Need help tuning Mikuni VM22

#1
Hey guys,
New member here from Va. I really need some help getting my minibike up and running for the first time since rebuilding.
My build is as follows:
Predator 212 Hemi
-governor removed
-Arc Billet Rod
-Arc Flywheel
-Mod2 cheater cam
-22mm valve springs
-header exhaust
-Mikuni 22mm slide carb with K&n pod filter

Stock Main Jet is 95...replaced with 110 size jet....Got to idle okay. but stall and cuts off immediately once i give it the slightest bit of throttle. I have very little carburetor tuning experience so and advice would be appreciated...

-Would going up in main jet size possibly fix the issue?
-Does gravity feed fuel tank deliver enough fuel?
-Would installing a pulse pump do any good?
 
#5
I know when I got my Chikuni, tuning it was a lot different than a stock carb. Different learning curve.
One screw does idle speed the other is an air bleed so it goes in reverse. Turn in to riches instead of out
 

delray

Well-Known Member
#8
I have run the mod2 cam with 1.3 rockers in a 196cc motor with,billet flywheel,arc .020 rod and flat top piston,milled head small narrow stem ss valves,25mm 24mm.
porting in the bowls into the runners little. with head cc it was just over 11.5 compression. the best I can do is 120 jet with a 15 pilot.
also seems to be the magic number for some other guys that have run this cam. i personally recommend going with the 1.3 ratio rockers with that cam. it looks like your running a torque conveter? you will need to install a higher stall springs and try to go with a larger rear sprocket. those two things will help greatly. that's a heavy bike and it will need it.....:scooter:
 
#10
I tried switching main jets a couple time...sizes 105, 120, 125..nothing seems to make it run better...still wants to cut off immediately giving the slightest throttle...i also switched out my fuel cut off valve and inline filter and it helped the flow immensely. It is a Chinese mikuki and someone wrote the it is opposite of the real mikuni as far as the air screw turns? i know the idle screw is turned out counter clockwise 2 and 3/4 turns on the stock 15 pilot jet. the main jet kit i have doesn't go up to a 130 jet..but i have seen precision drill bits you can buy online to make one bigger. Yes, I have a TAV30 torque converter with a size 40 chain and 50 teeth rear sprocket. The driven pulley has a red spring. Is the green spring the way to go?. Also, the torque convertor belt..seems a bit extra loose? i know its not suppose to be tight at idle, just seems more than it should be.
I attached a few pictures to show how much belt deflection it has.

The idle isnt the smoothest right now..surges a little..attached a video to kinda give you an idea.
 

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#12
Air screws generally work the same way, that is they control the amount of air that gets mixed with the fuel coming from the pilot jet. both the Chikuni and Mikuni airscrews work the same way just the Chikuni is underneath the main body and the real VM mikuni is on the side. There is a separate screw on the Chikuni and Mikuni that sets the slide position. If changing the main jet did not change your problem and as soon as you open the throttle it stalls something else is wrong. Sounds like an air leak or the float height is way off.
 
#19
Just curious, what cable are you guys using with your chikuni's? I just ordered one a couple of days ago, but I don't think it comes with a cable. Seems like I heard of some guys having to modify cables to work? Is it possible to order one that requires no modification?
 
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