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  1. #1

    Knock when chopping throttle

    I am making progress on my 750 SL jet jon carbs. Got the midrange flat spot solved (was a pop off spring not seated right). So much for skipping the pop off test

    I am getting a single knock when i chop throttle after high RPM run. It did this before I fixed the pop off spring too.

    Not done setting mixtures yet, was too rough & cold to deal with it during yesterday's test.

    I am using 91 octane no ethanol gas. Still using up premix + injection, so getting double oil.

    Just wondering if this is normal & if not how much I should be concerned.


  2. #2
    Click avatar for tech links/info K447's Avatar
    Join Date
    Jul 2007
    near Toronto, Canada
    Explain what this single knock sounds, or feels, like.

    Could it it be a driveline clunk as the torque on the impeller changes suddenly? Engine mount flex?

  3. #3
    Quote Originally Posted by K447 View Post
    Explain what this single knock sounds, or feels, like.

    Could it it be a driveline clunk as the torque on the impeller changes suddenly? Engine mount flex?
    It is definitely cylinder detonation. Can't tell if it is just 1 cyl or all of them, it is very brief, only happens when i let off of throttle, never when i nail it. Just in case I am wrong I'll get a sound clip on next test run.

    I have the hi speed mixture screws turned out 1/4 past recommended initial settings. Low speed screws are adjusted just rich enough to get out of initial bog.

    From your questions I gather it is not normal.


  4. #4
    Well, like normal I changed several things at once & the problem went away. So I don't know exactly what the cause was.

    I sync'd the carbs better, set the low speed mixture closer, & took off the rubber inlet elbow.

    Getting 6300 RPM WOT, tan plugs after throttle chop. Good to go there.

    It seems to run better without this elbow

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    Is this needed for the flame arrestor function? The intake screen is in place. 95 750 SL BTW.

  5. #5
    ripcuda's Avatar
    Join Date
    May 2010
    Newman Lake, WA
    That's just the air intake elbow that attaches to the flame arrestor. Without it... the intake will flow more air.

    I recall in '94 (earlier?) that there was a tech bulletin to trim the rubber elbow further up so the opening was a tad larger (elbow necks-down towards the opening). This affected air intake and thus A/F with the carbs.

    The factory carb settings for years that had that particular airbox/elbow are designed for that specific openings worth of intake air. If you run without it... you are leaning out the carbs somewhat. If your tweaking them anyways and carefully monitoring piston wash and plug color... you can probably adjust it to work without it.

    Others may have first-hand experience with it. I've kept mine on my 750s and used stock jetting to good results.


  6. #6
    BlueFishCrisis's Avatar
    Join Date
    Aug 2011
    Medina, Ohio, United States
    My understanding of the main purpose of the elbow was to direct the inlet to the bottom of the hull so that in case the ski flips it would be more difficult for water to be ingested into the intake tract. .....

  7. #7
    Thanks, that is what I was guessing for the elbow's purpose. It's running very well & is not running lean (tan plugs, good wash pattern), so I am going to call it good.

    Thanks for helping me. I could not have made this contraption work out without all the info I got from this site!

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