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Selecting main jet first for VM carb tuning?

niclpnut

niclpnut

Forum Mentor
Past Site Supporter
I've tuned my 1100's following the factory pro tuning guide selecting the main jet first and working progressively back to the needle then pilot circuit.

Does VM tuning follow the same principle?

I'm working on my 78 1000 with k&n pods and 4-1 pipe with a stage 3 kit. All mainainance has been performed including valve adjustments and a thorough carb cleaning. Upgrades include accel coils and dyna s. Timing is spot on.

I seem to be chaising my tail some with the needle transition.

I originally had it set up with 15 pilots, 1.5 turns on fuel screws, 1.5-2 out on air screws and needle on second clip from bottom.

Bike ran well but rich on transition, with black plugs, and a "burble" at cruise speeds and 1/8 throtle.

If I raise the needle will I also need to turn the fuel screws out some as well?

The idle mixture looked great just putting around and while syncing, with nice tan insulators.

since the idle mix was spot on I wasn't sure if I would need to change the setting with an adjustment to the needles.


Thanks,

Nic
 
VM require tuning from the pilot up as all circuits are metered seperately by their own jet. On CV carbs, the pilot is fed through a port in the tube the needle jet and main sit in, basically fed by the main jet which is why you tune the main first.
 
+1, the main jet has zero effect below about 3/4 throttle, you can even tune the low speed circuits with no main jets installed, it makes no difference.
 
Thanks for that.

Does the fuel mixture screw setting influence needle transition or is that a separate tuning function for idle mixture?

Guess what I mean is how much cross over is there from pilot to needle?

Nic
 
It in conjunction with the air screw adjusts the mixture only at the lowest throttle openings, like zero to 1/8 throttle or less. Start with the fuel screw 3/4 or 1 turn, the air screw twice as far open. Close the air screws a little at a time until the throttle response at the very tiniest openings starts to get a little bit bad, then go back in until it's smooth again. I go in 1/8 turn increments, turning all four screws at the same time, keep them all four at the same number of turns out. What I do is ride around slowly in first or second gear and go from closed throttle to just the slightest opening I can do, you want it as lean as possible without any missing or hesitation. Then fine tune the airscrews using the highest idle method.
Go on to tuning the needles, and eventually the main jets.

Mikuni has a lot more info on line someplace, if you search a while you will find it.
 
Thanks again. Definately simplifies things being able to start with the pilot circuit.

Guess I'm getting hung up with my CV carb mind set of trying to get the needle right before the pilot ciruit.
 
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