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Discussion Starter · #1 ·
Hi everybody,

I bought a broken KTM last year to rebuilt and i did a leak down test yesterday and I have alot of air going out of my carburetor. I dont have a clue what is wrong with it.

My piston was TDC.

Thanks guys. Il
 

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Welcome to forum.

That would indicate inlet valve leakage or valve timing wrong.
 

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Piston at TDC is correct position to test leakdown, but you should remove cam cover and inspect valve actuator to make sure valve is not stuck open or otherwise mis timed
You should check and make sure valve clearances are correct. You could have a damaged valve or valve train component or simply a tight valve requiring an adjustment.
 
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Dirt Wizard
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Make sure your not on rock either
 

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Discussion Starter · #6 ·
So today I check my valve clearance this is my result can you tell me if Im correct ? First time Im doing this.

Maintenance manual:
Intake valve clearance:
0.10mm - 0.15mm
Exhaust valve clearance:
0.12mm - 0.17mm

Intake:
Existing clearance: 0.102mm (tight)
Existing Shim: 2.2mm
Need shim: 2.172

Existing clearance: 0.102mm (tight)
Existing Shim: 2.0mm
Need shim: 1.972

Exhaust:
Existing clearance: 0.102mm (tight)
Existing Shim: 2.3mm
Need shim: 2.252

Existing clearance: 0.127mm (tight)
Existing Shim: 2.1mm
Need shim: 2.072

Also when i did the leakdown test without the cams cover and the cams i had no leak. That mean for me my clearance was not good is that make sense ?

Thank you guys.🤟
 

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Looking good so far. Make corrections and retest leakdown testing. Hopefully you are okay now. Be sure to report back with your good results
 

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The shim kits are an easy but expensive solution. But ordering one shim at a time is mistake prone and time consuming. Check the shim thickness available as they may not be exact calculated clearance required. Always safer to keep to larger tolerance on exhaust valves for additional safety but keep within recommended tolerances
 

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Dirt Wizard
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So today I check my valve clearance this is my result can you tell me if Im correct ? First time Im doing this.

Maintenance manual:
Intake valve clearance:
0.10mm - 0.15mm
Exhaust valve clearance:
0.12mm - 0.17mm

Intake:
Existing clearance: 0.102mm (tight)
Existing Shim: 2.2mm
Need shim: 2.172

Existing clearance: 0.102mm (tight)
Existing Shim: 2.0mm
Need shim: 1.972

Exhaust:
Existing clearance: 0.102mm (tight)
Existing Shim: 2.3mm
Need shim: 2.252

Existing clearance: 0.127mm (tight)
Existing Shim: 2.1mm
Need shim: 2.072

Also when i did the leakdown test without the cams cover and the cams i had no leak. That mean for me my clearance was not good is that make sense ?

Thank you guys.🤟
If you had clearance on all valves, regardless of being on the tight side, any clearance means your valves are closed, if removing the cams stopped the air leaking back into the intake you had a valve timing issue. You could have .05mm of clearance on the valves and you shouldn't leak air back into the intake.
 

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Dirt Wizard
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Or you were on rock instead of tdc when leak down testing before.
The clearances you have listed would not affect the running or starting whatsoever. That one exhaust valve is a little tight, but it would have no affect until the engine was warm with that clearance.
 
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Very good observations @DW
I missed the detail you skillfully observed when our friend mentioned the cams were removed when leakdown was restored. This could still indicate a mis-timed cam(s) causing a valve to remain open at TDC. Good catch @DW you are my hero
 

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Discussion Starter · #14 ·
Okay so i will keep those shims. I will time the bike and put a new kickstart lever and try to start it
 

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A lot of info here already. Your 4 stroke 250 KTM has a hydraulic cam chain tensioner (adjuster). If this one ha not been replaced it is now 12 years old. These hydraulic tensioners have a tendency to fail. When they fail the cam chain often jumps one tooth on the crank sprocket. Therefore make sure, when you check the timing of the cams, that the crank is also where it needs to be (TDC). If not please adjust. You might find that all you need to do is "jump" the cam chain one tooth on the crank sprocket. While you are inside the motor also check the two plastic wheels/sprockets that are part of the oil pump drive as these two are one of this motors weak points.
 

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Discussion Starter · #16 ·
A lot of info here already. Your 4 stroke 250 KTM has a hydraulic cam chain tensioner (adjuster). If this one ha not been replaced it is now 12 years old. These hydraulic tensioners have a tendency to fail. When they fail the cam chain often jumps one tooth on the crank sprocket. Therefore make sure, when you check the timing of the cams, that the crank is also where it needs to be (TDC). If not please adjust. You might find that all you need to do is "jump" the cam chain one tooth on the crank sprocket. While you are inside the motor also check the two plastic wheels/sprockets that are part of the oil pump drive as these two are one of this motors weak points.
Yes, Ive noticed that my chain adjuster fail when I put it back on the bike. So i will buy an other one for sure.
So about my leak i was talking about this is a photo. The leak is where the air filter.
95493

I'm under 20% i guess is very good.
And an other question about TDC. Look a the picture if Im correct. I align the mark with the hole infront.
95494


Thank you guys for your help. Very appreciated.
 

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Dirt Wizard
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Well I don't know about aligning that up, I lock the crank on the other side and make sure the cam lobes are pointing up they should be roughly at 10 and 2.
As trying to find the spot on the crank to screw the locking bolt in is not easy to do in a hurry if you're turning the crank to do a compression or leak down test and you have the flywheel cover off, lock the crank with the locking bolt (or look down the hole and line it up) then use a paint pen and mark the rotor and case on the opposite side
 

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Dirt Wizard
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Very good observations @DW
I missed the detail you skillfully observed when our friend mentioned the cams were removed when leakdown was restored. This could still indicate a mis-timed cam(s) causing a valve to remain open at TDC. Good catch @DW you are my hero
Hero? Calm down mate.
 
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