77 GS750 Why do I have 3 vacuum ports?

[ddominik]

I have vacuum ports on carbs (from left when seated) #1 #2 & #3. I just got this beast and rebuild the carbs (leakers) but can't figure out the vacuum hose connections.
I have a shop manual on the way but the threads (yup did a search first) on hose connections in the manuals is not encouraging.

Thanks
Dom
Bloomington Springs, TN

[onchiman]

Only #2 is vacuum. The other two are vents.

[t3rmin]

Where are the ports exactly? Like onchiman said, you should only have one vacuum port (on #2) and it'll be on the motor side of the carb. The vent ports are between #1-#2 and #2-#3 in the middle of the carb bodies on a T fitting. At least that's for CV carbs, may be different for your VMs...?

[mark]

Only #2 is vacuum. The other two are vents.

This is incorrect, at least if the numbers correspond to the carb number. A '77 has VM carbs and the vacuum port is on the #3 carb. #2 and #4 vent to air. The #1 carb does not have a port.

[ddominik]

(forhead slap) of course I knew that!!!!

Once I started the bike I could feel the vacuum on # 2

Thanks - guess I've been working on too many singles.

Dom

[ptm]

This pic of VM carbs from a '78 GS750 is looking at the back of the carbs (#1 on the left and #4 on the right)

#1 has nothing
#2 has a vent
#3 has the vacuum that goes to the petcock
#4 has a vent

[earlfor]

Mark is correct. The vacuum port on 77 through 79 VM carb's models is on the #3 carb. 2 and 4 are vents and left open.

The vacuum fitting on CV carbs is on the #2 carb, hence the confusion.

E.

This is incorrect, at least if the numbers correspond to the carb number. A '77 has VM carbs and the vacuum port is on the #3 carb. #2 and #4 vent to air. The #1 carb does not have a port.

[ddominik]

Vacuum hose .... which one is it????

Paul

Perfect picture .... even I can't screw this up!

Thanks again all

Dom

[ddominik]

Now .... where does this vacuum hose go?

Actually I know it goes to the fuel cock.

My new challenge:

I rebuild the fuel cock but can't get it to stop leaking. ....tried several orientations of the diaphram vent hole .... still leaking. I'm ready to cap off the vacuum manifold on the fuel cock.

Anyone else have this problem? Solutions?

maybe a photo of the correct stack up .... Paul?????

Thanks for listening

Dom

[earlfor]

Since the petcock leaks, capping off the vacuum will only defeat the run function of the petcock. It will still leak when shut off or not running, but without a vacuum to operate it, it will not open either. Buy a new petcock or get it fixed as it should be. If it leaks, making it not work at all isnt an improvement.

E.

Actually I know it goes to the fuel cock.

My new challenge:

I rebuild the fuel cock but can't get it to stop leaking. ....tried several orientations of the diaphram vent hole .... still leaking. I'm ready to cap off the vacuum manifold on the fuel cock.

Anyone else have this problem? Solutions?

maybe a photo of the correct stack up .... Paul?????

Thanks for listening

Dom

[schatenjager]

petcock

http://www.thegsresources.com/_forum/showthread.php?t=108441

You have to put the square plastic spacer between the two rubber "sheets" of the diaphragm, with the pinhole lined up on the spacer, one of the sheets (the other doesn't have one) and the hole in the metal petcock casing...

I'd take a picture of this thing but it's finally back together and working and I can't bear to take it back apart...lol

[jhnbrackett]

So does that mean 77-79 Carbs can't be synced? Y'all got me totally confused after reading some of the porst are vent & some are vacuum.

[schatenjager]

My understanding is that you sync the carbs from different ports than the vacuum/vent lines. A mechanic I know told me that there are four screws (one on each carb) that are used for synchronizing, not the vent/vacuum ports.

[ptm]

So does that mean 77-79 Carbs can't be synced? Y'all got me totally confused after reading some of the porst are vent & some are vacuum.

The ports on #2 and #4 are used to vent. The port on #3 is used to control the petcock - it ensures that fuel flows only when there is vacuum (the engine is running).

The sync gauge hoses are attached to ports that are on the side of the intake pipes that stick out from the head where the carbs attach. Each intake pipe has a small Allen-head cap screw which you take out and replace with the adapters for the sync gauge. Then attach the hoses from the gauge to the adapters.

[ptm]

Here's a pic from the factory service manual. The arrow points to an adapter on the #4 cylinder.