Close



Results 1 to 9 of 9
  1. #1

    Default High idle problem

    Hello all im trying to figure out where to start looking whats causing a high idle on my 1979 302, any ideas on what to look at? Im new to the carburetor world, thanks in advance

  2. #2
    FEP Supporter
    82GTforME's Avatar
    Join Date
    Feb 2013
    Location
    Alberta, Canada
    Posts
    4,855

    Default

    Is the high idle not kicking down on the choke? Vacuum leak? Is it the original or aftermarket carb?

  3. #3

    Default

    Its the original 2bbl carb but i plan on putting a 4bbl and i just bought it about 2 months ago, do the original 2bbl carbs have a choke on them?

  4. #4

    Default

    More than likely you are sucking air somewhere. Took me forever to remedy mine. It was a metal vacuum line that lays across the manifold and some how had a pin hole in it. Spray you some carb cleaner around the base of carb area and vacuum lines with engine running til you get a surge and that might help you narrow it down.

    Good Luck.

    Brant

  5. #5

    Default

    Quote Originally Posted by RazorbackGT View Post
    More than likely you are sucking air somewhere. Took me forever to remedy mine. It was a metal vacuum line that lays across the manifold and some how had a pin hole in it. Spray you some carb cleaner around the base of carb area and vacuum lines with engine running til you get a surge and that might help you narrow it down.

    Good Luck.

    Brant

    What he said

  6. #6

    Default

    First post to FEP. I had an identical issue with my 85 RS 5.0 and 84 GT 5.0. CO% and HC% and idle were also high with the idle and mix screws turned too far out/in by previous techs. The issue with both was the large vacuum hose running under the throttle linkage between the throttle plate and intake manifold areas. Hose was petrified on both cars from intense heat, the rubber slightly sucked in and leaking. The hose is approx 3/8" IIRC, and runs parallel with the carb/intake inlet to the area behind it. I tried checking both cars for leaks with propane and 2+2 carb cleaner, no luck. Pulling the carb for inspection plus looking all around the intake, where it meets the heads, carb, etc, the hose on both cars was pushed/sucked in allowing air to enter unmetered.

    A new hose and both cars ran better, but now very rich and stalling. I then readjusted the Idle Mixture and the idle speed.

    Repairing the leak dropped the idle speed to below 500rpm. Prior, the idle speed would not adjust below 800rpm before backing the adjustment screw out of contact with the throttle lever. Both were 5 speed cars, and IIRC, factory idle speed was approx 600RPM. Better still, the CO% and HC% dropped to nearly nothing at idle. I could lean the idle mix out enough to pass the idle emissions testing my state, this with all 4 cats and the air pump removed.

    Back then, they didn't to equipment checks for cats and emissions gear. Any un-metered air leaking into the combustion chambers from the intake area can wreak havoc on performance, especially on the EFI cars. Carb cars, people tend to start twisting idle-mix / speed adjustments, when that's not the issue at all. Hope this helps out.
    Last edited by RS_Capri_85_5.0; 09-22-2018 at 04:45 PM.

  7. #7

    Default

    Double Post; Mods please delete.
    Last edited by RS_Capri_85_5.0; 09-22-2018 at 04:41 PM. Reason: Double Post

  8. #8

    Default

    Hi RS,

    Have you checked with a vacuum gage yet? One of the best diagnostic tools to own. Not expensive either. Intake manifold vac should be north of 20" Hg. at 800 rpm if your engine is stock and tight. Anything less would indicate a vacuum leak. Like Razor says, carefully spray carb cleaner around the base of the carb. Best when somewhat cool as the EGR plate will be hot, and listen for idle improvement. After that, start disconnecting and plugging vacuum lines to isolate leaky circuits. I had a leaking air motor on my air cleaner that took me awhile to figure out. Car ran great without the air cleaner. Put it back on and whamo- poor idle and stumble when the throttle opened.
    W

    As always, "It ain't what you don't know that gets you, it's what you think you know that just ain't so."

  9. #9

    Default

    I used a carb spacer and it had a vacuum provision I did not see, it caused a big vacuum leak.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •