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Engine revs to the moon when I cold start the bike

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My 2001 250 sx revs to the moon when I cold start it. After approx 1-1,5 min it idles and run perfectly. The interesting thing is when I take a break in the track and i start the bike, it doesn't rev high. Last time it reved so high I can't kill the engine with the switch, it was like when you run out of gas and the mixture is to lean. Usually hard to cold start the bike to. For me it looks like it only do this when I start the bike with the choke, but as i know the choke should make the mixture richer, not leaner. Could the engine intake false air if the power control cover not seals perfectly on the clyinder? Actually the engine not runs lean, moreover it runs rich, because I changed the reed valve to a boyesen which made the mixture too rich, last time it fouled my spark plug. I rebuilded the top end and changed the reed valve before 5 engine hour, but problem occoured before the rebuild to! Any ideas? 

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a lean condition for sure.  you may say it runs good once it settles down, but I bet it doesn't.  I bet you are losing power and don't realize it.

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I would start with the easy things first. Sounds like it is lean on the pilot/dirty pilot or a/s not correct. Try cleaning the pilot and see how many turns out your a/s is. You turn it in to richen and out to lean. Should really be between 1 and 3 turns out with the correct pilot.. You can normally cure a mental over rev on start up with it adjusted correctly. People adjust the slide screw to get their bikes to idle when the a/s adjustment is the culprit. Try that first and see how you get on. Bob

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totally normal... on a correctly jetted and healthy bike.. .congrats on having good jetting

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Normal? We've raced 125 KTMs for years and they don't rev so high on choke start up that we feel obliged to use the kill button.  To be fair the choke is pushed off very quickly after start up . We also have a 2019 250sx which we've found needs longer on the choke after initial start up at track but it certainly doesn't rev as described by the OP. As usual he doesn't give a lot of details about current jetting. 

Edited by 7Tktm
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If the bike bogs when you whack the throttle open from low revs ,this says the pilot is lean , I suspect a little richer pilot would make it more responsive and not rev to the moon ,I've had many bikes do this , I have found them too lean on the pilot

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I would add mine have only done it when been sat for a while ( day a week or more ) 

Also more likely to do it when not started for a week and transported to track and then started . ( no idea why )  just stating my own experience . 

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25 minutes ago, noste500 said:

I would add mine have only done it when been sat for a while ( day a week or more ) 

Also more likely to do it when not started for a week and transported to track and then started . ( no idea why )  just stating my own experience . 

Hi yes I have heard of them revving higher on a choke start (maybe not to the point of hitting the kill button!) when not used for a while. We drain our carbs after use and never use mix over a week old. That's just how we do things and have never experienced this, but then again, our jetting is always as near correct as we can get it . We often change jetting several times over an mx weekend while some people never change it summer and winter lol

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This also happens when you have your idle set high.

Next time, when cold, turn your idle screw out 1.5-2 turns, turn on choke, and start.  Once warm your idle may be barely ok.

I like my idle really high.  So I got one of those easy adjust idle screws.  I turn it out 2 full turns, start bike on choke, warm up, turn idle back in 2 full turns, and all is good.   

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both ours do it without the choke. ( i dont drain them after use and its usually a week between )

i think it may be that fuel is bouncing around and getting in cases??

another guy once told me its fuel and oil residue blocking something and after being run for a while it clears. ( so bobs way of draining would sort it )

I will try draining down after use.

 

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I had a 2012 300 XC-W I bought new and had it 5 years and it never idled excessively high upon start up and it also started very easy hot or cold.  I sold the 2012 and bought a 2018 300 XC-W new in July 2017.  The 2018 never idled excessively high upon start up.  I installed the RK Tek head and it took even less choke before running with throttle and no choke or idle issues.  I next installed a new Keihin 36mm carb from JD Jetting.  Then I immediately noticed a change in idle at start up.  It will now idle very high immediately when the engine starts so I push the choke off which of course lowers the idle, I blip the throttle lightly a couple of times and if the engine starts to die, usually more likely in colder weather, I raise the choke up and let in back down not even clicking it into the up position, just a quick up and down.  The idle never becomes excessive after I have pushed the choke off and blip the throttle and only using the choke usually once or twice with a quick up and down coupled with light throttle use until the engine idles normal.  The engine idles great after warm up and starts immediately with the E-start after engine is warm.  I am currently running 42 pilot, JD Red #3, 170 main with AS at 1.5 -2 turns out.  Jetting seems really good.  I have tried richer needles as well as richer AS settings and the initial high idle remains consistent.  The only time it did not idle excessively high at start up was with the Mikuni carb using either the OEM head or the RK Tek.  It seems the combination of the RK Tech head and the Keihin carb is what resulted in a high idle at start up but this is the only negative I have found with the combo.  Everywhere else there is nothing but performance gain across the board including an easier start up without having to do the 2nd gear rock back and forth a few times to even get the Mikuni to start the engine. 

Edited by Gray

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Wonder if the choke is worn on the keihin ?

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My ktm 200xcw 2012 does that since new most of the time on the choke. I just intermittetly press the killswith button to keep the rev down. Bike run fantastic otherwise and has for 160 hours. Jd jetting in the bike.

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My jetting: As: 1 1/4 out, pilot jet:48, needle type: NOZD, clip position: 3, MJ: 172. 

Altidude: near sea level.

Temperature: When I last start the bike it was about 16 celsius, but now the weather is colder, about 3 celsius.

One thing what I dont understand is if the bike is lean, then the bike is Always is lean, not only when the bike is cold because when the bike not absolutely cold the bike idles and run good or even bit rich because it fouled my plug last time, but now the weather is colder so I think now it will be okay. At summer the bike not revved as high as now in the colder weather. Otherwise the engine had a bottom rebuild before 70 hour.

(Sorry for my english ?)

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When an engine is cold.. it is harder for the fuel to vaporize.   Also you have to have a combustible Fuel/Air Mix (or it will not light/combust)

In short, with the cold engine the AFR is too lean and will not ignite. You increase the fuel and the AFR comes into a state that will ignite. This is NOT due to any jetting set up but is more a result of the cold engine and the fuel droplets condensating on the internal engine parts.

The choke will add more fuel and less air and bring the AFR into ignitable spec.  The run on is due to the lean condition (again not from the jetting but from the internal "situation")..

Again, not a jetting issue (sorry Mog) just a result of the AFR not  being "right" at that moment.

Once everything "heats up" and the fuel can better vaporize, the choke is no longer needed and the engine will function as normal.

Please do not go and adjust any jetting to try and "cure" this scenario because it is normal and a result of the conditions INSIDE the engine.

Edited by HERA
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Someone said that if you leaned the bike over after you turn on the fuel and set the choke that the bike would start easier and probably not rev quite so high initially upon start up because you would be filling up the float bowl with more readily available fuel. Sure enough, when I tried this procedure a few minutes ago the engine did not rev near as high as it normally does.  The rpms began to lower much sooner and I didn't need to push the choke off immediately as a result.  Go figure.  Maybe it was the more fuel less air theory that HERA is referring to.  What I find curious is why my 2018 300 has this over rev issue at normal cold start up while my 2012 300 or 2009 250 XC-W did not.  All bikes have the same Keihin 36mm carb.  The Mikuni did not do this once it finally started, it just didn't want to start.  

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Lean pilot. If you have too rich of a pilot it will start without choke. If its lean itll rev high when cooler. Bet once you get it to operating temps you can turn your air screw all the way in and bike wont die. It runs....but itll run better with a richer pilot.

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