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Vacuum Petcock On/Off?

Discussion in 'The Garage' started by Paul75, Aug 12, 2017 at 5:13 AM.

  1. Paul75

    Paul75 XS650 Enthusiast

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    Guys I've searched but can't find the info I require. I've a '79 vacuum petcocks which I have just rebuilt, refitted and am testing. Simple but perhaps a stupid question; in the on and prime positions fuel flows freely from them, in the reserve position just a little flows out.

    I always thought that with a vacuum petcock the fuel would only flow when in the prime position and in the on position it would flow only when the engine was running? Fuel is effectively flowing in all positions.....surely this cannot be correct?
     
  2. YamadudeXS650C

    YamadudeXS650C Central New York XS650

    gggGary likes this.
  3. YamadudeXS650C

    YamadudeXS650C Central New York XS650

  4. gggGary

    gggGary Cara Mia!!

    Define rebuilt?
    This thread WAS useful but Photobucket killed the pics.
    http://www.xs650.com/threads/rebuil...rated-fuel-valves-petcocks.20662/#post-201190
    Some tips;
    Use only the genuine Yamaha branded 4 hole rubber "valve", the aftermarket are often not thick enough and seldom work,
    the back of the selector disk must be carefully flattened and polished. 600 grit on a flat surface carefully rub, then buff the face.
    There is separate passage down from the tank for the reserve position, check it is clear all the way to the "valve" .
    I have found aftermarket diaphragm shafts that were too short and just wouldn't shut off.
    I finally bought some orings to replace the usually hard one on the tip of the diaphragm shaft. Standard Size AS568 -007 B70 (NBR) O-rings this worked on recent rebuild
    Have you run the motor or pulled vacuum on these? There is a small rubber "one way valve" in the vacuum barb The barb can be unthreaded from the petcock body. I have run into these that work "too well" and don't let the vacuum leak off as designed. they then keep the diaphragm pulled and the petcock open, on period piece 3 days after the engine last ran it was still holding vacuum! I put a small scratch on the seat and it works correctly now.
     
    YamadudeXS650C likes this.
  5. gggGary

    gggGary Cara Mia!!

    OK someone had wisely created a PDF of the rebuild process. I downloaded it here. http://www.xs650.com/attachments/petcock-rebuild-pdf.104470/
    It shows the little one way valve in the vacuum barb, but doesn't mention that it can work TOO well LOL. I also edited the big repair thread to include the PDF link
     
    YamadudeXS650C likes this.
  6. Paul75

    Paul75 XS650 Enthusiast

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    No, I've not had the motor running yet. I went through and cleaned, polished, sanded everything as it should be done although the 2 rubber discs with holes in are not genuine yamaha.....maybe this is the reason? IMG_20170613_182859_840.jpg The only thing I didn't take out were the one way valves in the vacuum barb because the brass nut is so soft it was rounding off and I didn't want to risk damaging......
     
  7. gggGary

    gggGary Cara Mia!!

    Maybe but the crap disks tend to leak not "pour" fuel. If you reassemble and try WITHOUT pulling a vacuum, and they hold fuel there's your answer. Yeah the barb was not easy to unthread, fortunately I had a 6 point socket that was a good fit.
     
  8. Paul75

    Paul75 XS650 Enthusiast

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    Yes fuel was definitely pouring out. What do you mean reassemble and try without pulling a vacuum?
     
  9. gggGary

    gggGary Cara Mia!!

    Have you pulled vacuum on the vacuum port or not?
    You're not alone, I've fought vacuum petcocks several times, I'm just too danged stubborn to give up.
     
  10. Paul75

    Paul75 XS650 Enthusiast

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    Same here, I've spent far to long on this already so I can't give up! I must be really dumb here but what do you mean 'pulled vacuum'?
     
  11. Paul75

    Paul75 XS650 Enthusiast

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    No I haven't but I'll try....should it be open or closed when I suck?
     
  12. weekendrider

    weekendrider Iron Horse cowboy

    The spring on the back of the diaphragm should be holding it closed, at all times and selector positions. When you pull a vacuum (motor vacuum, a syringe, suck on a piece of tubing) the diaphragm is pulled back allowing fuel to flow through the hole in the center.
     
  13. gggGary

    gggGary Cara Mia!!

    If fuel's already flowing it doesn't make much difference, LOL
    Did you look at and follow all the steps in the PDF or thread? If you use a piece of tube to suck or a mighty vac type vacuum maker, you can watch the action of the diaphragm and oring seal with the selector valve removed
    PS we don't think you suck!
     
  14. gggGary

    gggGary Cara Mia!!

    i do tend to stretch the spring a bit and polish the beveled seat the o-ring sits on with a small bullet shaped Dremel buff with fine compound. A q-tip in a battery drill will do this also.
    [​IMG]
     
  15. weekendrider

    weekendrider Iron Horse cowboy

    gggGary I've never thought to use the mighty vac. Any idea about how much pressure is needed to over come the spring?
     
  16. gggGary

    gggGary Cara Mia!!

    No, but not much.....
     
  17. Paul75

    Paul75 XS650 Enthusiast

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    Yes I followed the PDF step by step, I also polished the seat and stretched the spring out a little. When I got a piece of tubing and sucked, it wasn't free flowing but it should be, that's weird!
     
  18. Paul75

    Paul75 XS650 Enthusiast

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    IMG_20170812_114523_490.jpg

    Tank came out pretty good!
     
    weekendrider and robinc like this.
  19. gggGary

    gggGary Cara Mia!!

    No it should pull and HOLD a vacuum. You should see the end of the shaft retract and if you stick your tongue on the tube it should stay sucked in. If it 's not you're having an issue with the diaphragms. they are thin and a bit fussy to get straight and sealed.
    Tanks does look sharp! Did you get a petcock kit? As above I found several with a too short diaphragm/plunger shaft. I am always super careful removing the original diaphragms and mostly can use them again. Very slow utility knife blade sliding around and lifting to separate the pieces. Hopefully the selectors weren't in too rough of shape and you didn't have to thin them too much. you can work the wave washer under the reserve- on-prime plate a bit so it pushes on the rubber valve a bit more, but that won't make up for a too thin after market valve. I also found some o-rings (from a metric assortment that swelled and would not stay sealed on the plunger shaft.
     

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