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Vacuum Sync on VM Carbs?

  • Thread starter Thread starter gmoore
  • Start date Start date
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gmoore

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I'm looking for a guide on how to vacuum sync the VM carbs on my 1979 GS 750? seems as though the majority of info I've found is related to CV type carbs where as the VM's are slide.

I've never done this before so I'm not sure of what hoses go where. pics with instructions would be ideal!

Also, if anyone knows where there is a "how to build a Manometer" thread/link that would be of interest as well.

Thanks!
 
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I can't help you with synching your carbs as I have CV kind but I will recommend you don't waste your time building a manometer. There are too many issues with man-made ones. You'll save yourself time, money, and aggravation if you get something like a Morgan Carbtune.
 
The dials will work. they're harder to read, and bounce a bit. Even the liquid tube style will bounce some without restricters. The best tool I have found is the Morgan carbtune. I owned one years ago, and have used different styles. (even home made) Nothing has been more effective than the Morgan.
 
I have had the pleasure of trying just about every commercially-available system there is to synch the carbs.

I still have, and use, my mercury sticks, but a Morgan Carbtune might be in my future, when funds become available.

It's hard to justify the expense when you already own the standard against which all the others are calibrated (but is generally not available any more).

If I currently had no gauge set, I would definitely go for the Carbtune.

.
 
So is there a how to guide on these slide style VM carbs kicking around here somewhere?

Where would I plug the vacuum hoses into on these carbs? & what screw would I turn to adjust them?

Sorry, I have never done this before.
 
There's a screw on the side of each intake manifold - remove it and screw in the hose adapter

Take off the top of the carbs (3 screws) and there's a screw and locknut. You loosen the nut and turn the screw to adjust the sync

Due to space restriction, some one invented a multitool long ago

http://www.motionpro.com/motorcycle/partno/08-0022/

a combination socket wrench/screwdriver

While I can understand the desire to save money, a good quality sync tool is not only easier to use, you don't have to calibrate it every time like dial gauges

I'm a dedicated Morgan user. Buy the little pack it fits in also
 
You can run the bike w/o the carb tops?

I also don't see where you would be able to put in a hose adapter w/o removing the pod filters. wouldn't that cause the bike to run entirely differently? are you talking about the actual air mixture screw?

I apologize if these seem like dumb questions.
 
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gmoore said:
So is there a how to guide on these slide style VM carbs kicking around here somewhere?

Where would I plug the vacuum hoses into on these carbs? & what screw would I turn to adjust them?
Click to expand...

Big T said:
There's a screw on the side of each intake manifold - remove it and screw in the hose adapter

Take off the top of the carbs (3 screws) and there's a screw and locknut. You loosen the nut and turn the screw to adjust the sync

Due to space restriction, some one invented a multitool long ago

http://www.motionpro.com/motorcycle/partno/08-0022/

a combination socket wrench/screwdriver

While I can understand the desire to save money, a good quality sync tool is not only easier to use, you don't have to calibrate it every time like dial gauges

I'm a dedicated Morgan user. Buy the little pack it fits in also
Click to expand...
Since you say you have seen the tutorials for the BS-series carbs, you should at least have seen where the gauges get connected to the bike. There are plugs in the intake boots for the VM carbs, just like there are for the BS carbs. That part of the process is unchanged.

As Big T mentions, remove the cap of each carb so you can access the locknut and adjuster. Unlike the BS-series carbs, there is no "master" carb from which to reference the others. I would suggest starting with the carb that has the lowest vacuum reading and adjusting it to get closer to the one with the highest, repeat as necessary. Note that as you make your second adjustment, the readings of the first two carbs that you balanced just a bit ago, will change. You will probably end up making several adjustments, not just three, before you have satisfactory results. Seeing how making one adjustment actually affects all four vacuum readings will quickly convince you why you need to have a gauge set that shows all four readings at the same time.

The manual suggests doing this adjustment between 1500 and 2000 RPM, but I had done countless number of carb syncs before reading that, so I still do mine at about 1100-1200 RPM. Keep a fan or two blowing on the engine to keep it from overheating, as this will likely take you several minutes to accomplish.

.
 
Here's an example

Here's an example

gmoore said:
You can run the bike w/o the carb tops?

I also don't see where you would be able to put in a hose adapter w/o removing the pod filters. wouldn't that cause the bike to run entirely differently? are you talking about the actual air mixture screw?

I apologize if these seem like dumb questions.
Click to expand...
picture.php


Bryan
 
Hi,

Have a look at these carb sync guides to get an idea:

Synchronize Carburetors
(With the Morgan Carbtune)

Synchronize Carburetors
(on bwringer's site)

The sync ports (where you connect the vacuum lines from the carb sync tool) will either be in the intake boots or the intake ports on the head, not the carbs. Take off the top of the VM carbs. The vacuum adjustment is on the top of the slides. You must adjust all 4 VM carbs. (The CV carbs use #3 as the "master" carb, it has no adjustment).

Oops, I see I'm late with this response. Mr. btbarb and Mr. Steve type quicker than I do. ;)

This is my fan setup.

carbsync-2fans.jpg



Thank you for your indulgence,

BassCliff
​
 
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OK, I see where the adapter would go now, it goes into the actual head right after the intake boots. It's a little allen bolt where the adapter for the carb sync Manometer screws into it.

I'm still confused that you need to have the carb tops open while syncing them, seems like it would make the bike run differently? I must be wrong though, there seems to be no other way.

Thanks!

haha, I wrote this before I saw the other responses, the reason I was having a hard time wrapping my brain around it was because I have the carbs on my workbench and I wasn't seeing anything that a vacuum hose could hook up to.

Thanks for all your responses! you guys are great!
 
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It will run fine without the tops installed.. You need them off to make the adjustment... IF you are going to spend the money ?? GET the morgan carb tune. The vacuum gauge setup is difficult to use !!!! as the needles bounce all over the place ..
 
SqDancerLynn1 said:
The vacuum gauge setup is difficult to use !!!! as the needles bounce all over the place ..
Click to expand...

If this happens you just dampen them down. It can be a little tricky to get them right down without stopping the vacuum altogether but they work fine. They are exactly the same as the Kawasaki factory tool of the day.
 
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