Sudden Low End Power Loss

Rooster

Today's Tom Sawyer
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Aug 24, 2000
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I just ran a search on this and didn't find quite what I needed.

2000 YZ250 Bone stock.

The last two rides I have noticed a power loss on the very low end. From a stop, I am having to use more clutch and gas to get rolling. The rest of the range is perfect, tons of power. I also noticed at a near stop landing on a cliff jump, that I had to slip the clutch to keep it from dying to complete the climb.. (Never had that problem before).

I have checked the obvious, no hanging up brakes, wheels spin free, no debris in the chain guide, etc. New filter and plug. The only thing I have changed before this started to happen is I dropped the pilot jet down one size because of plug fouling.

According to my sources, the jetting change should have, if anything, improved the low end by leaning it out.

Q. #1: Is my jetting information wrong, and the pilot jetting change is causing the power loss?

Q. #2: Could this be a compression loss problem? The piston and rings are original, but I don't have a lot of hours on the bike.

Q. #3: Could this be a power valve problem? I used to run bean oil through the bike and may have a load of crap built up in the power valve.

Thanks for your input!
 
T

TechKid

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Apr 30, 2002
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When youre working on anything, not just bikes, you should know that if it runs fine and you change something and it runs bad, it has to be something that you did.
 

RM_guy

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Check your reeds. While they can affect power everywhere there is a dramatic affect at the low end. Does it start OK?

Also, have you repacked the silencer lately? A dirty silencer can effectivly lengthen the stinger and hurt low end.
 
C

cr25096er

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Apr 16, 2002
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id say to check the powervalve then if that checks out put the other pilot jet in and see what it does.
 

Rooster

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When youre working on anything, not just bikes, you should know that if it runs fine and you change something and it runs bad, it has to be something that you did.
Click to expand...

Yes, that is common knowledge, but I am looking for more insight, as my manual suggests a smaller pilot to improve low end performance. The drop in performance didn't happen the instant I changed the pilot, it seems to have started about a week after that.

The reeds looked fine, no cracking, not out of tolerance. The bike will start first kick, almost every time.

silencer packing may be ready for replacement, but i usually do it once a month, for lack of better things to do with my time.

I do plan to check compression, and go from there.

It will be due for a top end soon, so I may just go ahead and do it and see what happens.

It's a real pain testing changes out, as I live smack dab in the middle of town, and can't really get it good and warmed up to see what it's doing. :ugg: I hate to mess with it out at the OHV park, because of all the dirt and dust flying around.

Thanks for the input. :D
 
R

Rcannon

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Nov 17, 2001
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I think reeds also. The part about having to change the pilot jet makes me believe this. The fact that I also went through the same thing makes me think it again.

Only one petal of my reed was bad and it made a giant difference in low end power. The bike was rideable, just not right. I think these break down over time so slowly that a person does not pay attention to it. My bike appeared to run lean with the reed screwed up. I added a rad valve and had to return to my stock pilot jet. It is very easy to check. I did not do this, but removing the carb and shining a flashlight in the area would have shown the damage on mine.
 

dklink2000

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Feb 18, 2002
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If you were fouling plugs you might have gunked up the power valve. I would check that.
 

RM_guy

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Unless they are stuck in the open position, stuck power valves wouldn't affect low end but it would kill your top end. Usually when they stick it is when they are closed, but not always so I'd check it out.
 

Rooster

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I've got super top end. That's making me think it may not be the powervalve. I remember my bud put his PV in his old YZ125 backwards once and it was a complete dog all the way through. My only problem is just from dead stop to a roll. I can launch it from second gear with no problem, but that's at a lot higher rpms.

I especially noticed it when playing around doing technical stuff at a snails pace.

I guess I'll put the original pilot back in and see what happens. I probably need to lean out the main to get away from the fouling problem, instead of the pilot. I know it's running too rich, but that's how I've always run my bikes, until now with all the fouling.
 
C

cr25096er

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Apr 16, 2002
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clean the pilot jet or better yet if u can buy the same size pilot jet. pilot jetts have little holes that cold easly cloge(sp?). and the fact that u have good topend is why w think its the powervalve bec it COULD be stuck open. just remove the cover and check. u dont need to start it just check.
 
I

IrishEKU

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RoosteR,
Does it bog a bit under load? Mine is due for a new clitch and it's doing the same thing. I replaced my top end and power like never before. All of the sudden she lost it, jerky PB and coming off a start she woul grab and open up. Just like the clutch in a car. If your fiber plates are glazed they will slip.
My .02 cents.
Phillip
 

Rooster

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I pulled off the power valve side dove last night. Perect smooth operation.

I pulled off the silencer, and the packing was spooge soaked, I must have missed the change last month. the pipe was pretty crudded up with spooge too. I cleaned everything out and will repack tonight.

The cylinder still has the hatch marks from the factory, and the piston looks like new, rings and all. (from what I could see through the exhaust port)

I thought about the clutch too. Seems to me that happened to a buds CR once, it was slipping just enough to cause a power bog from a dead slow start. I have been using the clutch a lot oon this bike, trying to control my power in corners and on nasty twisty hill climbs. Perhaps I am due. For no more than the cost, I will order one today and be done with it.

Then again, maybe this is all in my head, as I am getting concerned about the possibility of something going wrong right before dirt week.

thanks again for all the input, I will keep postiing until I have this critter bagged.
 
I

IrishEKU

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Originally posted by RoosteR13


The cylinder still has the hatch marks from the factory, and the piston looks like new, rings and all. (from what I could see through the exhaust port)
Click to expand...

A big red flag went up for me on that. On my last inspection prior to saying the heck with it and replacing the top end, I pulled the pipe too and took a peek at the piston and rings. They looked great! I wen't for a ride and didn't think much of it taking 5-6 kicks to start the old zinger untill she started running like a 4 stroke. When I replaced the top end the ring was so badly worn on the front it was about half the width as the rest of the ring! She was bleeding all the combustion force into the case and sucking alot out on the upstroke to foul the plug on starting and long downhills.
RoosteR check those rings real well, dollars to doughnuts depending on your last top end replacement I'll bet that has something to do with it ;)
 
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