1999 YZ125 breaking down at high rev's

jds535

Member
Aug 24, 2012
13
0
Hi Guys,
I rebuilt my sons 1999 YZ125 top and bottom. Brand New cylinder, piston, crank, crank bearings and seals. Rebuilt the carb with stock jetting. Reeds and Pipe are stock. The bike runs a little ragged and wihen topping it out in 4th down a straightaway it starts to break down and run a little ragged like the choke was pulled out or something. It runs pretty good down low and mid range. The plug color is tan after two tanks of gas thru it, but I have to run a plug from my KX250 in it to run. The stock BR9 plugs fouls out very quickly. I'm thinking I have weak spark but cannot afford to start swapping parts until I get lucky. Any advice?
BTW - filter is clean and oiled properly, exhaust is not plugged in any way.
 

jds535

Member
Aug 24, 2012
13
0
Thats what I thought too but the plug looks tan colored. And everything is stock ( and new ). So the stock jetting should run good, even if it doesnt give maximum expert style power.
I can try a leaner jet but going lean is risky business, I dont want to ruin my new topend.
 

dirt bike dave

Sponsoring Member
May 3, 2000
5,349
3
I think the decision is do you try one size smaller or two sizes smaller on the main for the first test run.

Seeing as the cylinder and crank are new, I can understand being cautious.

Back to back testing is going to help the most Get the next two sizes down on the main, and whatever you need to change them at the test site.

Get it up to temp and test with the stock jets. Drop to the next size down and repeat. Listen/feel for any signs of too lean. If none, drop to the 2nd size down. My guess is you will notice an improvment. If not, time to look for other issues besides jetting.
 

jds535

Member
Aug 24, 2012
13
0
The plug gap was very small ( I know I should look it up and set with a feeler gaude ) but when I cleaned it I opened the plug gap and now it runs even worse than before. Not sure if that makes it look more like weak spark or not??

I will try the jetting. They are cheap enough.

BTW - after increasing the plug gap and running it for half an hour, the color of the plug was still tab when removed.
 

jds535

Member
Aug 24, 2012
13
0
OK, so I finally got back to this. I removed the mainjet ( 350 ) and replaced it with a 340 that I had. No change to the way it runs. It must be electrical, right? Weak coil, or does it sound like stator ??

What do you guys think? I cant keep swapping random parts until I get it right.
 

High Lord Gomer

Poked with Sticks
Sep 26, 1999
11,790
34
I have had similar symptoms when my reeds were cracked.

Also when I put too heavy of a flywheel weight on, but you didn't mention anything about that.
 

BSWIFT

Sponsoring Member
N. Texas SP
LIFETIME SPONSOR
Nov 25, 1999
7,926
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Are our running out of fuel? If your float level is too low you'll start straving for fuel when you are on the main jet. Is your power valve working properly? Does the bike still have a noticeable hit? Try a BR8 or BR7 plug if the BR9 is not working but it sounds like a fuel starvation issue and would explain why the plug is still nice and tan.
 

BSWIFT

Sponsoring Member
N. Texas SP
LIFETIME SPONSOR
Nov 25, 1999
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Good point, _JOE_.
 

jds535

Member
Aug 24, 2012
13
0
I opened it all up ( again ). I checked / set the float height. It may have been a tiny bit off, but not by much. Based on what I was able to find I set it at 8mm. I pulled out the fuel petcock and cleaned it, it had some junk in the screen but not a ton. I Checked the reeds, the corner of one of the pedals on both sides were chipped and broken away a little. So, I ordered new reeds, ordered a new coil. I popped the head off to ensure that the rubber O ring seals were seated properly. ( I also wanted to get a look at the bore ). Everything looked very good. I am setting the jetting back to stock and will install the new reeds and new coil when I get them. I am gonna clean the carb again and put everything back together. Hopefully for the last time.
 

jds535

Member
Aug 24, 2012
13
0
ok, see if this makes sense. I put everything back together with stock jets and a Vforce reed cage I had pulled off the bike when we bought it. I thought if the chipped reeds were the problem, it would show up. Put everything back together and it still ratted out bad on the top end 4th or 5th gear with throttle 3/4 to full. The bottom and mid seemed to pull pretty good ( I'm a 250 guys so I'm guessing ). Went back and took out the BR8 plug and replaced it with the BR9 plug that was supposed to be in it. It has slightly less throttle response but screams on top the way it should. It must have been a combination of the jetting with the old reeds and the plug?? Interesting because the BR9 plug wouldnt even run in the bike before.
When I came back after a dozen laps I noticed spoog coming from the powervalve vent tube so it must be running a bit rich still. Based on the power deliver I think I need to go smaller on the Pilot jet or turn the air screw out a little bit.
What do you guys think?
 

jds535

Member
Aug 24, 2012
13
0
Filter is clean and oiled. Running 92 octane with Belray synthetic mixed 40 to 1. Come to think of it, I had been using 87 or 89 octane but bought better gas this time. Think it makes that much difference?
 

sharky243

Member
Dec 14, 2008
246
0
A couple things to check; Is the power valve operating correctly ? I had an issue once with my 96' yz250. Bike ran good but had no top end power. It ended up being the power valve governor. Another thing to check is your stator plate position. The ignition timing could be retarded too much. This can cause the symptoms you are describing. FYI; plug gap should be .024" for that bike. One other thing that may be the issue is if you have a crankcase air leak somewhere. I did read that you did a full proper rebuild, but if you are drawing in air from somewhere other then through the carb, it will lean.
 

jds535

Member
Aug 24, 2012
13
0
ok, here's my latest. The bike is running good at high RMP's now, even in higher gears. It rips on top like it should. It looks like it is running rich still though. I pulled the plug and confirmed this. It was wet and black. I also have black goo coming out of the powervalve overflow tube. I tried adjusting the airscrew but it didn't seem to have any effect at all. I noticed while on the stand that it bogs when you whack the throttle hard. If the rev's are high enough when you hit the throttle it will go right up, but if you start closer to idle and hit it, it will bog before it revs up. Everything points to pilot jet too rich right?
 

sharky243

Member
Dec 14, 2008
246
0
Something else to check, Is the bike smoking a lot like the fuel mix is too rich ? if this is the case, it is possible that you put the primary side crank seal in backwards and it is allowing trans oil to seep into the crankcase, which will make the bike load up like it is running rich.
 
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