98 yz125 problems

RM250racer
Edited Date/Time 10/23/2013 2:24pm
i bought a 98 yz125 that "needed a piston" because it wouldnt start. i took the cylinder off the night i got it and saw the piston was in backwards. i flipped it around and put on new gaskets i had laying around and threw a new br9eix plug in and it started first kick.

the problem is the bike has no power at all. there basically is no powerband. when i took the cylinder off i made sure the powervalve wasnt sticking and opened up. i took the carb off and dropped the needle clip down one because it seemed to be running too lean. its on the second from the bottom now. also the top of the piston was completely black. im not sure if its carbon from the exhaust not being able to leave with the piston backwards or if the top came black from the factory. its the entire top and i tried to rub some off and nothing happened. if it is carbon could that effect power?

im sure someone on here has an idea of what it could be. thanks
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bobby397
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bobby397 10/8/2013 3:54pm
10/8/2013 3:54pm
Might be wrong, but I believe it sounds like the exaust valves aren't working and I know on my old 99 yz125 the pin would work its way out over time.
RM250racer 10/8/2013 4:14pm
10/8/2013 4:14pm
bobby397 wrote:
Might be wrong, but I believe it sounds like the exaust valves aren't working and I know on my old 99 yz125 the pin would work...
Might be wrong, but I believe it sounds like the exaust valves aren't working and I know on my old 99 yz125 the pin would work its way out over time.
the pin in the linkage?
bobby397
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bobby397 10/8/2013 4:28pm
10/8/2013 4:28pm
No, the one that holds the exhaust valve in place.
bobby397
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bobby397 10/8/2013 4:29pm
10/8/2013 4:29pm

The Shop

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RM250racer 10/8/2013 5:49pm
10/8/2013 5:49pm

This is where the valve sits. It'll open all the way when I pull the linkage by hand and snap back to this spot
Radical
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Radical 10/9/2013 12:45am
10/9/2013 12:45am
Did you replace the ring(s)?
If the piston is shot, replace it too.
RM250racer 10/9/2013 5:47am
10/9/2013 5:47am
Radical wrote:
Did you replace the ring(s)?
If the piston is shot, replace it too.
no i didnt. the ring looked like it had no time on it and the piston doesnt look bad except for the black top. my guess is he did a top end and couldnt get it to run because he put the piston in backwards.
Mouse153
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Mouse153 10/9/2013 6:45am
10/9/2013 6:45am
Sounds like the power valve isn't opening or its opening at the wrong time. I would pull it all apart, clean and replace in accordance with the manual.

Also measure the cylinder and piston. Make sure they are within spec.
moore433
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moore433 10/9/2013 7:04am
10/9/2013 7:04am
Alot of Yamaha's have certain cylinder and piston combos. For instance, they have a : A,B,C & D cylinder. The cylinder should be marked as such. It could be that the previous owner not only installed the piston backwards, he could have the wrong piston all together.Just a thought..
czernik
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czernik 10/9/2013 7:07am
10/9/2013 7:07am
Is the muffler over packed or restricted
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---------- 10/9/2013 7:26am
10/9/2013 7:26am
When you put the jug on to the bottom end, there should be some sort of linkage that hooks to the powervalve that opens and closes it. That's how a mechanical powervalve works.

I think you probably slid the jug back on, and didn't line the linkage up. The the actual powervalve itself is probably fine, it's just disconnected from the arm/mechanism that opens/closes it.
JM485
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JM485 10/9/2013 7:27am
10/9/2013 7:27am
Dude, check your power band. They go bad after a while or it may not have been properly installed during the top end change.
motoxxx599 10/9/2013 8:11am
10/9/2013 8:11am
JM485 wrote:
Dude, check your power band. They go bad after a while or it may not have been properly installed during the top end change.
Im surprised we made it this long before hearing this ^^^ . In all seriousness im sure it wouldnt cost to much to get a motor guy to make sure your power valves are working correctly. Also, if you weight 200 lb + that may be a factor as well.
lostboy819 10/9/2013 12:01pm
10/9/2013 12:01pm Edited Date/Time 10/9/2013 12:07pm
moore433 wrote:
Alot of Yamaha's have certain cylinder and piston combos. For instance, they have a : A,B,C & D cylinder. The cylinder should be marked as such...
Alot of Yamaha's have certain cylinder and piston combos. For instance, they have a : A,B,C & D cylinder. The cylinder should be marked as such. It could be that the previous owner not only installed the piston backwards, he could have the wrong piston all together.Just a thought..
Makes no difference at all. You can put a D piston in an A cylinder or an A piston in a D cylinder and it will still be in spec. Yamaha does match them when the motor is new at the factory but the difference is so small that it's is almost a waste of time. I have seen many years of YZs that even though it has a A,B,C or D cylinder piston combo when new but when you go to do a top end all you can get is the B piston from Yamaha.

bobby397 has it correct,flat power is almost always a power valve problem. Fix the power valve operation and it will be good to go. Only other thing to check is make sure the stator plate or base assy is tight and the timing is correct I dont remember if the 98 has timing marks to line up, but I believe it does.
RM250racer 10/10/2013 4:34pm
10/10/2013 4:34pm
moore433 wrote:
Alot of Yamaha's have certain cylinder and piston combos. For instance, they have a : A,B,C & D cylinder. The cylinder should be marked as such...
Alot of Yamaha's have certain cylinder and piston combos. For instance, they have a : A,B,C & D cylinder. The cylinder should be marked as such. It could be that the previous owner not only installed the piston backwards, he could have the wrong piston all together.Just a thought..
lostboy819 wrote:
Makes no difference at all. You can put a D piston in an A cylinder or an A piston in a D cylinder and it will...
Makes no difference at all. You can put a D piston in an A cylinder or an A piston in a D cylinder and it will still be in spec. Yamaha does match them when the motor is new at the factory but the difference is so small that it's is almost a waste of time. I have seen many years of YZs that even though it has a A,B,C or D cylinder piston combo when new but when you go to do a top end all you can get is the B piston from Yamaha.

bobby397 has it correct,flat power is almost always a power valve problem. Fix the power valve operation and it will be good to go. Only other thing to check is make sure the stator plate or base assy is tight and the timing is correct I dont remember if the 98 has timing marks to line up, but I believe it does.
yea i had checked the timing before hand. it does have the marks. i took the cylinder off again after school today and gummed out the powervalve. before you had to use a wrench to open it and after it slid like butter. it didnt really shut by its self though but to my knowledge thats the linkage's job to close it back up. put the bike back together and still has no power. it wants to wheelie off the line then goes flat after 5 or so feet.

is there anyway the linkage could be affecting it? or is the a spec for the bolt that holds it together? with the linkage connected the only way to open it by hand is tighting to bolt so it spins. is this normal?

thanks for all the help
lostboy819 10/10/2013 5:40pm
10/10/2013 5:40pm
If it has power off the line and then flattens out that means your power valves are NOT opening. At low RPM they are closed which makes the exhaust port smaller for low end power and as the RPM climb the valves open up making the exhaust port larger for top end power. On the side of the engine there is a shaft that spins and the faster it spins it forces some ball bearing outward which moved the valve linkage to open the power valves. For some reason they are not opening. You can remove the side cover on the cylinder to look at the linkage and run it to see what is moving and what is not. you will need to pull the pipe to get the small cover off and the put the pipe on to run it.

Take some pics of the linkage when the cover is off and post it. Make sure you have everything in the parts list.



bobby397
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bobby397 10/10/2013 5:41pm
10/10/2013 5:41pm
So is it sputtering? If so check the reeds or maybe the jetting. Have you checked the compression? Still believe its a power valve issue.
bobby397
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bobby397 10/10/2013 5:45pm
10/10/2013 5:45pm
lostboy819 wrote:
If it has power off the line and then flattens out that means your power valves are NOT opening. At low RPM they are closed which...
If it has power off the line and then flattens out that means your power valves are NOT opening. At low RPM they are closed which makes the exhaust port smaller for low end power and as the RPM climb the valves open up making the exhaust port larger for top end power. On the side of the engine there is a shaft that spins and the faster it spins it forces some ball bearing outward which moved the valve linkage to open the power valves. For some reason they are not opening. You can remove the side cover on the cylinder to look at the linkage and run it to see what is moving and what is not. you will need to pull the pipe to get the small cover off and the put the pipe on to run it.

Take some pics of the linkage when the cover is off and post it. Make sure you have everything in the parts list.



Number 22 on the second diagram use to give me problems. The pin would come out easily

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IceMan446
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IceMan446 10/10/2013 5:51pm
10/10/2013 5:51pm
Replace the power valve springs and also the screw and bushings that hold it in place. Over time they will get loose and also the valve itself can stick if it sets for a long period of time. Also make sure the power valve actuating arm isn't bent or broken. I cant tell you how many times I have seen that happen.

Good luck man.
RM250racer 10/10/2013 5:53pm
10/10/2013 5:53pm
bobby397 wrote:
So is it sputtering? If so check the reeds or maybe the jetting. Have you checked the compression? Still believe its a power valve issue.
no its not sputtering. just flat. already checked the reeds and they were fine (and carbon fiber!) i played with the needle clip and the air mixture screw which normally does the trick, to no avail. pretty sure its a PV problem like everyone is saying.

@lostboy819 i will take pictures when i tear it back down probably tomorrrow. i know for sure that the washer on the linkage bolt (#1 on top print) is just a circle washer and the print shows a flat spot. that couldnt cause a problem could it?
lostboy819 10/10/2013 7:12pm
10/10/2013 7:12pm
I have seen the linkage put on wrong even thought it looks correct, I have seen the pins that are in the valve shear off #22 like bobby said.
RM250racer 10/11/2013 5:57am
10/11/2013 5:57am
lostboy819 wrote:
I have seen the linkage put on wrong even thought it looks correct, I have seen the pins that are in the valve shear off #22...
I have seen the linkage put on wrong even thought it looks correct, I have seen the pins that are in the valve shear off #22 like bobby said.
if it is that pin that fell out, shouldnt i have seen it when i took the cover off? or could it be sitting in my bottom end? Unsure
lostboy819 10/11/2013 6:12am
10/11/2013 6:12am
Run the bike with the cover #23 off and you will be able to see if the linkage is moving when you crack open the throttle. The pins could not end up in the bottom end. My guess is that where the #3 linkage bolts to the rod at the top of the cylinder is flipped over the wrong way.
YZDAD
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YZDAD 10/11/2013 9:33am
10/11/2013 9:33am
I don't know if this applies to this issue but I worked on a yz250 that was running as you describe and it turned out that the primary drive gear was put on the crank backwards. It has an offset in it and when it is on backwards it will not engage with the govenor, so the power vavle stays closed. Running it with the cover off will show you if the linkage is moving or not.
lostboy819 10/12/2013 11:54am
10/12/2013 11:54am
So............................put the pipe on and run it and see if its working. If the arm is moving the problem is in the power valves in the top part of the cylinder. If the linkage arm is not moving when you rev it the problem is in the governor set up under the right side engine cover.
RM250racer 10/12/2013 12:36pm
10/12/2013 12:36pm
I ran it and the arm didn't move. So pull the first case? I ran it pipe less if it makes a difference
lostboy819 10/12/2013 12:53pm
10/12/2013 12:53pm Edited Date/Time 10/15/2013 8:55pm
If the linkage didn't move then your problem is under the right side engine case. When the RPMs rise the linkage should open the power valve. Don't run it without a pipe. Put the pipe on and run it and then watch the linkage arm.
I am not trying to be mean but I think you are in over your head and should take it to your local shop and they will be able to diagnose it very quickley.
RM250racer 10/15/2013 5:20pm
10/15/2013 5:20pm
lostboy819 wrote:
If the linkage didn't move then your problem is under the right side engine case. When the RPMs rise the linkage should open the power valve...
If the linkage didn't move then your problem is under the right side engine case. When the RPMs rise the linkage should open the power valve. Don't run it without a pipe. Put the pipe on and run it and then watch the linkage arm.
I am not trying to be mean but I think you are in over your head and should take it to your local shop and they will be able to diagnose it very quickley.
i pulled the case and the sprung piece that opens the pv fell right out so i dont think it was connected right. but i put it back in and put everything back together and its still not working. nothing looked broken or warped.

just wondering, what makes you think im in over my head?
lostboy819 10/15/2013 8:55pm
10/15/2013 8:55pm
RM250racer wrote:
i pulled the case and the sprung piece that opens the pv fell right out so i dont think it was connected right. but i put...
i pulled the case and the sprung piece that opens the pv fell right out so i dont think it was connected right. but i put it back in and put everything back together and its still not working. nothing looked broken or warped.

just wondering, what makes you think im in over my head?
Because the power valve is a very easy thing to diagnose and fix, its not a hard thing to trouble shoot. Trust me and take it to your local Yamaha dealer and he will fix it right up. If the linkage is not moving when you rev it up the problem is under the engine cover. you had it apart and cant figure it out so let someone who has more experience check it out and it will save you some time and money.

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