Carb synchronising

Discussion in 'The Pit Stop' started by Leehine, Jun 22, 2020.

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  1. Jun 22, 2020 #1

    Leehine

    Leehine

    Leehine

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    Hi
    I’m having trouble synchronising my carbs. I can’t get the idle speed right, it races above 2000rpm then dies. I get 1 and 3 synced ok, but 3 and 4 won’t match, or sync with 1 and 3. It is a 16000 mile full power model 1999. Do I have to refurb the carbs? thanks in advance
     
  2. Jun 22, 2020 #2

    Fire-medic

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    Sounds like a vacuum leak. Spray starting fluid around the carburetors and the gaskets, to see where the rpm's increase, that's one of your leaks. Continue until everything from the VBoost manifold to cyl head area, to the rubber boots at the air filter are checked.
     
  3. Jun 23, 2020 #3

    Leehine

    Leehine

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    Thanks for the tip, I’ll have another go later. Cheers lee
     
  4. Jun 23, 2020 #4

    Parminio

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    Does it run rough at idle unless the choke is engaged?

    When you get going does it surge?

    If so, you've got a bad air cutoff valve. You'll have to pull the carbs and find out which one it is. If you're going that far, might as well change them all.
     
  5. Jun 24, 2020 #5

    msteele930

    msteele930

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    I’ve benefited from the knowledge of Sean and many others for years without posting a single comment but it’s time. I’ve had similar problems, cold nature starts in the Deep South, misfire until warmed up and dead spots in throttle response. Cleaned carbs many times over the years and problem still persists. Found the problem by checking the compression “without opening the throttle”, the cylinder that had the intake leak, right rear was 40 PSI higher than the other three cylinders. The engine was going to get air somewhere & it did and caused the compression reading to be much higher than the others.
    Then, when I rechecked the compression “with the throttle open” like the test requires, all four cylinders were the same. I replaced the rubber hoses and clamps from the carbs to the manifold and the bike runs like a NEW one. Maybe this will help find your problem.
     
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  6. Jun 24, 2020 #6

    desert_max

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    This isn’t quite right. The symptom he’s questioning, if it exists, is more likely due to a blocked pilot jet. Air cut off is typically intended to eliminate an overly lean condition on deceleration.
     
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  7. Jun 25, 2020 #7

    Parminio

    Parminio

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    That's what was wrong with mine when it did that.
     
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  8. Jun 25, 2020 #8

    Fire-medic

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    After 27 years of multiple VMax ownership, the #1 problem with carbs is obstructed pilot jets. #2 problem is/are vacuum leaks due-to deteriorated rubber components. I've replaced the VBoost to cyl head o-rings, the carb bottom to VBoost rubber donuts, and the airbox to carburetor rubber donuts. I have never had to replace an air cut-off valve. That doesn't mean someone else didn't have that problem, of one malfunctioning, it's just anecdotal evidence of longitudinal experience with VMax carbs.

    I'd be interested to hear what DannyMax, Sean or CaptainKyle has to say.
     
    Traumahawk likes this.
  9. Jun 25, 2020 #9

    Toolman

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    You may want to double-check your synchronizing sequence

    It should be 1-2 then 3-4 and finish by balancing left to right or 1.2 to 3.4
     
  10. Jun 26, 2020 #10

    Nubicon

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  11. Jun 27, 2020 #11

    Toolman

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    If in doubt check the manual
     
  12. Jun 29, 2020 at 8:30 AM #12

    dannymax

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    I have replaced dozens of non-functioning air cut off diaphragms and most exhibit what appears to be damage from carb cleaner....diaphragm material is hard and wrinkled up at the bottom. Carb cleaner left behind from the shotgun procedure might be responsible for this.
    BTW, the air cut off diaphragms (valves) are sometimes referred to as anti-backfire valves...which may better explain their function.
     
  13. Jul 1, 2020 at 9:47 AM #13

    CaptainKyle

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    I have replaced a bunch of them to & I kind of lean towards it being from carb cleaner & shotgun procedure to.
     
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