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Yet Another Petcock Question...

  • Thread starter Thread starter peterpressure
  • Start date Start date
P

peterpressure

Guest
So when I got my Mut of a GS 750 I had a mechanic look it over durign the winter months and he wrote down, and I quote.

The Fuel shut off is not correct, it needs to be on PRI to run - turn to on or reserve for "off" position.
Click to expand...

Sure enough he was correct, what gives?

I'll go take some pics of the Petcock...
 
peterpressure said:
So when I got my Mut of a GS 750 I had a mechanic look it over durign the winter months and he wrote down, and I quote.



Sure enough he was correct, what gives?

I'll go take some pics of the Petcock...
Click to expand...

Maybe he doesn't realize it's a vacuum operated petcock. If he were bench testing it, gas would run out in the "prime" position but nothing would flow in either "reserve" or "on" position....(unless, of course, it was a 1980 model, in which case it wouldn't have a "reserve" position and gas would flow out of it no matter which position it was in.)
 
Well the problem is, the bike only runs on PRI, I have accidently left it on RES, and after a half mile or so I run out of gas. It has Res on the far left, On in he middle and PRI on the right.
 
Vacuum

Vacuum

Is the vacuum hose hooked up? (I'm assuming the vacuum port is plugged with something.) If it is hooked up, you'll need to get a new petcock...or rebuild the old one; (which might be effective and it might not.)
 
Put it in the "on" position, hook a vacuum hose up to the smaller of the two nipples (heh.. heh.. heh...) and suck on it. If gas flows out the other nipple you're good. If it doesn't buy a new petcock.
 
chuckycheese said:
Is the vacuum hose hooked up? (I'm assuming the vacuum port is plugged with something.) If it is hooked up, you'll need to get a new petcock...or rebuild the old one; (which might be effective and it might not.)
Click to expand...

doctorgonzo said:
Put it in the "on" position, hook a vacuum hose up to the smaller of the two nipples (heh.. heh.. heh...) and suck on it. If gas flows out the other nipple you're good. If it doesn't buy a new petcock.
Click to expand...

Ok, so the petcock Vaccum port is not hooked to anything, only the fuel port is, which explains why it only runs on PRI. When I hook a hose to the vaccum port and suck with the petcock set to "ON" gas start coming out.

Here is the new problem I have, how come my carbs do not have a vaccum port? Someone must have put non 78 GS750 carbs on this bike, because I SEE NO vaccuum port on my carbs. I have the main Fuel port in the middle and 2 Breather tubes. That is it...

Now I need to identify what type of carbs these are, I know they are Mikuni something...
 
Vacuum take-off is on the #3 carb...check the photo of my 550 carbs (same take-off as yours).

Picture017.jpg
 
So like this:

I swear I could not find it yesterday, Let me go take another look with your pic...

petcock_gs550.jpg
 
You got it Peter! Hopefully the vacuum nipple is there on your carbs...
 
Nessism said:
You got it Peter! Hopefully the vacuum nipple is there on your carbs...
Click to expand...

I do not see it at all, I took the tank off and took a few pics. They did not come out very good, it is raining like crazy out right now and really dark in the garage.

My Idle knob and throttle are in between the #2 and #3 Carb, so that is different from your pic. But my breather tubes are on #2 and #4 like yours and my fuel is also in between #2 and #3 like yours...

I swear someone put some other type of non vacuum fed carb on this bike!

bad_carb_pic.jpg



bad_carb_pic_2.jpg
 
I've heard of people putting Kawasaki Mikuni's on GS bikes...

If the vacuum port is truly missing you might have to fabricate something or get a manual petcock and turn it off each ride.

Edit: just checked the 750 factory manual and it shows vacuum ports on the cylinder head where the rubber boots attach. You might want to check there...
 
Nessism said:
I've heard of people putting Kawasaki Mikuni's on GS bikes...

If the vacuum port is truly missing you might have to fabricate something or get a manual petcock and turn it off each ride.

Edit: just checked the 750 factory manual and it shows vacuum ports on the cylinder head where the rubber boots attach. You might want to check there...
Click to expand...

When the weather is better ill take it out of the garage and take better pics. I just got some new O-rings for the intake, so I'll have ample time to take the carbs out...

As for the suzuki petcock, I just run on PRI for driving around, and set to ON or RES when I am not running, without vacuum its essentially OFF.
 
Looked though my 850 and 750 manual and it shows the vacuum port fittings for sync'ing the carbs on the cylinder head (on the 550, the sync fittings are on the carb boots). I'm guessing that the vacuum take-off for the petcock might be on the head as well but I'm not sure. It's got to be there somewhere...
 
doctorgonzo said:
Put it in the "on" position, hook a vacuum hose up to the smaller of the two nipples (heh.. heh.. heh...) and suck on it. If gas flows out the other nipple you're good. If it doesn't buy a new petcock.
Click to expand...
saying nipple is funny8-[
 
Where is my Vacuum?

Where is my Vacuum?

New pics of my Carbs, replaced the O rings today!
Any idea if these are Suzuki or Kaw carbs? MIC?

Carb1.JPG


Carb2.JPG
 
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peterpressure said:
New pics of my Carbs, replaced the O rings today!
Any idea if these are Suzuki or Kaw carbs? MIC?

Carb1.JPG
Click to expand...

The little flat right above the fuel filter in this pic should be drilled and a nipple should be there, that is the vacuum port.
Since you don't have one the carbs must be off something else.
 
It really looks like suzuki to me. But I couldn't tell you for absolute sure. What also could have happened was there was a problem, such as a broke pilot screw or something, and the PO replaced just the 3rd carb body. He could have done this with a Carb from a different cylinder (possibly have to drill for vacuum or fuel lines) and therefore you don't have the vacuum port. If you really wanted the vacuum, you could just replace the 3rd carb.
 
Those are the small 22mm vms- for 550. On yours(26mm) Look just below the idle adjustment on the cylinder #3 carb that is the vacuum connection It does look like it was plugged? You could dril into any one of the carb boots & install a fitting. Another way Think fitting for carb sync!
 
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