Bench syncing VM carb

[Guest]

I'm putting my 79 GS850G back together. I'm having troubles bench synching. Can anyone give me a detailed description of how to do it. I followed the VM rebuild pdf. file from BikeCliffs site but the slides just drop all the way to the bottom to begin with. I am wondering if I am supposed to have the large spring on or not to do the bench sync? helpppp............thanks

 

[tkent02]

With the idle screw under the rack loosened, turn the throttle screw on top of each slide until there is just the slightest crack of light visible under each slide when it is down all the way, set the locknut lightly. The idle screw under the rack will bring all of the slides up together after that. This should be close enough to run. There is an adjustment in the manual for how high the slides go at wide open too.

 

[SqDancerLynn1]

To go one step further. Back out the idle screw so it doesnt touch. Then adjust the #3 carb from the linkage on the top of the #3 carb to where it just fully closes. then adjust the idle screw up until you can slide a 1/16" dril bit under the #3 slide from the intake side. Then adjust the other three to match. Will be very close

 

[hampshirehog]

Another way that I use is to measure from the top of the carb (tops off) to the top of the slide - same place on each carb - using a caliper accurate to 100th of a mm.

I've only started using this technique recently so it's only tried and tested on around a half a dozen sets of carbs but I've been surprised how accurate setting up using this method appears, with only the deftest tweak required on a gauge synch i.e. they've really been good enough to not need the gauge synch.

I'd be interested to know if this is just luck or whether Mikuni really did have good tolerances on their carb bodies; anyone else measured like this?

 

[ryonker]

Wow, nice idea!

I found on mine (even though I have a sync tool) the engine ran great after bench syncing the visual way.

 

[49er]

Wow, nice idea!

I found on mine (even though I have a sync tool) the engine ran great after bench syncing the visual way.

I agree with you. I didn't vacuum sync the VM's on my 850 the last time I had them off because I found that on previous occasions, the bench sync was accurate and required mininal or no fine tuning.

I do think that you would need a vacuum sync when fine tuning CV carbs though!

 

[tomo]

A tried and tested trick I learned a few years back, is to get four lengths of metal rod, welding rod for example. Fit these under the carb slides. Turn the carbs so the rods are pointing down then use the throttle linkage to very slowly raise the carbs while observing the rods. At first you`ll probably find that one or two slide out first. Adjust the linkage of each carb slide accordingly. Ideally, you want to get em set so that all the rods slide out at the same time. This has worked well for me for bench synch, but obviously once on the bike synch the carbs up, preferably with a Carbtune. Have fun !.

 

[alke46]

Another way that I use is to measure from the top of the carb (tops off) to the top of the slide - same place on each carb - using a caliper accurate to 100th of a mm.

I've only started using this technique recently so it's only tried and tested on around a half a dozen sets of carbs but I've been surprised how accurate setting up using this method appears, with only the deftest tweak required on a gauge synch i.e. they've really been good enough to not need the gauge synch.

I'd be interested to know if this is just luck or whether Mikuni really did have good tolerances on their carb bodies; anyone else measured like this?

I hope this works as I too am going to use this method. Question is though, how much distance to the tops if the bottoms are off a bit?

 

[hampshirehog]

The distance to the top of the slides doesn't matter as long as it's the same on all the carbs. Just so long as the slides have a bit of lift and aren't bottoming out on the body of the carb you should achieve a good synch.