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Thread: Part throttle issues.

  1. #1
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    Default Part throttle issues.

    Despite my best efforts, i cannot seem to get them to run as one would hope they should. At hard throttle, i have zero issue, and at idle and very light throttle, i have no issue, but part throttle cruise just starts to burble and pop, like it must be over fueling or something.
    With application of carb stray to the throttle shaft, you can hear a slight rpm change, and have begun to think that its air leaking past and changing the mix for me as I drive. I have done the whole all the way in, 2.5 turns out thing, there is proper oil, they are equal flow at 1500 rpm, but they just start bogging till i push through. Could I doubt highly the dashpots are getting hung up, but its something that has crossed my mind, but how to test that, as i cannot really see them while i'm driving
    The car is a 1970 240z, with factory l24.
    1970 240z
    HLS30 03017

  2. #2
    Registered User BadDog's Avatar
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    Default

    I was having a bugger of a time figuring out why I had lean ping (sounds like shaking a spray paint can) under load until I found that my front crab's float level was way lower than the rear. All these years I've been adjusting my fuel mixture knobs the same amount, I forgot to check them individually for mixture. Try the tips on this page: Technical Information Page ..... Carburation
    The technique recommended there for checking mixture by raising the lift pin in the underside of the dashpot mount is where it clicked for me. That's when I found that my front carb needed over 2 full turns out more than my rear carb needed.
    I still have to do something about my float levels, but this helped so much its not even funny.
    1973 240Z Silver, 5 spd., 4.11 diff., 150,000+ miles
    2013 Subaru Impreza 5-door

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    if fueling was an issue, would it not affect at all speeds, and loads. This is kinda a specific range. When cruising in 4th no real issues, and hard throttle no issiues. But it just starts backfire and burble at part. I'll look into the stuff on that page, but man, this is driving me nuts.
    1970 240z
    HLS30 03017

  4. #4
    Registered User BadDog's Avatar
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    I missed that full throttle had no issues. What are you using for damper oil?
    1973 240Z Silver, 5 spd., 4.11 diff., 150,000+ miles
    2013 Subaru Impreza 5-door

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    Default

    What color are your plugs? This will tell you which carb needs adjusting. If the mixture screw adj. is substatially different between the two the float level is off. The best source of info on SUs is the SU DVD from ZTherapy.

  6. #6
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    Im using the "su" oil from MSA, and my plugs are fine. I'm fairly sure air is leaking right on past the throttle shaft, as when i give it a little bit of propane, rpm changes. I'm thinking I may just pull the trigger on sending them off to z therapy.
    1970 240z
    HLS30 03017

  7. #7
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    After watching "Just SU's" for the first time, your symptoms are describing a leak in the throttle shaft to a T. I'm a novice for sure, but from watching the video that is what he describes exactly.

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