Yet another petcock question

[BoardBandit]

I have recently purchased a 1979 GS750L with a mere 7,000 miles on the clock. Clearly, she's sat a while. I am slowly making my way through the systems but I admit I'm stumped by the petcock/carb area. In disassembling the petcock, there is clearly something that looks like a vacuum diaphragm on the back inside. The hose from it goes to a nipple on the fuel gauge sending unit cover, vented to open air. So it is doing nothing in terms of opening a vacuum diaphragm. In the petcock, there is an o-ring that would normally be present if it were intended to shut off fuel, but there is no spring to hold it closed. Which is fine, since there is no vacuum to pull it open. On the front of the petcock, there are the typical 3 positions: on, reserve, and an unmarked position that clearly shuts off the fuel, so no "prime" setting. There is no open vacuum port on the carburetor.
Does this mean it is NOT a vacuum petcock and if so, why the diaphragm and o-ring inside? Am I supposed to shut the fuel off after riding, since I can't expect the float needles to hold back the fuel?
I assume everything is stock; nothing looks out of place or cobbled together. I have searched the forum and can find nothing related to this. Any insight would be appreciated. Thanks!

[Big T]

You'll have to post up a picture of your petcock for review.

The vacuum line should connect to carb #3

So, the previous owner was a hack, be prepared for more surprises

[tom203]

Oh look, someone in my neighborhood! Yes post a pic. The fitting under the fuel sending unit just goes to a drain hose in case the unit leaks-
The stock layout is like this fiche

https://www.partzilla.com/catalog/suzuki/motorcycle/1979/gs750l/fuel-tank

[BoardBandit]

Hey Tom. Yes, looks like I'm just down the road. lol
Yes, I realize the sending unit fitting should just be a drain. I get that. What is puzzling is that the petcock has no prime feature. If it is a vacuum petcock, why is there only on, reserve, and off? And all 4 carb boots, have screws plugging the vacuum ports. All screws are identical so it doesn't look like someone blocked off the one that should do to the petcock. It's all truly baffling. There does not appear to be any empty port anywhere else to plug into. I'll try to get some pics posted so you can all see.
These carbs are something. 5 out of the 8 air ports on the intake side were blocked with insects/nests.
And just so there's no confusion; I am not a novice, just new to these bikes.

[Steve]

There should be a spring to push the diaphragm and o-ring into the port to close off fuel flow.

The smaller nipple on the petcock is a vacuum port. It should be connected to a similar port on carb #3. Note that carbs are numbered from left to right, as you sit on the bike. Carb #1 is under your clutch hand, carb #4 is under your throttle hand.

The nipple on the drain for the fuel sending unit should run back, over the airbox and down to below the swingarm pivot, it is simply vented to atmosphere.

A stock petcock (which you may have) does have three positions. With the lever pointed forward, it is in REServe. With the lever pointed down, it is in RUN. With the lever pointed back, it is in PRIme. There is no position labeled "OFF". When the petcock is in RUN or REServe and the engine is not running, the spring that should be inside the petcock will shut off fuel flow.

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[BoardBandit]

Thank you all for your replies and I don't mean to sound ungrateful but I know exactly how vacuum petcocks work. What I don't understand is why/how this one is the way it is: it does not make sense. Someone has removed the spring. But with a spring in place, there is no prime position, so no way to prime the carbs or drain the tank. I have disassemble the petcock and there in no configuration that would allow fuel to flow without vacuum if a spring were in place.

[Steve]

The PRIme function is accomplished with a curved spring behind the lever. When the lever is turned to the PRIme position, a block on the back pushes a tab on the spring against the pin on the diaphragm, allowing fuel flow. It is not readily obvious and takes a bit of examination to understand the function. The actual lever has a couple of pins on its back side that engage a plastic block. That block has the holes that select which fuel port is engaged. The back of that plastic block is what has the tab (projection) that pushes on a flat, curved spring that should be in the back of the cavity in the petcock body. The spring spirals around the circumference to hold it in place, and has a pin in the center that projects toward the diaphragm. The tab on the plastic selector block will push that pin against the diaphragm to un-seat the o-ring to allow fuel flow.

We are not belittling you by detailing how the petcock works. As of right now, you have three posts to your credit, so we have absolutely no idea about your background and experience. Also consider that someone else just might read this that doesn't know how it works, and it will educate them in the process. I know quite a bit about these bikes, but I don't know everything about every model, so even I learn something on this forum almost every day.

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[GSXR7ED]

I was interested in this subject so I read it...

It's always entertaining to read people's reaction to their questions.

Ed

[BoardBandit]

Steve, Thank you for that explanation. I believe it is starting to make sense now. It looks as though more pieces have been removed from the petcock than I had originally thought. I will have a good look at it all when I get home from work.
And I am sorry if my frustration comes across as harsh. Lack of knowledge frustrates me and when I go seeking further knowledge but come back with everything I already know but very little new information....well... Not your fault, I know. But frustrating nonetheless.
So now my question is: where does the vacuum line go? I know it has been said, "carb #3" but where? There is nothing on the carb body itself. If it goes in the vacuum port on the head ( I know earlier I said "carb boots". That was wrong. The ports are on the head) how does it attach? There are threaded holes, each blocked with a screw.

[pdqford]

I stopped by for an education.
I always get one when on this site.

The “vacuum port”s on the head/boots is prolly there for the carb sync.

[Baatfam]

Don't know if this helps, as the vacuum port on my '83 GS1100E is on carb #2, and I'm pretty certain, your carbs are different than mine, but I grabbed a photo anyway.
I was thinking it should be located on yours, somewhat the same way, and might point you in the right direction.
Not using it on my bike, (see Pingel), and it is under the little yellow cap, near the center of the photo.
(Guess I could've taken the cap off for the photo )

[BoardBandit]

OK, OK, I think I may have this figured out. I watched a video on Youtube and looked at a couple Google Images and.....what I have on my bike are 3 lines going to the carbs. I had (mistakenly) assumed they were all vent lines. I now feel there are 2 vent lines and the 3rd is the vacuum line. I will confirm this evening. Thank you all for helping point me in the right direction. It is so frustrating dealing with a bike that you have never seen run, most parts are there and seem to be hooked up to things but logistically i

[BoardBandit]

Baatfam, thanks but I think I have it now. I also see you are a Triumph fan. I had a 2001 Trophy 1200 and my current ride is a 2013 Trophy. I just sold the Sprint GT.

[BigD_83]

The PRIme function is accomplished with a curved spring behind the lever. When the lever is turned to the PRIme position, a block on the back pushes a tab on the spring against the pin on the diaphragm, allowing fuel flow. It is not readily obvious and takes a bit of examination to understand the function. The actual lever has a couple of pins on its back side that engage a plastic block. That block has the holes that select which fuel port is engaged. The back of that plastic block is what has the tab (projection) that pushes on a flat, curved spring that should be in the back of the cavity in the petcock body. The spring spirals around the circumference to hold it in place, and has a pin in the center that projects toward the diaphragm. The tab on the plastic selector block will push that pin against the diaphragm to un-seat the o-ring to allow fuel flow.

That is the best written explanation of the design I have ever seen. Thanks, Steve. Oh yeah, and it doesn't take much mis-shaping of the spring to bugger up the movement of the petcock handle, either.

[Steve]

... what I have on my bike are 3 lines going to the carbs. I had (mistakenly) assumed they were all vent lines. I now feel there are 2 vent lines and the 3rd is the vacuum line.

That would be correct. The lines on carbs 2 and 4 are vent lines, the one on carb 3 is the vacuum port. Identical locations, different functions.

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