasket tolerances mine is 0.044 - 0.048" after each rebuild.

What jets are in your bike?

i just tryed checking it with solder and i only had a roll that was .08 thick and .125 thick. .080 was not enough to get crused. .125 was a litte to thick and crushed down to .109 but i could not get it to reach tdc. so its somewhere between .08 and .109 that was measured off the ends. that is being measured with the old piston in. new piston should take up some space i would think. i guess i need to get the piston in befor i can tell how much needs to come off. if i were to have .045 cut off the head would i still need to have some cut off from inside the dome? or could i just jet it a little bigger and have it a little higher compression. jet should be stock as far as i know. ill pull the carb and check

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Your comp would be too high without removing material from the bowl.

Say with a new piston squish falls at 0.090" and you removed 0.045" (1.143mm).

Stock bike has 21cc head volume and a (static) compression ratio of 10.9:1. Figuring this out means that stock there is a total of 25.25 cc above the piston when at TDC. But you just removed 3.96 cc, leaving 21.29 cc chamber at TDC.

Now your compression ratio is 12.7:1 (give or take for quick math and rounding errors).

Way too high for a 2 stroke, unless you only ride at 12,000 feet elevation, or plan on running race fuel. You need to open up the bowl!

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and if i were to leave it the way it is and just hone it and do a piston and rings what would it do... just a little less power... and as for the detonation part from to much gap i dont see how that would be. less gap would = more comp and wouldnt more comp = higher chance for detonation

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A tight squish band reduces the chances of detonation, with compression being equal. It does so by forcing the air fuel mixture towards the center of the cylinder and increasing turbulence. This keeps combustion more controlled to the center of the cylinder until the piston starts the downstroke.

A wide squish allows a lot of air fuel mixture to remain out at the edge of the cylinder. As top dead center is approached and the plug fires cylinder pressure rises quickly. The extra compression causes this extra mixture to ignite ahead of the flame from in an uncontrolled manner, detonation. It's death for an engine, literally the equivelent of jackhammers on the piston crown. Rewrote pistons, heads, cylinders, can kill rod bearings, blow orings, etc.

Tight squish band and proper compression is good!!!

I've got a really good link I recently found I'll try and put up tomorrow (pain on the phone).

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The piston will have a blackened spot inside of it a easy tale/tale of running too lean as well.

As much as bikes cost running leaded fuel keeps the motor running longer and extends engine life.

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Found my camera cord. Here's how mine came out.

DSC00559.jpg

Head as removed

DSC00565.jpg

Head after wiping with a dry paper towel (damage was done sometime during the bike's life before I got it. When torn down the piston was fine but head hammered up, smoothed the high spots and have ran it for years)

DSC00566.jpg

And underside of piston, with just a teeny tiny hot spot. Pretty happy with that, as I bet I know right when it happened (50F day, should have went up a main but was riding "tight" trails and didn't worry about it. Got there and found that guys definition of "tight" must have meant you couldn't fit a truck down it. All 3rd, 4th, 5th gear sandy stuff, lots of WOT).

DSC00578.jpg

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Probably a better, if not more expensive, method to reduce the squish to the non-detonation zone (.035-.045") and then open up the combustion chamber properly. It can be done at the same time.

I had a late 70's open class YZ, finally did the head to .035 clearance and just about all of the jetting/pinging/other problems went away instantly. Power, which was already abundant, became even better. I wish I still had that bike. Retarding the timing is correcting the symptom, not the problem. Plus, it is the cheapest 5-20% power increase you'll find anywhere. Can't tell you how much you'll get, but you will get something. Instead of wasting fuel out the pipe, you'll be putting it to work.

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I think you will find that most yz (125 and 250) when jetted correctly and ridden hard ie

sand or mx racing need slightly better than 93 octane fuel. I am running straight 100 octane and like they way my pistons look now

much better than when I ran straight 93.

Read the back of your owners manual on jetting and tuning and its is really spot on.

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Found my camera cord. Here's how mine came out.

DSC00559.jpg

Head as removed

DSC00565.jpg

Head after wiping with a dry paper towel (damage was done sometime during the bike's life before I got it. When torn down the piston was fine but head hammered up, smoothed the high spots and have ran it for years)

DSC00566.jpg

And underside of piston, with just a teeny tiny hot spot. Pretty happy with that, as I bet I know right when it happened (50F day, should have went up a main but was riding "tight" trails and didn't worry about it. Got there and found that guys definition of "tight" must have meant you couldn't fit a truck down it. All 3rd, 4th, 5th gear sandy stuff, lots of WOT).

DSC00578.jpg

That is a very good looking set of engine internals. The smaller the heat spot on the underside, the longer you have full power during a ride.

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Click-free

IMG_0336.jpg

That's my kind of trail. Michigan has some tight stuff, but 3 years in Arizona showed me what real technical single track was. I need to get some pics of my skid plate. :smirk:

Many times, my photo just won't show up, sometimes it does, no pattern. My photobucket account shows my skidplate, but not after this ride. It was whacked several times, pretty hard. Actually, a trials bike would be ideal for this.

That is about a two foot wide trail, max. Certainly can't pass anyone.

This and other shots are from a section of State Forest set aside specifically for motorcycles. Great riding over there, and it looks like I'll be there Monday, as it has finally stopped raining. I saw the mythical yellow orb out today!:smirk:

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i had detonation around the circumference of the piston with the stock and wiseco. You could try retarding the timing too.
Probably a better, if not more expensive, method to reduce the squish to the non-detonation zone (.035-.045") and then open up the combustion chamber properly. It can be done at the same time.

I had a late 70's open class YZ, finally did the head to .035 clearance and just about all of the jetting/pinging/other problems went away instantly. Power, which was already abundant, became even better. I wish I still had that bike. Retarding the timing is correcting the symptom, not the problem. Plus, it is the cheapest 5-20% power increase you'll find anywhere. Can't tell you how much you'll get, but you will get something. Instead of wasting fuel out the pipe, you'll be putting it to work.

Not to get too far OT, but yamahasocal450 is correct.

The '99+ YZ250 engine has an issue with improperly set timing

from the factory, and, the combination of an aggressive timing curve and high compression.

That's why this particular engine can benefit from a 1-2 degree timing retard to

help with detonation, jetting and the also recognized mid-rpm torque spike.

Add the fact that many states are/have gone to 91oct. max fuel and the timing is an issue.

With squish set and proper jetting, it has allowed detonation free running on California 91 pump fuel.

Each engine/setup/fuel has to be taken as a specific case.

In the case of the '99+ YZ250 engine, timing retard should be looked at.

how do you change the timing?

Timing Thread

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