Is this really going to be my first post?

[dshir61]

Sadly, yes...

I was hoping my first post would involve how much fun I had out on my "new" '78 GS750, or posting pictures of it after I started cleaning it up. However I have not been afforded the luxury of any prolonged rides due to a rather annoying issue.

When I got the bike I noticed it had been running rich. Not much of an issue. Dipped and rebuilt the carbs, everything was looking peachy. Went to install them on the bike and came to the rather head scratching realization that there is no vacuum port on the carbs or the manifold... No wonder it was running rich the PO had been running the original vacuum petcock on Prime, and in my naivety overlooked a rather glaring issue.

My question is: what is my best course of action? Do I drill out a new vacuum port and stick with the stock petcock? Or do I just go manual and be done with it? Its rather frustrating because I have this awesome little driver and I am really reluctant to take it out until I find a fix...

Thanks all for your input!

Derek

[allojohn]

First off, welcome to the forum Derek glad to have you.

Can you post a photo or two of the carbs? Lets see what we're dealing with here.

[dshir61]

Will do. I will run home on my break and snap some real quick. Should be up within the hour

[Seana23]

Hi babes..
What carbs do you have that don't have a vacume port?
How did the PO ever get it to run right?

We really need pics.ov this annomoly...
Thanx.
Xxx

[MisterCinders]

You are mixing terms here I think.

Your petcock has a vacuum port that connects to the No. 3 carb to provide suction that triggers the fuel flow.

The 1978 GS750 has vacuum ports on the head next to each intake. There should be some allen head screws in them that you remove to connect a vacuum synch gauge for synching the carbs. Other bikes have these ports on the intake boots instead of the head.

If your head does not have these ports, then some PO must have swapped heads with a different model that requires intake boots that have these ports.

The lack of synch ports is not catastrophic, but you won't be able to do more than a bench synch on the carbs, which kind of sucks. Regardless, the synch ports have nothing to do with running the petcock on prime. They also have nothing to do with your richness issue. Open or poorly sealed synch ports will cause an air-leak and lean condition, but that's it.

HTH

[dshir61]

Here are the pictures. Sorry they are not the best. I remembered I had taken some for the sake of before and afters, so this is as they were before being dipped.

And I apologize for mixing up terms. I knew there were sync ports on the heads themselves, and they are still there. I had just seen a picture somewhere of a carb intake, or mounting plate?, with a vacuum port for the petcock (on the backside nearest to the heads).

As far as I understood it the vacuum port (to the petcock) was supposed to be facing forward on carb #3. Where it is supposed to be (as far as the pictures I have seen) is just solid carb body.

[MisterCinders]

That is bizarre. Your No. 3 carb body looks like a No. 1 carb. I assume you are still getting fuel in the bowls on 3 and 4.

There should be a nipple on the No. 3 carb that looks like the breather point on 2 and 4. Yours is sealed off like a No. 1 carb.

If the PO shuffled in an extra No. 1 carb body, you would only run on 2 cylinders, since no fuel would enter the right hand carbs. So if you have fuel flow, then you have a mutant No. 3 carb, or some PO tried to jury rig a No. 1 body by drilling it out? Not even sure that is possible. Neither situation is good.

Until you can rustle up a replacement carb, you will be forced to use the PRI setting to run the bike, and will need to turn it off (set it to ON) when you park. With no vacuum, "ON" = "OFF." It's like 1984, ON is OFF, LOVE is HATE, PEACE is WAR. :P

BTW - your intake boots look pretty shot. That wiring above the head needs some insulation. The pod on No. 4 is ragged for some reason.

[Dogma]

Could it be that this rack of carbs was lifted from some other bike that didn't use a vacuum petcock? If so, the foreign jetting could explain the rich mixture.

[allojohn]

That is weird and I can't think of anything which would lead someone to make that mod. From time to time you do see strange fixes on here but normally you can see the reason.

There's an interesting story here somewhere.....

As MisterCinders suggests, look for a replacement #3 carb body.

[MisterCinders]

I have some spare carb bodies from a rack that I raided for parts. IIRC, I bogarted the no. 3 body when I broke a pilot fuel screw in one of my carbs. If not, I can send it your way. PM me a reminder to check my bits bin.

[dshir61]

Thanks all for the replies! I knew I could get some insight lol. I knew something wasn't quite right, just figured I would ask those who knew more than I and get the affirmation. Oddly it seems like both 3 and 4 are getting fuel, so there may be something strange afoot that I missed during the tear down (it was my first attempt at this kind of a build, but not to dissimilar form the old car carbs I used to work on).

As soon as I saw the intake boots I knew I had to replace them. Even though the rubber was still soft I went ahead and put new ones on any way. And the wiring makes me weep tears of pain. There are definitely some screwy fixes on this bike, but it was a good price and I knew I wanted something I could put some time into either way. (And the random different pod on #4 confused me a bit, hopefully i can find a single K&N one to match, lol).

And MisterCinders, thanks for the offer. I'll send a PM your way for sure

[niclpnut]

Are those 29 smooth bores?

Look at the bottom of the bowl. Looks like a large drain bolt, and not a drain plug on the side with a nipple like stock VM 26's.

You might have a good score.


Nic

[niclpnut]

Look at these 29's...


No vacuum port on 3

[dshir61]

Are those 29 smooth bores?

Look at the bottom of the bowl. Looks like a large drain bolt, and not a drain plug on the side with a nipple like stock VM 26's.

You might have a good score.


Nic

I just checked them when I got home and they look exactly as you describe them. I'll yank the pods off to double check, but seems to match your description to a "t"

[niclpnut]

Probably Over doing it on the 750...hence the rich condition, but good score none the less!

Oh and DONT drill out anything on those

Nic