Register now to get rid of these ads!

Technical Cadillac 500 weird low power issue

Discussion in 'The Hokey Ass Message Board' started by abe lugo, Nov 28, 2021.

  1. abe lugo
    Joined: Nov 8, 2002
    Posts: 2,533

    abe lugo

    Well. I will try and see what you guys have as advice.
    Just recently started getting a weird almost “dieseling while giving gas” very very lightly. While driving. It also king feels like it a trying to go into low or passing gear but I know that is not the case.

    The engine was relatively cool. Or running temp. I am mostly using high octane gas.
    Cadillac 500 engine, AVS2 650 Edelbrock electronic ignition. Fuel Pressure valve with a return. Set about 2-3lbs.

    while I’m giving gas it seems to drop into this effect. If I let of the gas and roll a bit it goes back to normal and I can drive on.

    I thought it might be vapor lock but I don’t think that is the case.
    I ended up adding heat shield to the inlet lines.
    The coils is new and runs nice in the early mornings. We were running at about 70 all the way to car and coffee this morning.

    Any thoughts or suggestions would help.
  2. Hemi Joel
    Joined: May 4, 2007
    Posts: 1,211

    Hemi Joel
    from Minnesota

    What is the compression ratio?
  3. abe lugo
    Joined: Nov 8, 2002
    Posts: 2,533

    abe lugo

    Not sure actually it’s pretty much a stock engine. No funny cam or domed pistons. Just stock. Let me know how that would help anyhow? Thanks.
  4. wheeldog57
    Joined: Dec 6, 2013
    Posts: 2,202


    Would you say it is surging? If so, that usually means lean condition. Good luck
    VANDENPLAS and abe lugo like this.

  5. abe lugo
    Joined: Nov 8, 2002
    Posts: 2,533

    abe lugo

    You know. I don’t ever think I have experienced surging. I will have to check into that. If anything it might be rich. I believe I set the initial time per the Edelbrock guides.

    I do have a clear plastic filter and don’t see any sediment and the fuel is moving as needed.

    it was running without any hiccups a few months ago, I went back and forth to work for a car show and a later dinner. In different weather conditions. Even as hot as it was when it first did this. Stop and go traffic a few mile then the freeway and back again.
    I will take a look at the Edelbrock manual on this one. I kind of suspect something got past the filter and is plugging some place or an accelerator pump issue.

    I checked the Edelbrock videos. It seems to be more like hesitation initially. I may try the different setting on the accelerator pump. But like I said. I didn’t seems to be an issue before. BAd Gas??
  6. Ebbsspeed
    Joined: Nov 11, 2005
    Posts: 5,666


    Stock compression on the first year (1970) of the 500 was 10 to 1, and it dropped to 8.5 to 1 in 1971 when everything built in Detroit went to shit.

    If you have an engine with 10 to 1 compression you may well be hearing detonation, which is quite likely to occur when you lightly touch the throttle. The reason is that when you are cruising down the road at a steady pace, the vacuum is high and so the distributor is at maximum advance. Touching the throttle a bit adds just enough combustible mix to cause some knocking. Push the throttle a little more and the advance will back off a bit and the knocking may subside.
    My old '64 Olds with the "Ultra High Compression" 394 did that. I could be rolling down the road with the engine just running nice and smooth, and touching the throttle a bit would cause it to knock for a bit until the advance backed off a ways and the engine accelerated a bit.
  7. oldiron 440
    Joined: Dec 12, 2018
    Posts: 2,552

    oldiron 440

    Is the timing controlled by the computer?
  8. scrappybunch
    Joined: Nov 16, 2011
    Posts: 359

    from nj

    The stock Y-pipes were well known to collapse inside. It's a double wall pipe. That fooled many mechanics back in the day.
    loudbang and perry parsons like this.
  9. Hemi Joel
    Joined: May 4, 2007
    Posts: 1,211

    Hemi Joel
    from Minnesota

    I was kind of thinking along that line. If it is a high compression 1970, it could be detonation. You could test by backing off the timing about 5 degrees and see if it has an effect.
    ottoman, loudbang and abe lugo like this.
  10. gimpyshotrods
    Joined: May 20, 2009
    Posts: 19,181


    Fuel pressure should be 5 to 5.5.
    wraymen likes this.
  11. Mr48chev
    Joined: Dec 28, 2007
    Posts: 30,192


    If you are on the throttle and it stumbles and falls on it's face and then picks up and goes after you let off the gas for a second (about like letting off the gas to shift a stick) it may be running the bowls dry in the carb and the engine is running out of gas. That 650 is pretty small for a 500 Cad that came with a Qjet rated at 850.

    I'm thinking that you are just starving the engine for fuel on acceleration both with the little carb and low fuel pressure.
    wraymen, abe lugo and Tow Truck Tom like this.
  12. indyjps
    Joined: Feb 21, 2007
    Posts: 4,541


    The stumble wasn't there before and recently developed. What changed?

    Go over the fuel system and replace rubber line and filters. Alcohol in fuel breaks down old rubber lines and swells them shut or causes then to flake off and clog up filters and get into carbs.

    Test the fuel pump, if the diaphragm is breaking down with ethanol fuel.

    Clean carb out, best you're able.

    If that isn't making a change, drop the tank and ensure the fuel sock isn't disintegrating and clogging the pickup.
    abe lugo likes this.
  13. I Know I Know Listen very Carefully to the.................................................!
    Monkey in the Trunk.!
    and if that Dosent work Did You Change the Fuel I Run High Test Gas

    in my Merc. 327/300 hp. Stock engine!
    Just my 3.5 Cents

    Live Learn & Die a Fool
  14. gimpyshotrods
    Joined: May 20, 2009
    Posts: 19,181


    Before you do anything, set your fuel pressure to 5.5psi, and your timing to 8º BTDC at idle.

    Test it.

    Then, pull the vacuum advance hose and cap it at the suction end, and test again.

    These are free and easy things to do.
    Last edited: Nov 29, 2021
  15. krusty40
    Joined: Jan 10, 2006
    Posts: 849


    And replace the plastic fuel filter with metal one. Only you can prevent engine compartment fires.

  16. Almostdone
    Joined: Dec 19, 2019
    Posts: 636


    I know you said it just recently started, but as @indyjps asked, was anything changed beforehand?

    indyjps likes this.
  17. Jessie J.
    Joined: Oct 28, 2004
    Posts: 396

    Jessie J.

    Depending where and how long it sat, mice or chipmunks can pack exhaust and mufflers full causing drivability problems.

Share This Page

Register now to get rid of these ads!


Copyright © 1995-2021 The Jalopy Journal: Steal our stuff, we'll kick your teeth in. Terms of Service. Privacy Policy.

Atomic Industry
Forum software by XenForo™ ©2010-2014 XenForo Ltd.