'75 500 in a coupe with bad mileage and hard starts, Ideas?
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500, 472, 425, 368 Discussion, '75 500 in a coupe with bad mileage and hard starts, Ideas? in Cadillac Engine Technical Discussion; ...
  1. #1
    HAZZARDJOHN's Avatar
    HAZZARDJOHN is offline Cadillac Owners Enthusiast
    Automobile(s): 1975 Coupe deVille d'Elegance, 05 Lesabre,91 Suburban 4x4
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    '75 500 in a coupe with bad mileage and hard starts, Ideas?

    I have been reading about people getting all kinds of better MPG out of their 500’s then I have, I get 10 MPG and once got 11 MPG. This is almost a standing rule, very rarely fluxuates Highway or city. I have owned it for ten years I run on 89 octane gas. It has a new exhaust with a new cat. The carb has been rebuilt at least three times in the last 9 years. I have had it done by various mechanics as I hate rebuilding carbs. I have new plugs and wires twice. It has good compression on all cylinders.

    Now for the symptoms: Very hard starting cold. I have to turn it over a couple times to prime the carb with fuel pressure, then pump it three times. Then It will start but I have to hold the gas a little to keep it idleing up (like the high idle does not work) It has been that way since the first day I have owned it and never changed with any carb rebuild, even though I feel it has to be in the carb. If you put it in gear before it runs for a minute it will die. After it is warm it runs awesome and will start right up. You only have to do that routine when it has been sitting over night. It is 100% stock with emissions equipment and all.

    I want to have it mechanically sound before I go ahead with the body work and paint.

    ~HJ

    Oh, The heat riser is not working, I have it wired open and the lines cut off, for 2 reason. It rattled after I had new exhaust put on, and I could not locate a new one. Not sure if that should make a difference in middle of august, but it always has a tough time starting cold.

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  3. #2
    cadillac_al is offline Cadillac Owners Connoisseur
    Automobile(s): Cadillac 78 Seville, 78 Coupe de Ville, 92 Sedan de Ville
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    Re: '75 500 in a coupe with bad mileage and hard starts, Ideas?

    It sounds like the carb is leaking the gas out of it. You would think one of the three guys that rebuilt it would have epoxied those well plugs. Tell the next guy to do it. The heat riser doesn't help all that much so I wouldn't worry about it. If the choke is adjusted correctly and the carb holds it's fuel, it should start easily.

  4. #3
    bicentennialcadillac is offline Cadillac Owners Enthusiast
    Automobile(s): '76 Fleetwood Brougham, '79 Sedan deVille
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    Re: '75 500 in a coupe with bad mileage and hard starts, Ideas?

    How is the vacuum?

  5. #4
    steelybill is offline Cadillac Owners Enthusiast
    Automobile(s): '85 Fleetwood FWD
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    Re: '75 500 in a coupe with bad mileage and hard starts, Ideas?

    Sounds like the last guy who did the carb didn't set the choke correctly, and the fast idle cam position. It has two vac pull-offs if I remember correctly. It's always best to replace those at overhaul. The leak is common on Q-jets and should have been fixed on the very first overhaul.

    Your running/stalling after cold start sure sounds like the choke is set too lean.

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