Two right-handed Mikuni VM34 carbs

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Scotchmist

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Hi all,

I've been offered a pair of brand new Mikuni VM34 carbs that seem to be well jetted for an XS650 in standard trim. The price is good, but they appear to both be right-hand carbs. From what I can see the only difference between RH and LH Mikunis is the placement of the idle/slide screw. I'm wondering if anyone knows any other differences between them. By my reckoning, the only issued would be access to the inner idle screw on one of the carbs during initial syncing.
 
For the right/left Mikunis, left and right slides are different, mirror images of each other. Guide slots and idle screw ramps on opposite sides. Avoid intermixing...

http://www.xs650.com/forum/showthread.php?t=25079

Attached is a VM parts listing. See the two different slide/valve part numbers for the VM34s.
 

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Yes a pair of matched rights will work fine. The inner idle speed screw CAN be turned, it's just a bit fiddly.

If they don't have cable chokes, one choke is a bit hard to get at also. I've found one choke is all that's needed to fire it up.
 
You can get a pull rod choke for that one side. You do need to sync at idle, then match the cable freeplay for sync at speed. Syncing multiple individually cabled carbs is actually a two step operation - match and sync idle speeds, then match and sync speeds above idle once the throttle is opened and the slides are lifted some.
 
You're doing it backwards and your carbs will not be perfectly synced that way, ever. The idle speed adjusting screw on the VMs lifts or lowers the slide a little and that changes the idle speed on that carb. When you change the slide height, you change the amount of freeplay in the cable. That changes the sync at speed between the carbs, the sync you've been setting first. The way you've been doing it, you may have them synced at speed but you throw that off if you then mess with the idle speed screws. This also means that if you ever need to make an idle speed adjustment in the future, you must tweak each carb's idle speed screw the same amount in order to maintain sync (matched cable freeplay). If you adjust only one carb, you'll throw it off.
 
There's three types of sync, in two zones.

Mechanical sync - When the physical positions of the valving matches
Vacuum sync - To accomodate differences in induction.
Run sync - To accomodate differences in cylinder power.

Zone 1 - Idle
Zone 2 - Cruise throttle position.

Mechanical sync is done at the bench, as a starting point. If all is well with the world, and the engine is in good shape, this may suffice.

Vacuum sync is done on the running bike. If all is well with the world, and the engine is in good shape, this may suffice.

Run sync is also done on the bike, but with the goal of achieving smooth operation on worn engines, especially noisy early 4-bangers, and to sometimes to appease difficult customers.

Zone 1 is done first, followed by zone 2.
 
I understand what you're saying, I just don't agree with the method you use. Idle sync is always done first. If the motor isn't 100% healthy (and what motor is), you will most likely end up with idle settings that have the slides slightly mismatched to achieve matched RPMs. Then you can proceed to match the cable freeplays and get your "at speed" sync. Using your method, you may have "at speed" sync and matched slide heights at idle, but the idle RPMs may not match. Match the idle RPMs and the slide heights may not match. If the idle slide heights don't match, your previously set "at speed" sync will now be off.

I've been syncing carbs this way for many, many years. It works and I'm not going to change the way I do it. You've been doing it your way for a long time as well so I doubt you will change either. As 2many said, in a perfect world with a perfect motor, your method might work. In the real world, not so much. If anybody should be selling tools, it's you, lol.
 
5T, since we're on the subject of VM's here, just wondering your opinion of running a single carb conversion, or anyone else's opinion for the matter, lol.
 
I really don't go for that too much. I feel it's a step backwards in terms of performance. A 2-1 carb intake works best if both runners are the same length. That's very difficult to accomplish on the XS. The carb has to angle out the side because of the frame backbone directly behind the engine. Yes, one carb is easier to adjust and there is no carb syncing to worry about, but a dual carb set-up really isn't very complicated or hard to learn how to tune.
 
solo2racr said:
... it DOES NOT involve idle screws...
Click to expand...

What I've found here is that the senior members have been using a terminology that tries to clear-up some of the confusion caused by some misprints and errors found in some of the old manuals of the '70s. So, to avoid confusion:

The 'idle' screw refers to the 'idle stop' screw.
The 'mix' screw refers to the 'idle fuel delivery screw', even though it doesn't control air/fuel mix ratios on the XS650 BS series carbs...
 
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figure8 said:
5T, since we're on the subject of VM's here, just wondering your opinion of running a single carb conversion, or anyone else's opinion for the matter, lol.
Click to expand...

Hi fig8,
most folks use an asymmetric manifold to avoid the frame tube and that's gotta eff up the carburation.
A symmetrical manifold and a single carb WILL fit but it brings the carb intake far too close to the frame tube to fit an intake bell on it, let alone an air filter.
Same deal as on my 1953 plunger frame BSA Gold Flash.
What BSA did was to let a forged inlet into their frame tube that allowed a rubber sleeve through it and connected to an air cleaner behind the frame tube.
Do the same with the XS650 frame tube except all you need is a short length of steel tube welded in to make the eye and a Britbike style pancake air filter behind it.
 
Just like a v8. run a tee from the exhaust under the manifold at some point to keep any puddled fuel boiled off.
 
I can tell you from experience that a single carb just doesn't run properly. I have used both manifolds that are available to us xsers. With a keihin cvk34 it wouldn't run at all. With a single vm34 it ran well anywhere but idle. Idle was rougher and had some stalling issues. Pretty sure the long side of the intake had fuel pooling as it blew the carb off once. . It really just doesn't work worth a shit unless both intake tubes are short and equal. If I could do it that way and run it....I would gladly run a single vm34.

I don't believe the power loss from a single carb is anything big though. Triumph sure sold alot of tigers and bsa thunderbolts...
 
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