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Discussion Starter · #1 ·
Hi, I recently purchased a 1972 CB350 that has VM30 carbs installed with unifilters. I was able to start the bike, with the choke off - lever horizontal. When I gave it a little throttle, the motor died. Supposedly, the P.O. states that he had ignition set and carbs synced to idle better.

I’ve been going through all the stickies and old posts to try and learn everything to get the carbs back in order. Does the slide opening seem excessive?
 

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Too many people 'want' VM carbs even though they need a ton of work (and money/parts) The setting given by Cycle World in 1972 don't work with 'modern' fuels, plus, they were more prepared to 'ride around' any problems with pick up.
Modified stock carbs are almost always better for street use
 

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Not sure i would agree with the previous post.

In any case, it's what he has so while editorial commentary is appreciated, doesn't help. i spent the time dialing in my Mikuni carbs and they work fine for me, i'd do it again in a heartbeat. my bike runs on modern gas. any jetting issue won't be with the needle or needle jet, but pilot or main. let's start at the pilot.

it sounds like you need to get back to basics, starting with float level - pull them off and re-check, reset as necessary. reassembly won't affect synchronization if you dont move or damage anything on removal/reassembly. if they're off, don't change anything else before running again to check.


if the float level is correct, then, go up a size, or two on the pilot jets as the bike should take some throttle without dieing - this is to try to caburet up to 1/4 throttle. fwiw, idle screw adjustment is worth about one pilot jet size in its adjustment range. at three full turns, there is no more effect to be gained so change the jet.

check the idle mix screws, set them at 1 turn out only as a starting point (or simply verify they're not turned fully in and seated).
 

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slide opening looks ok.
are these a right/left set or both left-handed? (idle speed adjustment screw is available on the left / outside)
 

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ensure your fuel is fresh within a month and battery well charged.

as noted above, today's fuels are different than yesterday's. they're much more unstable (will not store at all) but the octane rating should be equivalent.
 

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go online to the Mikuni site, they publish a VM series tuning manual (really a booklet).
 

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The pilot adjustment with just air screw is pretty crude compared to carbs with fuel screws discharging a fuel/air emulsion. I have a set here I swapped for stock CB360 carbs (modified to work with K&N filters) I know some had screw in air jets but not the 30mm I have here. I will probably modify them sometime, remove main air jet and tap hole to ~5mmplus tap a thread in pilot air jet so I can fit restrictors. These carbs have 52 pilot, 200 main jet, 6F5 needle and 176-P6 EMULSION tubes (not usual primary discharge nozzles) Slides are 2.5, pretty sure they can be modified to 2.0 or maybe lower? Personally I've never had them on a bike but previous owner never got them adjusted well enough to use bike. I just checked, the air jet is screwed in, size 2.0. Looks like a 4x0.7mm thread so easy enough to make odd sizes for testing. A 145 or 150 main jet will probably be closer to what it needs and around a 30~40 pilot jet. I just haven't had time or inclanation to mess with them
 

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Discussion Starter · #10 ·
Thanks for all the help everyone. I believe the problem was a combination of a not fully charged battery, throttle cable not routed properly, and I was turning the air screw the wrong direction. I’m happy to say that I have the bike running- was able to ride around the block a few times. Thanks again!
 
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