1. Hello Guest. You have limited privileges and you can't "SEARCH" the forums. Please "Log In" or "Sign Up" for additional functionality. Click HERE to proceed.

Petcock Vacuum

Discussion in 'XJ Technical Chat' started by Jerrod Robinson, Apr 7, 2021.

  1. Jerrod Robinson

    Jerrod Robinson Member

    Messages:
    150
    Likes Received:
    16
    Trophy Points:
    18
    Location:
    Cardington, Ohio
    Hey guys what's up, I have an 82 750 Maxim that I'm dealing with. I got it running and it sounds normal, but it has no acceleration Power and it will not idle. I've rebuilt the synced the Carbs, replaced the Valve stem seals, and have the valves adjusted with no change. I was all outta idea's when I discovered I have a Broken Petcock Vacuum. I've ordered a Replacement, but I've never delt with a Vacuum Petcock before, could this have been the problem this entire time? What kind of problems is no petcock vacuum known to cause?
     
  2. k-moe

    k-moe Pie, Bacon, Bourbon. Moderator Premium Member

    Messages:
    19,613
    Likes Received:
    6,703
    Trophy Points:
    113
    Location:
    The City of Seven Hills
    It depends on where the break is. If it’s the vacuum diaphragm you’ll get excess fuel in one cylinder (and possibly into the crankcase). If it’s the vacuum line you’ll have a big vacuum leak in that cylinder. In either case she’ll run like poo, exactly as you describe.
     
  3. Jerrod Robinson

    Jerrod Robinson Member

    Messages:
    150
    Likes Received:
    16
    Trophy Points:
    18
    Location:
    Cardington, Ohio
    The Vacuum Prong on the Petcock has been broken off, so there's no Vacuum line running from the Petcock to the Carb boot. There just the Fuel line running to the Carbs.. Thank's for the Knowledge Brother. I appreciate it.
     
    Beck likes this.
  4. Minimutly

    Minimutly Well-Known Member

    Messages:
    1,746
    Likes Received:
    748
    Trophy Points:
    113
    Location:
    West Wales, uk
    Pity you never made clear that bit about the broken petcock barb earlier, would have saved some guessing.
    And, yes, unless you plug the vac line to the inlet rubber it will run like crap. However, this would generally only be at low speed, once on full throttle I wouldn't expect it to make a huge difference. However, I'm not sure if prime allows ll
     
  5. Polock

    Polock Well-Known Member

    Messages:
    9,751
    Likes Received:
    2,093
    Trophy Points:
    113
    Location:
    Beaver Falls, PA
    i think you loose the reserve setting, thats good to have
     
  6. Minimutly

    Minimutly Well-Known Member

    Messages:
    1,746
    Likes Received:
    748
    Trophy Points:
    113
    Location:
    West Wales, uk
    Sorry, still having freezing up issues..
    I meant to say, not sure if prime allows full flow? Even if not, I would expect full performance for a little while before the fuel in the bowls drops off.
     
  7. Huntchuks

    Huntchuks Well-Known Member

    Messages:
    951
    Likes Received:
    395
    Trophy Points:
    63
    Location:
    East Rochester, NY
    Prime allows full flow. Remove you fuel line and try it.
     
    Beck likes this.
  8. Minimutly

    Minimutly Well-Known Member

    Messages:
    1,746
    Likes Received:
    748
    Trophy Points:
    113
    Location:
    West Wales, uk
    I'll take your word for it:)
     
  9. Huntchuks

    Huntchuks Well-Known Member

    Messages:
    951
    Likes Received:
    395
    Trophy Points:
    63
    Location:
    East Rochester, NY
    Of course you would need to add a temporary line to empty into a container. No one need gas all over the engine, frame and floor. A good way to make sure no fuel flows on reserve or 'on' positions too.
     
  10. XJ550H

    XJ550H Well-Known Member Premium Member

    Messages:
    13,199
    Likes Received:
    3,854
    Trophy Points:
    113
    Location:
    Great North Woods
    could always set to prime and ride..
    there is a thread on converting petcock to non vacuum
     
  11. Jerrod Robinson

    Jerrod Robinson Member

    Messages:
    150
    Likes Received:
    16
    Trophy Points:
    18
    Location:
    Cardington, Ohio
    New Petcock arrived, And now it will idle with the Choke on. But with no choke I have to apply slight pressure to the throttle or it will instantly stall out. And it still feels very weak at higher RPM's.
     
  12. Jerrod Robinson

    Jerrod Robinson Member

    Messages:
    150
    Likes Received:
    16
    Trophy Points:
    18
    Location:
    Cardington, Ohio
    I had brand new Plugs in it, and I checked them after about a half hour of accumulated running. And the No 1 plug was a little dark but looked pretty good overall. The number 2 plug still looked pretty much brand new and according to it's pipe it was firing. But 3 and 4 had heavy black sooting, and looked to be close to fouled.
     
  13. Jerrod Robinson

    Jerrod Robinson Member

    Messages:
    150
    Likes Received:
    16
    Trophy Points:
    18
    Location:
    Cardington, Ohio
    Oh, and I thought I should mention that the factory exhaust was toast when I bought it. So it now has a 4 into 1 header with a slip on muffler. It's running the factory airbox with a new filter..
     
  14. Jerrod Robinson

    Jerrod Robinson Member

    Messages:
    150
    Likes Received:
    16
    Trophy Points:
    18
    Location:
    Cardington, Ohio
    Alright, found my issue. Been studying up on syncing the Carb's. And the pilot screw in my in my number 3 Carb is seized and stripped out pretty good. And that's the piston that has the brand new looking spark plug with no sooting. So I'm guessing that it's seized all the way closed.
     
  15. Jerrod Robinson

    Jerrod Robinson Member

    Messages:
    150
    Likes Received:
    16
    Trophy Points:
    18
    Location:
    Cardington, Ohio
    Been doing some research on my power loss at high RPM's, And I'm pretty sure my valve's are floating. It's the only thing making sense at this point. So it looks like I'm going back in...
     
  16. Simmy

    Simmy Well-Known Member

    Messages:
    2,261
    Likes Received:
    1,126
    Trophy Points:
    113
    Location:
    Waterloo Ontario Canada
    So you've gone from not running too good to now running so good you think it's valve float.
    Sorry, Jerrod I can't agree with your diagnosis.
    Where are you with this?
    You need to extract this pilot screw. Sounds like you might even need a replacement carb body for #3 if you're not successful.
    Get that sorted. If it was me I would start with a complete rebuild of all 4 carbs.
     
    Beck likes this.
  17. Simmy

    Simmy Well-Known Member

    Messages:
    2,261
    Likes Received:
    1,126
    Trophy Points:
    113
    Location:
    Waterloo Ontario Canada
    Sorry Jerrod, I just went back to your original post,
    Explain "rebuilt the synced carbs", if you rebuilt the carbs you would have spotted #3 pilot screw seized then.
    What has been done to the carbs so far?
     
    Beck likes this.
  18. Jerrod Robinson

    Jerrod Robinson Member

    Messages:
    150
    Likes Received:
    16
    Trophy Points:
    18
    Location:
    Cardington, Ohio
    Yeah, I asked the guy I payed to adjust my Valve's and sync my Carbs a very similar question. He's not responding.
     
    Beck likes this.
  19. Jerrod Robinson

    Jerrod Robinson Member

    Messages:
    150
    Likes Received:
    16
    Trophy Points:
    18
    Location:
    Cardington, Ohio
    He had the Bike last Fall for like 2 Week's cause he said he had to order shim's. I bought the Carb rebuild Kit myself so he probably put the new Jets and Gasket's in it and called it a Day. But Im not sure cause he won't respond.
     
  20. Jerrod Robinson

    Jerrod Robinson Member

    Messages:
    150
    Likes Received:
    16
    Trophy Points:
    18
    Location:
    Cardington, Ohio
    The Float Gasket's were replaced, and the screw that was seized was Stripped too. Which I think he did, but instead of telling me he couldn't get the Screw to break loose and working out a refund. He just gave me the bike back and kept my money.
     

Share This Page