Engine Overrev run hot after Carb Rebuilt KTM 300

KTM Two Stroke
Edited Date/Time 1/27/2021 4:22pm
After a Winter Rebuilt on my KTM 300 SX 19 i make a 20 minutes Test Ride yesterday. It runs lean but Engine Sounds good . Outside Temperature was close to Zero.
As i stop at the Garage it revs out and was not able to stop it by Killswitch .

I dissconect the Spark Plug Connector and it doesn‘t stop after i switch of fuel it takes Two minutes to stop

Before it happens i did the following Mods:

New Throttle Cable
New #4 Mikuni Slide (Leftover Honda CR 125)
Slide and Drain Plug Mod according to Thumpertalk
New Aluminium Throttle Grip

I thought it was a stuck Throttle but i found nothing today when i dissasemble .
Could it have something to do with the Slide Mod ?Jetting was according to KTM Manual

Appreciate every help





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FGR01
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FGR01 1/24/2021 8:11am
1/24/2021 8:11am
That's a pretty big notch on the front of that slide and it looks like your idle screw is turned all the way in and holding the slide lifted a good bit.

When the engine runs away like that, put it in 3rd or 4th gear, hold the brakes hard, and slowly let the clutch out.
KTM Two Stroke 1/24/2021 10:28am
1/24/2021 10:28am
Thanks You are right the Idle Screw is completely inside but the Overrev came after 20 Minutes and not after the first Start Up

Good Idea to shift in fourth and kill the Engine it was a scary Moment when a 300 revs Up that high but for the next time i am prepared 😂

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Paw Paw 271
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Paw Paw 271 1/24/2021 1:28pm
1/24/2021 1:28pm
You are indicating you have a very lean condition. Either a plugged jet or and air leak will cause this.

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rjg
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rjg 1/26/2021 8:01am
1/26/2021 8:01am Edited Date/Time 1/26/2021 10:47am
How high was it revving? Like almost like it was wfo or hanging idle?

Too much fuel some how. Air screw position

It likely ran until the float bowl was empty.


Crank seal , that plug doesn't indicate a lean 2stroke. I think it was overly rich and
you could built up alot of fuel in the crank case. You burned that off with a leaking crank seal. Possibly

Maybe your petcock wasnt actually off possibly.

It could only keep running after you pull off the plug and shut off the gas if it has it other sources of both air and fuel
boothead359
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boothead359 1/26/2021 4:55pm
1/26/2021 4:55pm
I’ve experienced this with a 90 hp evinrude 2 stroke outboard. Runaway motor , revving high with the ignition shut off and the key in my hand. Had to disconnect the fuel line to shut it down. My first and only experience with a motor running with no spark for combustion , I guess the cylinder was hot enough to promote ignition. At any rate no fun at all. And still dont know what caused it, never happened again and I had that motor for many years after.
KTM Two Stroke 1/26/2021 10:34pm
1/26/2021 10:34pm
rjg wrote:
How high was it revving? Like almost like it was wfo or hanging idle? Too much fuel some how. Air screw position It likely ran until...
How high was it revving? Like almost like it was wfo or hanging idle?

Too much fuel some how. Air screw position

It likely ran until the float bowl was empty.


Crank seal , that plug doesn't indicate a lean 2stroke. I think it was overly rich and
you could built up alot of fuel in the crank case. You burned that off with a leaking crank seal. Possibly

Maybe your petcock wasnt actually off possibly.

It could only keep running after you pull off the plug and shut off the gas if it has it other sources of both air and fuel
Removed the Ignition Cover looks good found no leaking under the Flywheel , is it posible to remove the left Crank Seal from outside without to split the Cases ?

rjg
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rjg 1/27/2021 4:22pm
1/27/2021 4:22pm
Hmm cant remember but what you could do is spray quick start at the crank seal while its idling. You will know its leaking when the rpms change significantly.

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