setting idle speed

Discussion in 'Technical' started by fullcm, Aug 6, 2017.

  1. fullcm

    fullcm Doug

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    Hello,
    I got around to giving my 74 maverick 250 a tune up this weekend. I picked up a nice used Sears engine analyzer and timing light.
    I just have a question on setting the idle speed. Mainly on how to set up the dash pot. As I understand it the purpose of the dash pot is to reduce the engine idle to less than the setting when the key is turned off to help prevent dieseling.
    So here is what I did:
    With the dash pot unplugged (fully retracted) I set the hot engine idle speed using the idle adjusting screw on the carb. to 600 rpm in drive with lights off (like the label said). Then I energized the dash pot so the solenoid is fully extended and set it to just touch the throttle arm. Then I backed off the idle adjust screw a couple of turns so that arm is only held to the right speed by the energized dash pot.
    It seems to run ok and does not diesel when I turn it off.
    Does this sound right?

    Thanks for reading.
     
  2. Crazy Larry

    Crazy Larry Member

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    I'd just get rid of the dashpot. Set the idle to 750 RPM in Neutral, and call it good.
     
  3. fullcm

    fullcm Doug

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    I think your right, it doesn't seem to do much. Maybe mine is bad I think is supposed to have a spring loaded effect when its energized to act like a shock and slow down the throttle from closing too fast. Mine when its energized it holds the throttle arm out at about 1000 rpm. I went ahead an reset the idle speed so it rests on the idle screw and backed out the dashpot, If my idle speed is still too high from it after a drive I will just unplug it.
    Thanks Larry.
     
  4. Krazy Comet

    Krazy Comet Tom

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    If it's original the dash pot was probably bad 30 years ago...
     
  5. jbrich

    jbrich Johnny

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    Hi fullcm,
    You have A solenoid throttle positioner.
    An "anti-stall dash pot" is what provides the cushion when the throttle closes quickly. It isn't powered, it just bleeds air from A diaphragm sealed chamber when the throttle lever hits it.
    If the plunger on the solenoid extends when energized, it is working. It can help driveability if it is adjusted correctly.
    Is this your carb ?
    idle 4.jpg This is from the 1972 FSM. Yours may be similar.
    idle 3.jpg idle 2.jpg idle solenoid 1.jpg
     
  6. fullcm

    fullcm Doug

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    Mine is similar, photo is attached and the solenoid is in the lower left corner. I'm not sure what this solenoid is actually supposed to do, I read that it may be there to help stop dieseling.
    The solenoid will extend when energized (any time the key is on) it does not have enough power to actually move the throttle arm out but the first time the pedal is depressed and the throttle arm is moved out the solenoid will move out further then it stays out, preventing the throttle arm from moving back to the idle screw when the pedal is released causing the idle rpm to be around 1000.
    I was wondering if maybe I am supposed to adjust the solenoid so that when extended the rpm is at 600, basically taking over the job of the carb idle screw. Then I can back off the idle screw so when the solenoid deenergizes my idle speed will drop to something less that 600 to help prevent dieseling when this engine is shut off.
     

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  7. Krazy Comet

    Krazy Comet Tom

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    You have a idle solenoid that can serve two purposes, both increase idle... With A/C it increases speed with it operating so the load of compressor doesn't drag down idle...

    Without A/C it's more of a emissions/anti dieseling item... In this mode usually idle is set to approx 500 RPM, with it deactivated and maybe 750 activated... On shutdown it reduces carburetor opening to lesson chance of run-on(dieseling) when hot...

    There was one on my Comet, went away when I swapped to 4bbl carb...
     
  8. rthomas771

    rthomas771 Member

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    I removed it from my '74 250

    043.jpg
     
  9. jbrich

    jbrich Johnny

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    In this case, the solenoid has an anti-dieseling function. As you thought.

    If you can achieve A satisfactory idle with no "run on", then it isn't absolutely necessary. As evidenced by the folks who have removed them without issue.

    The description and adjustment steps in the manual are easy to follow if you decide to keep it
    Basically, the curb idle screw is set for A 500 rpm idle, in neutral, everything else off. Then the solenoid it plugged in and allowed to extend fully. With the automatic transmission in drive and the headlights on bright to load the alternator, adjust the idle speed by adjusting the solenoid.

    The solenoid determines the curb idle speed.

    When the key is turned to off, the solenoid de energizes and allows the throttle to close more.

    There are other types called throttle kickers, strong enough to push the throttle open when they are energized. Used to compensate for the load of an A/C compressor.
     
  10. burndamage

    burndamage Member

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    Following this thread, How does one get the perfect idle settings on a carburetor? My 73, 4.1 250 runs, brakes and shifts but she turns off at idle. She has a A-Team Performance 162 Carter carburetor.
     

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  11. Jaybee

    Jaybee Member

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    As others have alluded to, a dashpot and an idle stop solenoid are different things, even though both have plungers that contact the throttle lever contacts. A dashpot is an air-operated damper which slows the throttle closing just before it hits the stop in order to prevent stalling. The idle stop solenoid extends when the key is on, and withdraws when the key is off in order to close the throttle further and discourage dieseling. It's something which was a common problem in the early 1970s due to changes in tuning to accommodate emission controls.

    Now, as to setting the idle, start by GENTLY turning the idle mixture screw all the way in until it's seated. It's the screw that goes into the carb body with the spring around it, located near the throttle shaft. You'll damage the seat if you crank down on it more than just enough to feel the resistance increase. Back it out 1.5 turns. Set the idle just high enough that it's smooth. Now, turn the mixture screw 1/4 turn one way or the other. If the idle slows down, turn it the opposite direction and you should hear it speed up. Turn the screw just to the point where the idle is most smooth and fast. It really shouldn't be more than about 3/8 turn from that 1 1/2 turn base setting. If you want to get it finely tuned, put a vacuum gauge on it and tune for maximum vacuum. Once you have the mixture set properly, you may need to turn the idle speed down a little. If you have a tach, you're looking for about 750rpm. At that point it should really purr, silky smooth and not sound at all like it's racing.

    If the idle mixture screw adjustment doesn't seem to make a difference, pull the screw all the way out and shoot some carb cleaner through that passage. If everything is clean and you're in the ballpark it should almost be like a rheostat, you'll be able to hear the difference and see it on your vacuum gauge with even an eighth of a turn difference.
     

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