Airwolf

Member
Jun 28, 2003
7
0
Several months ago I purchased a 95 YZ 125 that needed a top end rebuild, or so I was told by the previous owner. As such the motor was disassembled and in a box. After sorting through all the nuts, bolts and various top end components, I purchased and installed a new piston, rings and gaskets as needed. I also drained the gas and radiator. After getting everything back together it started on the first or second kick. I then rode it for a minute or two then shut it off and let it cool down. Since that first start it continually got harder and harder to start until it finally just would not. ( The new piston did not even get an hour of ride time).

Frustrated I let the bike sit for several months- today I dissassemble the top end to find the piston pretty chewed up (right and the ring gap), thus the low compression. I happened to have a new ring in my shop so I cleaned up the old piston and installed it with the new ring. The bike now seems to have decent compression but will still not start. It seems to be getting gas (I tried starter fluid). It is getting spark (altough it looks weak).

My two stroke skills are limited- Is there anything I can do to pin point the problem? How do I know if the spark it too weak. The compression will blow your finger off the plug hole. I am sure some of you two stroke gurus will have some insight and it will be much appreciated.
Also carb has been cleaned and reeds are new.
 

SMAKme2X

Member
Jul 13, 2003
25
0
probly need to fix the jetting, or try throwing a new spark plug in there, or try jump starting it.
basic stuff to do but it might help
 

reelrazor

Member
Jun 22, 2004
340
0
Can you further explain "pretty chewed up and(I assume that is supposed to be 'around') the ring gap".

You need to find out WHY that happened. Did you check the ring gap when you put it together the first time? You didn't by chance install the piston backwards?

To pinpoint the problem...... the basics;a) checking compression with your fingertip isn't enough-beg borrow or steal a compression tester and use it. b) what does the plug look like when you have kicked it a few times w/o it starting? c) do you have FRESH fuel in it?-you said the bike sat for several months before you got back to it
 

CaptainObvious

Formally known as RV6Junkie
Damn Yankees
LIFETIME SPONSOR
Jan 8, 2000
3,331
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If the piston is "chewed-up", chances are the cylinder and/or ports may be damaged too.
 

matt-itude

Member
Jul 6, 2004
293
0
go back to the basics as mentioned. they teach for an engine to even try to run you need 3 things compression spark and fuel. if the plug ends up wet after trying to start it we have fuel(now on the fuel we could still question quality as previously stated). we need roughly 90 psi of compresion to even fire decently although it should have a fair amount more. and you need spark you could take an old plug and open the gap up a little more say 50 to 60 thou total and plugging that on and holding it to ground on the cyl head while kicking the engine (be carefull not to ignite the air and fuel that comes out of the spark plug hole or leave a plug in it) if it will jump the wider gap it should atleast be strong enough to start the engine. figure which is not there and that will give you some direction. also make sure you havent totally flooded the crank case trying to start it.
 

Airwolf

Member
Jun 28, 2003
7
0
Thanks for all the replies.
With regard to ring gap- I did not check the gap when I originally installed the new piston and ring.( I did not know I needed to)

Also - to clarify what I mean by chewed up. The aluminum is slightly melted/distorted on the piston where the gap in the rings was. I might also note that when I opened the motor up the ring gap was very nearly facing the exuast side. ( I had installed the ring with the gap toward the intake side originally. I used a Weisco replacement piston and it did not have a ring gap "locating pin" that the manual said it would , just a mark indicating where it should be. I have to assume that the lack of a pin allowed the ring to work its way around to the exuast side and then catch on an exhasut port, thust the damage.

The cylinder does show some wear right inbetween the exuast ports, but I am hoping it is not enough to hinder function.
The plug is wet after starting attempts and the gas is new.(drained the carb bowl)
Thanks again for all the help and I will try the spark plug test ASAP.
 

reelrazor

Member
Jun 22, 2004
340
0
Airwolf said:
Thanks for all the replies.
With regard to ring gap- I did not check the gap when I originally installed the new piston and ring.( I did not know I needed to)

Also - to clarify what I mean by chewed up. The aluminum is slightly melted/distorted on the piston where the gap in the rings was. I might also note that when I opened the motor up the ring gap was very nearly facing the exuast side. ( I had installed the ring with the gap toward the intake side originally. I used a Weisco replacement piston and it did not have a ring gap "locating pin" that the manual said it would , just a mark indicating where it should be. I have to assume that the lack of a pin allowed the ring to work its way around to the exuast side and then catch on an exhasut port, thust the damage.

.

***Yeah, checking the end gap is a requirement. It lets you know for sure that you aren't putting the wrong piston/ring set in the bore.

The missing locating pin is THE problem. Rings WILL rotate unless prevented from doing so. It's pretty critical that it stay in the position it needs to.

Since you said that you used a new ring on the old cleaned up piston(with no locating pin!), I am gonna hafta say, if you want this bike running you may as well tear it down again, order up the necessary parts and/or machine work. The last means get the bore replated/sleeved. Don't eeven bother checking the spark.

More details on the mark where the pin should be, please? It wasn't, by chance, an arrow on top of the piston?
 

matt-itude

Member
Jul 6, 2004
293
0
I think by the info now revealed reelrazor is right. My guess is you will find good spark and when you get a compression guage on there it will be substandard numbers. If there is enough damage you are saying you can see it there there is almost certainly too much damage. sorry to hear this.
 

Airwolf

Member
Jun 28, 2003
7
0
Thanks for the help guys. I guess I was hopeing to get by without too much more money in this thing. I will check into a re-sleeve and new piston, although I am not sure how much this bike is worth to begin with. 95 YZ125 in "OK" condition???

I am perplexed as to why the Weisco replacement piston that I originally installed did not have a locating pin for the ring. It makes sense that it needs one, so why would Weisco sell one that did not have one???? It seems that this is what caused all the trouble and subsequent damage to the cylinder.

Do either one of you know off hand what a re-sleeve might cost?
Thanks again for the help-
Aaron
 

reelrazor

Member
Jun 22, 2004
340
0
I have installed literally hundreds of Wiseco pistons and never have I received one that came like that. I know you can special request piston 'blanks' for specialty tuning, but yours must have slipped through the quality control cracks. It happens. We have to take responsibilty for knowing what is acceptable and what isn't, though.

I guess I woulda gotten on the horn with the supplier explaining the situation and requesting a replacement BEFORE installing a piston that wasn't complete. Live and learn.

http://www.mototricks.com/resleeving.html

They say $170+. There may be a machine shop near you that will bore the cylinder and press the sleeve cheaper. Bare sleeves usually run about $110.

As far as "is it worth it?"........well, what did you pay for it? Can you get your money back with it not running?....not likely, right? Can you get another for less than the work will cost?...not likely either. Will you ever make that mistake again?.....prolly not.

Hell, college is $90 a credit hour these days....chalk up your expense to learning.
 

Airwolf

Member
Jun 28, 2003
7
0
$110 is not too bad. I will give them a call in the AM- What is your opinion on going back with just a cast iron sleeve(rather than Nik-a-Sil)??

Had I been more familiar with two strokes, particulary this one, I would have definitely sent the piston back prior to installation rather than endure the present carnage.

Thanks again for all the help.
 

reelrazor

Member
Jun 22, 2004
340
0
Yeah, put a steel sleeve in it. Gets you back to stock bore with a room for a few cheaper mistakes in the future.

If you buy a sleeve and have someone install it, make sure they have exp. doing two strokes. The ports need to be lined up well, and then blended, etc. Mototricks might be worth it @ $170.
 

viking20

Sponsoring Member
Aug 11, 2002
428
0
If I were you , I would not consider a steel sleeve....Spend a bit more , and have the jug exchanged for a 133 from Eric Gorr´s Forward Motion. Link is on the left , prices on the web-page...
 
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