13 kx250f help

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2/23/2018 1:53 PM

Little info, I have a buddy that I ride with I do all of his bike work and maintenance. He has a 13 kx250f. I rode it and it felt super slow like almost 125 slow. I was struggling to hit jumps in gears I should be able to hit them in. I thought it was just that I have not rode a 250f in quite some time and I'm at 5300 ft elevation and I weigh 170lbs.

When I was a kid and at sea level I used to like to have a 14 tooth front sprocket so all the gears would pull longer. I did that and it helped some but still seems really slow.

Then I rode my other friends 12 kx250f and this thing was what I remembered a 250f being like. Hit all the jumps with ease and almost as fast as a 250 2 stroke. 3rd gear with stock gearing would pull longer than the other bike geared up.

My friends 13 250f seems like it is down at least 20% power over the other 250f. They should ride basically the same.
This bike starts fine, idles fine and revs out fine.

What would you guys think it could be? I thought maybe low compression or valves need adjusted but it starts and idles fine.

Then I was thinking maybe the clutch is fried and it is slipping a bit. I had a 250f that I fried the clutch on and it was kind of like this but mine was way worse.

What do you guys think, and what do you guys think I should do?


2/23/2018 2:03 PM
Edited Date/Time: 2/23/2018 2:04 PM

Ok, so the bike is 5 years old now....
Has it ever had any work done to it?
If so, what?
Last top end done when?
Last valve clearances checked when?
Timing chain replaced when?

In short, more info needed.

Paw Paw


2/23/2018 2:19 PM

If everything pawpaw suggested is good, id look into a remap for the elevation.


2/23/2018 2:22 PM

Have no clue, used bike bought about a year ago from craigslist.

My plan was to do compression test, and check valve clearances if comp test was good.

Just kind of thought it was odd that it starts with no issues and seems to rev and ride fine it just does not "go" like the same bike does.

This is why I thought maybe clutch plates because everything seemed to work well.

Just like any mechanic knows check the easy/cheap things first so wanted to hear what people thought just by hearing the symptoms or if somebody has ever has something like this happening where it was something stupid or easy.


2/23/2018 2:25 PM

It came originally from a fast racer so because of the name that is still on the graphics. No way he would not have it set up correctly for the elevation.

I mean the motor has got to be on its last leg if it is motor related because this thing is bad slow.


2/23/2018 3:57 PM

Again....The bike is 5 years old and with no knowledge of the maintenance history, everything is on the table to be looked at.
RE: Clutch question....Has the oil been changed in the time frame of the current ownership? If so what oil was used?
In this bike the oil for the engine and the trans are the same and it could be causing issues if the incorrect oil is being used or if it has not been changed by the current owner.
FYI: A compression test on modern 4 strokes is almost useless due to them having auto decompression devices built into the cam shafts.

Paw Paw


2/24/2018 8:19 PM

Thanks the oil was changed by a “mechanic” when he bought the bike. Not sure what he used. Only has like 3-4 hrs maybe on the bike since he bought it. Oil needs to be changed though again.

I figured about the comp test but was not sure.

I wanna check valves for him and see what the clearances are. Guess that would be the first step. Guess next would be pulling cylinder?


2/25/2018 4:40 PM

Ok update I have some measurements.

Checked valves. Manual says intake .10-.15mm Exhaust .17-.22

The right number I could not get to fit and the left number fit or was a touch loose. So somewhere in between these ranges.

Left Intake=.05-.07mm Right intake=.07-.10

Left Exhaust.15-.17 Right is .15-.17

So all of them are too tight. With the intake being worse. With this information what do you guys thing. I know re-shim but do you think this could cause loss of power by 20ish%.

Measured the clutch friction plates, The fiber pieces on the plate are kind of off set so you can either measure fiber to fiber or fiber to metal.
Fiber to metal was 2.35mm and fiber to fiber was 2.85mm. I will have to check if that is in spec. They looked ok to me



2/25/2018 5:33 PM

Tight valves will cause performance issues for sure. Adjust them to spec before you move on. Do a leakdown test to make sure theyre sealing too. If you adjust them and it takes more than 4 kicks to start then you prob have valve seat issues.


2/25/2018 6:13 PM

Thanks yeah I’m gonna get a 250f shim set and do his.

It starts in 4 kicks right now in 30 degree weather without starting in like a month.

Can I pull the head to inspect instead of a leak down test. I would rather not build the whole contraption to do that if I don’t have to.


2/26/2018 1:16 AM

No you cant pull the head. You need to get the valves in spec BEFORE ANYTHING ELSE.

Then go get a leakdown tester from harbor freight. Youll need one anyways. Welcome to 4 strokes. Compression tests arent good enough.


2/26/2018 5:57 AM
Edited Date/Time: 2/26/2018 6:36 AM

Yeah I know I need to re-shim but did not know if once you do that you could look at the valves with the head off and tell.

Looking up the leak down test, seems that you don't have to plug the intake and exhaust?

Looks like the procedure is piston at TDC and no air can escape anywhere except the valves.

I think on 2 strokes you always see guys build contraptions to block the exhaust and intake and I really did not wanna do that and the harbor freight tester is cheap.

Is there some psi that it should hold for a certain amount of time?

Wondering if it would be benifical to pull the head when adjusting the valves to see the cylinder walls. A new head gasket is 30 bucks and you already have to take the cams out. Does not seem like too much more work?


2/26/2018 7:43 AM

Pull it down and check cylinder/ piston/ valves etc. Might just need a top end. Easy peezy.


2/26/2018 8:03 AM

May have to do that to get a look at it. Thanks guys for the help.


2/27/2018 6:07 AM

Talked to my buddy last night. He has two bikes so he does not need this one back up and running quick. But we decided to buy the shim kit and adjust the valves. Then we will take the head and cylinder off to inspect and buy more parts if need be from there.

My one question is do you have to have the cams back together to do the leak down test. Would be nice to do the leak down test after we adjust the valves but before we take the head off but I would rather not take the cams out to adjust then put them back in,leak down test then take them back out again?


2/27/2018 6:25 AM

Adjust the valves, reassemble, do the leakdown test. Cams should be installed.


2/27/2018 6:35 AM

That sucks but I guess its not really that hard. Thanks