Vacuum Petcock

[Gpaws1968]

Hi everyone, my 1981 XS650 Special II has a vacuum petcock gas tap, when I was growing the ones we had were On, Off, and Reserve.

What is the advantage of having a vacuum style?

 

[Jim]

You don't need to turn it off when the bike's off. In fact, it doesn't have an "Off" position. The "on" position is actually off until engine vacuum pulls it open.

 

[jetmechmarty]

It’s also a safety device. When the engine quits, the fuel shuts off.

 

[Melnic]

And the old ones, the diaphram rubber can rupture. If this happens, the fuel can leak directly into the vacuum line and into the cylinder. Its good to inspect them and if needed rebuild. If rebuilding, be careful about looking at the diaphram rubber and the holes in it. I've seen different types, Mine has 4 holes on each side and the spacer has 2 holes per side to go into those holes. Petcocks on the bike when I got it were not the stock petcocks and one was different than the other and was not rebuildable with XS650 rebuild kits. Both petcocks on the bike when I got them had leaking diaphrams so I"m temporarily using an on/off/res type (not a vacuum type). Will put the rebuilt petcocks on the tank when I paint it.

 

[jetmechmarty]

I have heard the rupture concern many times. I have never heard any evidence of a vacuum diaphragm rupture while in use. The fail mode is failure to seal OFF.

 

[gggGary]

I have heard the rupture concern many times. I have never heard any evidence of a vacuum diaphragm rupture while in use. The fail mode is failure to seal OFF.

Not common, but it happens, why there are TWO diaphragms. There's also a tiny one way vacuum valve in the diaphragm cover.
I like vacuum petcocks but have no problem using the manual petcocks on bikes so equipped. Either type often needs a good refurb 40 years on. The Yamaha are better than period Kaw petcocks that just sucked (on or off)

 

[jetmechmarty]

Not common, but it happens why there are TWO diphragms. There's also a tiny one way vacuum valve in the diaphragm cover.
I like vacuum petcocks but have no problem using the manual petcocks on bikes so equipped. Either type often needs a good refurb 40 years on. The Yamaha are better than period Kaw petcocks that just sucked (on or off)

My biggest issue is mixing them up. Trained by vacuum petcock means walking away from the manual petcock with it left on.

 

[XSTime]

I really like this original 79 set but they are starting to leak past the “off” time for a looksie why?

 

[jetmechmarty]

View attachment 255330
I really like this original 79 set but they are starting to leak past the “off” time for a looksie why?

Some of the kits contain an o’ring that has a cross section too small. The sealing face has to be polished and the o’ring has to make good contact.
My XS650SK has never had the petcock tampered with. It still goes positive off when the engine is off.

 

[XSTime]

Some of the kits contain an o’ring that has a cross section too small. The sealing face has to be polished and the o’ring has to make good contact.

That O’ring is the suspect for failing to seal off the operation of the lack of vacuum “off” when the engine is off.
When I opened the vacuum chambers this evening to look, I found O’rings misshaped from age. The originals were mushed flat on the sealing face so I looked into replacing with what I found local.

Took a chance and installed new O’rings that were close in size. Going to let these new O’rings settle in place awhile The rest of the petcocks look so clean that I didn’t even go further than to clean, dry, and lightly grease and reassemble.
Have all winter to decide on which to prove out first. I must admit the Mikes XS petcocks appear slender and well designed.
The 79 Special Petcocks are the old Fav and will be stored for future use.

 

[ckahleer]

Does Yamaha have a PRI position on all its petcocks? I'm not sure if all Hondas are like this, but my 500 Shadow has a vacuum petcock, and the positions are ON-OFF-RES. The problem is, after the bike sets a few weeks, you have to crank the hell out of it to get it to start.

 

[mrtwowheel]

I saw an early '80s Honda CB900C vacuum unit fail once. The engine was still running while gas was being sucked into the cylinder that the vacuum was connected to and pouring out the exhaust at a fast rate. It was an alarming and scary sight. This is a big unit separate from the petcock and has earned the name " spawn of satan", SOS for short. Easily removed and trashed. I cringe at the sight of them. The same unit was used on all DOHC's of that time.

 

[AUTOMAN]

Not common, but it happens, why there are TWO diaphragms. There's also a tiny one way vacuum valve in the diaphragm cover.
I like vacuum petcocks but have no problem using the manual petcocks on bikes so equipped. Either type often needs a good refurb 40 years on. The Yamaha are better than period Kaw petcocks that just sucked (on or off)

Spot on!
I am not fam. with the Yamaha, but the Kaw sucks.(by-pass mod)

cliff

 

[jetmechmarty]

Does Yamaha have a PRI position on all its petcocks?

RES-ON-PRI. That’s the setup on the vacuum petcocks of the era. That’s not 100% of the bikes.
My XS1100SG has manual petcocks and there is a single vacuum shutoff in the fuel circuit. The fwd position is OFF-ON-PRI-RES. That’s CCW.

 

[Wingedwheel]

I must admit the Mikes XS petcocks appear slender and well designed.

They will work fine and as I said before I have used the repops as well. The only issue I had was the valve body was the same for both petcocks so the side was determined by which end the barb fitting was installed. The OEM are mirror similar to the vacuum petcocks you have. It was just a little confusing at first after learning the originals.

 

[Niels B]

I’ve refurbished 3 sets of ‘78xsE petcocks like yours - they all work fine after a good clean and new seals & membrane

 

[5twins]

The original vac petcocks on my '78 Standard look different than those pictured above. The center round portion of the arm is smaller and the fuel outlet faces straight back. I think they were a one year only thing .....

I've never replaced any rubber parts in them but did have one start leaking. It wasn't from any of the rubber bits but rather from one of the cross-drilled passageways. The epoxy Yamaha applied to it fell off, lol. I fixed it with JBWeld. That was several years ago and it's still fine, leak-free.

 

[Niels B]

5twins, all mine are like yours - facing straight back, actually never seen the one pictured on the green tank, mine are from 2 original green and 1 original black tank - all 3 1978 tanks

 

[XSTime]

I really like this original 79 set

78E ‘s did come with the version seen on 5twins tank.
78-79 “Specials” came with the slightly different version. The outlet pipe pours at an angle as shown. Slightly different style

 

[Niels B]

Thanks for the clarification - learned yet another thing about these bikes