Yamaha Dirt Bikes Forum

I have a 1999 yz 125 and it is having some issues. First off, I do not know the last time the piston and rings were changed. It is pretty hard to start. Like 5-10 kicks when cold. Once started it doesn't seem to idle at all. I am new to these, but they should idle without being on the throttle right? Second I took it for a ride and you really can't feel the powerband kick in.

Are these all symptoms of a bad piston and ring. Also where is the cheapest place to buy the piston and ring and does anyone know a good tutorial online somewhere of how to do this.

Thanks  

Yeah I would say it would be a good thing to replace the pistin and rings. a lot of ppl have their 2 stroke motocross bikes (especially 125s) setup so they dont idle well. this way they dont foul plugs easily.If your bike isnt hitting powerband though that means you need to replace la top end.  
2006 YZ250

Well I just rode it again and powerband is kicking in and then it just kinda stutters and comes back. Most of the time however it feels like powerband is not working.  

Okay, To start the bike you want choke out, and NO throttle. Then ones it starts you want within a few min, Snap throttle controle then choke off and have the same. You don't want or need the bike to idle after warm up for long. After stall when hot, One hit of the throttle and kick or 2 should bring it to fire ( NO CHOKE). As all others said, I'd do a top end and start Freash. Great bike, But gotta have a new top end. Dont run the bike till you get the nessesarey fix. Its easy to do with the manual if your mech inclined. If not its only 250 at a dealer. Worth every penny as the bike will last and be good to you. Just gotta be good to it first.  

Yeah Id say that it needs a new topend put in. A topend kit is not that spendy though and you should be able to do it by yourself. then it will be a hole new bike. You might have to have a shop work on your cylinder wall(hone it or bore it if it needs it) but everything else can be done by you.  
2006 YZ250

Being a used 125 (1999 to boot); it would be worth while to pull the top end and freshen it up.  Its possible its never been done!

The symptoms you described can be from a worn out top-end.  Could be bad jetting as well, but since the cost is relatively low I'd to the top end and see how it runs after that.

Hopefulley you have a manual, but also verify if the jetting is stock or not so you know where you are there.

While doing the top end, make sure the powervalve is clean and operates OK as well.  A sticking or non-operating one can have a large affect on how the motor runs.  
Dave H 2006 YZ250
(2008 YZ 125 son)

I just did a compression test and it read a 115. Is that an acceptable value? Also, what is the stock main jet size for this bike?  

O  

I have no idea I think 115 should be close to acceptable. Check your manual it should tell you these things or call a dealer and ask the mechanic.  
2006 YZ250

Dealer told me today that 140-150 would be more ideal. So that being said I am going to continue with my tear down of the top end. I am going to have to really inspect the cylinder and make sure it is okay.  

yeah, feel it and make sure it is flat. no scracthes or cracks. When my KX100 lost compression, we found two small cracks in the piston! That's bad. hopefully there isn't too much damage in your bike and it gets running good again. let us know how it goes.  
2005 YZ250F
2006 RM250
Tragedy and hardship are the forge that produce men of steel

Okay, piston has some damage and the cylinder looks to be okay. I see what looks to be a tiny scratch near where the power valve is but you can't even feel it it is so tiny.  

so were the cracks in my piston. Throw in a new piston and rings and then check teh compression. You may have to get a new cylinder head. This can be kinda expensive. not compared to 4-strokes though. Yeah, i would just throw in a new piston and rings, check the compression and if it is still low, pop for the cylinder head.  
2005 YZ250F
2006 RM250
Tragedy and hardship are the forge that produce men of steel

Do you know how I need to break in the new piston? I hate to just jump on it and gun it, but what else can you do?  

I just posted this in another post, so i will post this, then go to the other thread and put it in this one too.  
2005 YZ250F
2006 RM250
Tragedy and hardship are the forge that produce men of steel