VM34 pilot jetting

I have passed on my Mikuni VM to a friend and he is struggling to get it running without hunting. he has the jetting and slide as per the figures in the INOA tech digest i.e.. 230 main, 159 P2 needle jet,6DH3 needle, 40 idle jet, 2.5 slide, 2.0 air jet. The manifold is a sudco and has been matched to the ports on the head. Any ideas on this problem. (The bike is a Mk3 850) he has reset the float level.

Is he sure there are no air leaks? Does the sudco manifold use 4 screws or 2 ?

the current jetting from MAP is a #3 slide series 159 P-0 needle jet

The 159 P-0 is what I ship with, you may want to try a #35 pilot jet if the air screw ends up being out near or more than 3 turns. Needle clip in mid (3rd) position; all assuming primary running ground at sea level to 5000'. A 2.5 slide compliments the P-0 needle jet, your P-2 may need the 3. slide you have. If you want to experiment with the P-2 needle jet try lowering the need clip to the 4th position. I am happy to chat with you about dialing in the Mikuni; I get feedback from some very sharp tuners.

Float arm set parallel to the float bowel seating surface; no caliper need here.

Do check for air leaks, the reinforced rubber adapters don't live forever; if you put some tension on it and it shows any cracking, best to replace it. The carb being unsupported in the rear acts like a moment arm and adds operational stress to the adapter.

Thanks gents, the manifold has four screws and the rubber is new. Some figures to work on

Hunting is normally a sign of richness.

I notice you have a #40 pilot. I had a Mikuni fitted to my Commando by a PO, that had a #25 pilot.

I'm not suggesting you should have a #25 specifically, but the scenario does seem to point to richness IMHO.

those settings are ok, how are the spark plugs, are they fouled, put some new plugs in, is it an electronic ignition that requires resister plugs
we may need more information as just saying it hunts is not much to go on

After resetting float level, now no fuel in breather. pilot now adjusting with 35 jet 2 turns out. Main jet 250, 159 P 0 needle jet 6DH3 needle middle position ,2.0 Air jet , 2,5 slide. Hunts when slowing down (rich) pulling up hill better when closing to 3/4 throttle. When running very rich pulled to 90 plus. No perceivable air leaks. I was going to try a 3 side and raising the needle one groove. Slide for weaker lower down, needle raise for richer 3/4 up.

gripper's friend, Mr Angry

cwinsby said:

After resetting float level, now no fuel in breather. pilot now adjusting with 35 jet 2 turns out. Main jet 250, 159 P 0 needle jet 6DH3 needle middle position ,2.0 Air jet , 2,5 slide. Hunts when slowing down (rich) pulling up hill better when closing to 3/4 throttle. When running very rich pulled to 90 plus. No perceivable air leaks. I was going to try a 3 side and raising the needle one groove. Slide for weaker lower down, needle raise for richer 3/4 up.

Click to expand...

The #35 pilot is a good choice, but with the air screw out 2 turns, I'd say your on the lean side. I suggest that you try 1 1/2 turns out and adjust the speed screw to bring the idle speed back; start the interplay there, when you change one, you'll need to change the other.

The 250 man is too rich try a 240, maybe even a 230 unless you get a lean surge, then back to a 240.

The 2.0 air correction jet is a good choice.

The idle jet (35), needle jet (159 P-0) and the needle (6DH3) work well with a 2.5 slide.

What type of ignition system does your motorcycle utilize?