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1997 YZ125 powerband problem

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Hi there,

I just picked up a 1997 YZ125 with a supposed top end rebuild and hour ago...

I rode the bike for about 6 hours the first weekend I had it, went awesome! 

The next weekend I went on a trial ride that was slow paced and not hitting the powerband too often, I noticed that if I didn't hit the powerband for a while it would then not want to again until it had been rev up more for a while then seemed to come right. It got to the point where the bike completely died and I had to put a new plug in, new plug in and she went perfect again, but the plug did come out very black and wet, almost like the oil at the end of a 2 stoke exhaust. 

I figured I would leave it and see how it goes next ride, the next ride it was really strugling to hit powerband and it sometimes wouldn't hit powerband at all, even after running it hard for a while.

I'm assuming it's to do with the condition the plugs are coming out? 

I have read on here about re-jetting the carb?

Stuck power valves, which I doubt seen as it ran so good a couple of weeks ago.

or the top end rebuild is a yarn and the piston and rings need replacing?

What do you guys reckon? 

Cheers

 

 

Edited by Caleb Riordan

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Hi there,
I just picked up a 1997 YZ125 with a supposed top end rebuild and hour ago...
I rode the bike for about 6 hours the first weekend I had it, went awesome! 
The next weekend I went on a trial ride that was slow paced and not hitting the powerband too often, I noticed that if I didn't hit the powerband for a while it would then not want to again until it had been rev up more for a while then seemed to come right. It got to the point where the bike completely died and I had to put a new plug in, new plug in and she went perfect again, but the plug did come out very black and wet, almost like the oil at the end of a 2 stoke exhaust. 
I figured I would leave it and see how it goes next ride, the next ride it was really strugling to hit powerband and it sometimes wouldn't hit powerband at all, even after running it hard for a while.
I'm assuming it's to do with the condition the plugs are coming out? 
I have read on here about re-jetting the carb?
Stuck power valves, which I doubt seen as it ran so good a couple of weeks ago.
or the top end rebuild is a yarn and the piston and rings need replacing?
What do you guys reckon? 
Cheers
 
 

I would start by taking a look at your reeds and check your jetting and what plug are you running
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58 minutes ago, twostroke-notbroke said:


I would start by taking a look at your reeds and check your jetting and what plug are you running

BR9EIX plug

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Just did a bit of work on it then, so now its got even worse. I empty all the fuel and put fresh fuel in it to make sure fuel is fine. I kicked it over and it idle really good until it warmed up a bit and then died like it had run out of gas, so I left it for a bit, pulled the spark plug, the plug looked still perfect, checked it had spark, plenty of spark, drained the carb, kicked it over, started up, idle then again exact same thing, died like it had run out of fuel after a few minutes. What on earth is going on! now I can't even get it to start at all.....

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Just did a bit of work on it then, so now its got even worse. I empty all the fuel and put fresh fuel in it to make sure fuel is fine. I kicked it over and it idle really good until it warmed up a bit and then died like it had run out of gas, so I left it for a bit, pulled the spark plug, the plug looked still perfect, checked it had spark, plenty of spark, drained the carb, kicked it over, started up, idle then again exact same thing, died like it had run out of fuel after a few minutes. What on earth is going on! now I can't even get it to start at all.....

Check your comression

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I don't own a compression tester but ill go buy one soon, I did check compression with my finger and it felt pretty good.

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Compression test came out at only 100psi so I have pulled the head. Piston and cylinder look in fairly good condition, it has an ART branded piston, the cylinder walls still have the crosshatched hone marks. The piston ring gap seems to be waaaaay to much though....

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Geez I wish folks would check the simple stuff first!

Put the bike back together and install a brand new BR8ES plug.  If you are going to trail/woods ride, you may need to rejet, but often times a hotter plug will do the job.  This plug costs $2.49 and can be found at any auto parts store.  And by the way, there is no need to run fancy plugs in your bike, unless you like to spend money.  If you want a "9" heat range, run the BR9ES.  I do occasionally need to tinker with the jetting on my YZ 125s due to seasonal temperature changes, but I also change plugs for the type of riding--"8" for woods and "9" for track or sandy conditions. 

If the plug does not fix it, you may need to rejet.  BUT, before you do that, try fixing your riding style.  You should never be "off the pipe" for any length of time on a race bike, no matter what kind of terrain you are riding.  If the terrain requires a slow speed (like tight single track), down shift and ride in first gear.  The vast majority of my riding is tight eastern single track, and I use 1st gear constantly on my YZ 125s.  Keep the motor spinning!

Edited by rpt50

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Went to the bike shop today and their opinion was a hotter plug (BR8EG) and a new spark plug cap. Wasn't expecting it to work but I got home and boom back to normal. I haven't swapped the other plug back to see if it was the spark plug cap or spark plug but if anybody else runs into this issue give it a try.

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