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Thread: Smooth Idle ,got Smoother???

  1. Default

    Strange occurence today. While warming up my truck today this morning I started fooling with the windshield wipers. They have strong vacuum and I noticed that when I disconnected the hose it made the idle smooth out considerably, and I wouldn't say it was rough before!
    What gives? Is it a sign of Fuel Pump problems to come? Pump works fine and has good flow. Anyone else ever noticed this?
    1969 M715

  2. #2
    Join Date
    Dec 2003
    Location
    parsons Kansas
    Posts
    476

    Default

    sounds like it may be idling a little rich. have you tried adjusting carb mixture screw to see if it makes a difference?
    67 M725, 67 M715, 68 M715

  3. #3
    Join Date
    Dec 2003
    Location
    Giddings, Texas
    Posts
    7,729

    Default

    My idle goes rough when I unplug the wipers. My fuel pump side of the vacuum pump has been broken since I got the truck. I don't know why a seperate accessory should change the idle. Good question.
    Remember if you didn't build it you can't call it yours.

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  4. Default

    Dave-I have adjusted the carb.(and completely rebuilt it) on the lean side and on the rich side before coming to what I thought was the smoothest idle I could achieve. Then I unplugged the wipers?
    Does the vacuum work off of two sides of the same diaphram or are there two diaphrams on the stock fuel pumps? I haven't ever had one apart, anyone?
    1969 M715

  5. #5
    Join Date
    Dec 2003
    Location
    Lone Pine, CA
    Posts
    451

    Default *update* M715 spotting on MCAS in North Carolina

    I'd guess it's a touch rich at idle and unplugging the hose creates a vacuum leak that'll bring it to being lean.

    The pump has two seperate sections. The vacuum side kicks in when it senses a drop in vacuum like when you accelerate so it can keep the wipers up to speed.

    Chris

  6. #6
    Join Date
    Mar 2004
    Location
    Texas
    Posts
    13

    Default

    I have noticed the same thing with mine. It runs better with the wipers disconnected. FYI, I am using the mechanical fuel pump side still. I thought that this was a totally seperate pump, anotherwords having nothing to do with crankcase vacuum.
    Coby

  7. #7
    Join Date
    Sep 1998
    Location
    North Central Wisconsin
    Posts
    11,520

    Default 100 amp rectifier

    There is an input line to the vacuum pump from the motor...at least stock there is...might have been changed later if you dont have it now...
    brute4c
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  8. #8
    Join Date
    Dec 2003
    Location
    Lone Pine, CA
    Posts
    451

    Default

    Like Jon said,

    It has an "in" side from the intake manifold and and "out" side to the wipers. When the vacuum signal at the "in" side drops, the pump will keep the vacuum up high enough to run the wipers.

    Chris

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