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Author Topic: Adjusting idle rpm on KX250A7 ?  (Read 3926 times)

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Offline John Orchard

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Adjusting idle rpm on KX250A7 ?
« on: June 27, 2013, 05:51:55 pm »
Hey Guy's, I'm just playing around with different carbs, I have a Mikuni VM38 from an '81 KX250A7, it does not have an idle adjusting screw, how do you adjust the idle? ...... mixture screw?

Does anyone have some jetting specs that they are running that they could share with me?  Looks like I'll start with the following ...

MJ:  170
NJ:  R4
PJ:  20 (might need a 25 but it has a flat-bottom slide that requires smaller pilot jets)
N:   6FL28
S:   2.5
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Offline Slakewell

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Re: Adjusting idle rpm on KX250A7 ?
« Reply #1 on: June 27, 2013, 06:44:09 pm »
The idle screw really only raises the slide a bit , you can file the slide or change stuff to raise it.
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Offline Stevo17

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Re: Adjusting idle rpm on KX250A7 ?
« Reply #2 on: June 27, 2013, 06:50:17 pm »
Hi John,

Similar motor to my B1 with a VM38SS.  Mine has an idle adjusting screw tho...  Standard specs are as follows:

MJ:  165
NJ:  R8
PJ:  15 (might need a 25 but it has a flat-bottom slide that requires smaller pilot jets)
N:   6L2
S:   3.0

Not sure what mine actually has in it but if you get desperate I will pull it apart and let you know.
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Offline John Orchard

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Re: Adjusting idle rpm on KX250A7 ?
« Reply #3 on: June 27, 2013, 08:23:58 pm »
Thanks Guy's, it gives me something to think about.
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Offline oldyzman

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Re: Adjusting idle rpm on KX250A7 ?
« Reply #4 on: June 27, 2013, 08:35:20 pm »
i believe some old mikuni carbs have a blanked off hole where the idle adj screw would normally go. If your slide has the chamfer on the side then you could drill and tap the hole.
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Offline John Orchard

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Re: Adjusting idle rpm on KX250A7 ?
« Reply #5 on: June 27, 2013, 09:00:34 pm »
No chamfer on the slide, I have a spare slide with a chamfer but I'm keen to try this slide with flat bottom (same as the optimiser).  The slide optimiser helped my RM125B pull stronger off the bottom (as much as a 125 can lol)
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Offline evo550

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Re: Adjusting idle rpm on KX250A7 ?
« Reply #6 on: June 27, 2013, 11:14:03 pm »
It's not part of the choke, like Keihin PJ's ?
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Offline John Orchard

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Re: Adjusting idle rpm on KX250A7 ?
« Reply #7 on: June 28, 2013, 09:04:27 am »
Quote from: evo550 on June 27, 2013, 11:14:03 pm
It's not part of the choke, like Keihin PJ's ?


Nope
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Offline JohnnyO

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Re: Adjusting idle rpm on KX250A7 ?
« Reply #8 on: June 28, 2013, 09:17:02 am »
Must be the same carb as on the 84 RM250, no idle also.. 2 stroke mx bikes don't really need to idle anyway
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Offline Bitten

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Re: Adjusting idle rpm on KX250A7 ?
« Reply #9 on: June 28, 2013, 09:43:21 am »
As mentioned the early RM's have no idle screw also and I was told I needed to adjust the idle via adjusting the slack in the throttle cable - a bit fiddly and hit and miss as it never quite stayed correct for long!

Ended up needed a new carbie and the new Mukuni's have the idle screw so in my case all was well
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Offline John Orchard

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Re: Adjusting idle rpm on KX250A7 ?
« Reply #10 on: June 28, 2013, 09:48:13 am »
I'll adjust the idle by filing the slide.
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Montynut

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Re: Adjusting idle rpm on KX250A7 ?
« Reply #11 on: June 28, 2013, 10:06:43 am »
I always have my 250 and open class MXers set to a dead trottle (not idling) anyway. 250 and in particularly open class bikes tend to push (understeer) in slow corners when they idle. Just my opinion from a mid fielder ;)
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Offline JohnnyO

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Re: Adjusting idle rpm on KX250A7 ?
« Reply #12 on: June 28, 2013, 11:03:49 am »
Quote from: Montynut on June 28, 2013, 10:06:43 am
I always have my 250 and open class MXers set to a dead trottle (not idling) anyway. 250 and in particularly open class bikes tend to push (understeer) in slow corners when they idle. Just my opinion from a mid fielder ;)
Exactly
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Offline JohnnyO

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Re: Adjusting idle rpm on KX250A7 ?
« Reply #13 on: June 28, 2013, 11:05:10 am »
Quote from: John Orchard on June 28, 2013, 09:48:13 am
I'll adjust the idle by filing the slide.
Filing the slide cutaway?
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Offline John Orchard

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Re: Adjusting idle rpm on KX250A7 ?
« Reply #14 on: June 28, 2013, 11:45:03 am »
Quote from: JohnnyO on June 28, 2013, 11:05:10 am
Quote from: John Orchard on June 28, 2013, 09:48:13 am
I'll adjust the idle by filing the slide.
Filing the slide cutaway?


No that'd lean it out at 1/3 throttle.
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