mikuni carb question

jprice

jprice

OldMiniBikes.com
#1
#1
I know nothing lol
there is a air and fuel mixture screw the one with the spring wrapped around it ?
and there is an air screw near the back of it ?
in order to get the sled to run good at idle I have to turn the air screw all the way in and back it out 1/2 turn does that sound right?
thanks in advance ...
 
W

wjustice

Well-Known Member
#2
#2
On the mikunis I've seen, it's should be 1.5-2 turns out. Adjust it to highest idle speed then turn in just till the idle starts to drop. That puts it on the rich side. If it's too far in, then you should use a smaller pilot, too far out (should be less the 3 turns or it can fall out) you should use a larger jet.
 
bikebudy

bikebudy

Banned - Must pay $500
#3
#3
Jet with spring wrapped around it is large and on the side.

Idel screw...

small screw is air screw, as said 2 turns out max..
 
ole4

ole4

#5
#5
Guys Something does not sound right. Increasing pilot jet size richens the idle circuit correct? Opening up the idle air mix screw (the one on the bottom back) allows more air to mix with the fuel supplied by the pilot jet. So if you have to open the adjustment more than three turns you should use a bigger (IE: higher number jet) Pilot jet. Am I wrong in this assumption?
 
H

Havasu Dave

Moderator
#6
#6
ole4 said:
Guys Something does not sound right. Increasing pilot jet size richens the idle circuit correct? Opening up the idle air mix screw (the one on the bottom back) allows more air to mix with the fuel supplied by the pilot jet. So if you have to open the adjustment more than three turns you should use a bigger (IE: higher number jet) Pilot jet. Am I wrong in this assumption?
Click to expand...
Got me by the balls. I have a Mik 22 all adjusted out, and that bottom air screw does very little. :shrug: It never has. 17 idle jet, and 120 main, center slot on the needle on a cam'd 5 FH.

The side screw just places the slide in a different position for idle adjustment, no mixing. The slide needle with the five slots effects both low and high side settings, and the pilot jet affects the total high end effects.

I'm running a bit lean actually, but I don't mind the bluing on the stainless, or the throttle blip as I cruise past hot looking chicks. :biggrin:
 
D

danssoslow

Member
#7
#7
ole4 said:
Guys Something does not sound right. Increasing pilot jet size richens the idle circuit correct? Opening up the idle air mix screw (the one on the bottom back) allows more air to mix with the fuel supplied by the pilot jet. So if you have to open the adjustment more than three turns you should use a bigger (IE: higher number jet) Pilot jet. Am I wrong in this assumption?
Click to expand...
I believe I've read that all slide crbs are like this, but I'm using the Walbro Briggs carbs as an example. If the idle screw is before the slide, as in the PZ22, it is an idle air circuit, introducing air into the circuit; if it is aft of the slide (as I believe the op is describing here), like the PZ26, it is an idle fuel adjustment, and introduces fuel into the circuit.

Ifit is true that these slide carbs work in this manner, and the op's idle circuit is behind the slide, then opening the screw will introduce more fuel, richening the circuit. If he can only turn the screw a 1/2 turn out and cannot introduce more fuel, the pilot would be on the large side for his application.

Here's a copy of Briggs' quick reference guide for tuning.
http://www.briggsandstratton.com/en...ing Guide/~/media/Racing/carbtuningguide.ashx
 
ole4

ole4

#8
#8
The pilot jet is for the idle circuit the main jet is for 3/4 to full throttle. The air adjustment screw is just that it adjusts the amount of air to be mixed with the fuel that passes thru the pilot jet. That amount of fuel is fixed by the jet the screw is in the air path not fuel.
see section 3.1
http://www.mikuni.com/pdf/vmmanual.pdf
 
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