Mikuni vm34 carb tuning help

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So setting up a mikuni vm34 on my 74 850. Setup as delivered with pilot 40, main 240 , slide 2.5, needle position middle. Started off with warmed bike and set idle as slow as it would idle and with air/fuel at 1 1/2 turns out. So at 1 turn out the bike will stall, Idle will pick up at 3 turns out and then no difference the rest of way out. Shouldn’t the bike idle stumble again somewhere after 3 turns out? I should be somewhere between 1 and 3 turns total from what I’ve seen online. From what I can tell I would need to go to a smaller pilot jet to add more air and less fuel. Does this sound right? By the way bike runs great with no dead or flat spots. Thanks for any help or advice.
 
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What needle do you have? A 6dh3 with clip in position 3, middle groove, is a good starting point.
The rest of the jetting also a starting point.

Check the full advanced timing while slowly opening the throttle. Keep opening the throttle until it stops advancing. If you have points and mechanical advance unit I suggest you remove it give it a good cleaning, check the springs to see if they completely retract the bob weighs.


A 159 0-8 a 230 main a 35 pilot can be tried, but do a lean surg test any time you lean the mixture,

Best
 
if it idles better at 3 turns out it appears the 40 pilot is admitting too much fuel. try a 35 as suggested or even a 30. one of these will likely give you the best idle within a half-turn of 1.5 out, which is the goal.
 
if it idles better at 3 turns out it appears the 40 pilot is admitting too much fuel. try a 35 as suggested or even a 30. one of these will likely give you the best idle within a half-turn of 1.5 out, which is the goal.
Yea that’s what I thought also. Appreciate all the help.
 
There is an interplay between the two adjustments. You want the idle mixture a bit rich because air can enter the game faster than fuel this aids in a, or should, seamless transition from the idle circuit to the needle coming to bat.

A 35 pilot jet should be tried if a 40 doesn't provide such a transition.

Don't try a plug check if the engine is allowed to idle, the plugs will always appear black.

Best.
 
My Mikuni has plagued me for years...... mostly my own fault for having other bikes to ride. But recently I broke down and decided to resolve it once and for all......

My bike acted, rode and sounded like it was too lean, but the plugs were always black and sooty. My bike came with a 2.5 slide, 35 pilot and 260 main.... I bought a wide selection of pilot and main jets.... went to a 30 pilot and 230 main and it has been flawless since. To be as honest as I can, I found an air leak where the rubber boot mounts to the intake, I suspect that has been 90% of the problem all along. Don't be like me. The Mikuni is not exciting, but the bike is now like a sewing machine. $0.02
 
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