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  1. #21

    Join Date
    Jul 2018
    Posts
    9
    Location
    Minnesota
    Mower
    LawnBoy

    Re: F-340 Experts - Altitude Screw Adjustment Question

    I was referring to the D-600 fuel system with the "Altitude adjustment screw" pictured on PP 4-15 and 4-16 of the shop manual that I mentioned. My carb is the modular plastic model as shown in the diagram. Please enlarge the carb diagram shown to about 200% and explain to me that if turning the screw CLOCKWISE to Lean the fuel mix, then how does the fuel even gets to the adjustment screw to be restricted?

    The altitude adjustment screw is in the air path of the carb bowl vent, not in any fuel path. The two black arrows on the diagram lead from the air vent on the air filter side of the carb to the Altitude adjustment screw.

    I agree on the rest of points made on the fuel levels of carbs, and wish that the float level on my carb could be adjusted. My 1973 Yamaha snowmobile had little clear pipes on the sides of the Kehin (Spelling?)
    carbs to see and adjust the fuel levels. This modular carb is almost too basic, and it seems they either work well or not at all close to what I know LawnBoys can run like.

  2. #22

    Join Date
    May 2019
    Posts
    24
    Location
    Delaware
    Mower
    LawnBoy

    Re: F-340 Experts - Altitude Screw Adjustment Question

    Fishnuts2,
    Good to hear back from you. I do not pretend to know many details re: these plastic carbs, but I do have some recent experience to share.
    Last week, I adjusted my altitude screw on my 1980 Utility model (~#4502) 1.5 turns OUT from seated. It ran best ever, even on lousey 10% ethanol pump gas.
    It started immediately with only two primer pumps and stayed running. Also, no more idle "hunting" when hot.
    I'm convinced I previously had it too lean (i.e. difficult cold starts and hunting/surging idle).
    I'm all set, hopefully someone else can mimic these results w/their F-340 motor.
    Lawn-Boy Rocky

  3. #23
    Teds's Avatar
    Join Date
    Jan 2017
    Posts
    76
    Location
    Iowa
    Mower
    LawnBoy

    Re: F-340 Experts - Altitude Screw Adjustment Question

    Quote Originally Posted by Fishnuts2 View Post
    I was referring to the D-600 fuel system with the "Altitude adjustment screw" pictured on PP 4-15 and 4-16 of the shop manual that I mentioned. My carb is the modular plastic model as shown in the diagram. Please enlarge the carb diagram shown to about 200% and explain to me that if turning the screw CLOCKWISE to Lean the fuel mix, then how does the fuel even gets to the adjustment screw to be restricted?
    I have one of the modular plastic carbs, I don't like them either. I'll take a look at it, the diagram doesn't look too clear to me. The text right below it clearly states to turn altitude screw counterclockwise to richen fuel mixture. I guess it could be a typo, wouldn't be the first time in a service publication.

    OK, a quick glance at the plastic modular carb shows the altitude screw doesn't factor into the venturi, there is a small passageway into the fuel bowl. So it works "as advertised" in the text near as I can tell.

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