Tuning the mikuni low speed fuel circuit with air screw

I am running a VM34 on my 850 and I recently went from a 35 pilot jet to 30. (I did thus as I could unscrew the air bleed screw with the 35 jet, but the idle speed did not drop.)

When I unscrew the air bleed screw with the 30 jet, I get a progression from
- too rich and slower idle speed to
- speeding up idle to max speed
- then dropping very slightly but not a lot.

What I notice is that if I unscrew the idle air jet a little at the point where idles starts to drop slightly, I get exhaust popping through the muffler on overrun (ie if I close throttle at 2500 rpm and let engine braking slow the bike down).

So to tune the low speed circuit, I unscrew the idle air screw until it the exhaust pops on overrun, then I screw it in a quarter turn at a time until there is no exhaust popping on overrun.

I would really appreciate your comments on this

Thanks

Dennis
Vancouver

What air jet and slide are you running?

I usually lean the idle speed up with the air screw the the highest rpm and the richen 1/4 turn. Then set the idle to the desired location with the slide screw.

I have
Slide 2.5 cutaway
Needle 6Dh3 (It is set to the middle notch)
Main Jet 250
Pilot jet VM20/210 #30
Air jet BS30/97 #2.0

Dennis

change the air jet to a 1.1 or 1.0 the 2.0 is not for a four stroke. You will probably need to change the pilot jet back, but tuning can be bike specific.

DennisMo said:

I have
Slide 2.5 cutaway
Needle 6Dh3 (It is set to the middle notch)
Main Jet 250
Pilot jet VM20/210 #30
Air jet BS30/97 #2.0

Dennis

Click to expand...

And your needle jet is ?

It is 159 PO

The idle should be set with the slides at bottom. The rest is slide cutaway and needle jet. The pilot jets usually feed fuel when the slides hit bottom and there is a lot of vacuum - they are probably designed to stop two-strokes from seizing, at the ends of long straights. If you get popping when you shut-off - probably lack of enough fuel in the mix, or a leaky exhaust. Needle jet might be more significant. You are using a very lean needle. I would raise it one notch.