OzVMX Forum
Clubroom => Tech Talk => Topic started by: John Orchard 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
-
The idle screw really only raises the slide a bit , you can file the slide or change stuff to raise it.
-
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.
-
Thanks Guy's, it gives me something to think about.
-
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.
-
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)
-
It's not part of the choke, like Keihin PJ's ?
-
It's not part of the choke, like Keihin PJ's ?
Nope
-
Must be the same carb as on the 84 RM250, no idle also.. 2 stroke mx bikes don't really need to idle anyway
-
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
-
I'll adjust the idle by filing the slide.
-
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 ;)
-
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
-
I'll adjust the idle by filing the slide.
Filing the slide cutaway?
-
I'll adjust the idle by filing the slide.
Filing the slide cutaway?
No that'd lean it out at 1/3 throttle.
-
I'll adjust the idle by filing the slide.
Filing the slide cutaway?
No that'd lean it out at 1/3 throttle.
That's right.. I'm interested to know which part of the slide is filed to raise the idle? I can't see how filing it will lift it
-
I'll adjust the idle by filing the slide.
Filing the slide cutaway?
No that'd lean it out at 1/3 throttle.
That's right.. I'm interested to know which part of the slide is filed to raise the idle? I can't see how filing it will lift it
A carburettor is just a block of alloy that mixes air & fuel together, there'll be a way ;-) Maybe modification of the bottom of the slide to allow some air through and pick up some fuel on the way?
-
You file the front to richen and the rear to lean. So front for idle
-
You file the front to richen and the rear to lean. So front for idle
Interesting.. One of the bing slides I have for my 490 maico has a small notch filed out of the front of the slide but I haven't tried it
-
IMO that would be to help it from stalling in heavy breaking.
-
I think I'd rather run a small groove from one side of the slide to the other (in line with the port), above the fuel outlets, varying the depth depending on the desired idle rpm.
-
Just for convenience, either putting my goggles on at the line or if I crash in a race, those few seconds of idling can save some hassle.
-
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 ;)
You idle through the corners and are still a mid fielder....