Truck is the high altitude/California emissions model, 318.

Carburetor has been replaced with a 4 barrel Holley 4160 I believe, although it might be a 4150, intake manifold was replaced to fit the new carb, nothing fancy though.

Truck was tuned and up and running fine when I was driving it weekly.

I recently had a prolonged illness and the truck sat up for two weeks straight. When I got back in it, it was idling extremely high, probably 1000rpm or so higher than normal, but also dieseling very badly at shut off.

I hooked up my vacuum gauge and timing light. Timing with the spark advance disconnected is about 10 degrees (I believe 8 is factory spec but it's an old vehicle), and the vacuum gauge showed steady vacuum at about 17Hg, down to about 6 or so Hg when I hit the throttle. It is not however jumping higher after it sinks to 6 or so.

I believe when I originally set it up it used to sit around 15Hg, jump down to 5Hg and then back up to 20Hg or so before returning to normal when the throttle was activated.

The throttle plate is sitting correctly, the plate for the secondaries is closed, and the choke plate appears to be moving correctly as well.

What else could cause a constant, very high idle as well as very prolonged dieseling at shut off?

I will attempt to re-tune the carb this evening but I'm worried I'm just compensating for a larger problem. I'm thinking of removing all of the unnecessary emissions/vacuum stuff and rebuilding the carburetor if I can't find the specific cause of the recent high idle/dieseling.

  • 3
    Try squirting water around the base of the carb and where the intake manifold meets the heads. If you hear a difference in how it's running, you could have a vacuum leak there somewhere. Dec 21, 2015 at 18:24
  • Could an exhaust leak cause anything like this? I thought I heard some hissing from my passenger side exhaust manifold around where the rail goes in at the back
    – E-Rock
    Dec 21, 2015 at 18:48
  • No, an exhaust leak would cause issues with fuel injected vehicles, but not with a carb'd one. It's not a good thing, mind you, but not going to cause you runability issues. I'm still thinking that 2-weeks of non-use would not be causing your issues. I'm wondering if there was an underlying problem which was exacerbated by the non-running status. You might also try filling the tank up to the brim with fresh gas, as this may help if there is a problem with the gas itself (gas going bad/water absorption), though I don't think that would cause the issues you're experiencing. Dec 21, 2015 at 18:54
  • 1
    Will do. I added some Sta-Bil and some Lucas just for the hell of it about a week ago when I first noticed the problem, but I'll try giving it some fresh gas as well. If I can't identify a vacuum leak this evening I'm going to simplify the vacuum fed emissions systems, rebuild the carb, and inspect for any other leaks, abnormalities etc. while I've got it removed. If I find a vacuum leak I'll let you know so you can post that as an answer. Thank you sir!
    – E-Rock
    Dec 21, 2015 at 19:52
  • Add some fresh gas to water down the additives. I've had issues when putting too much Sta-Bil on carburated engines. :)
    – race fever
    Dec 22, 2015 at 15:24

1 Answer 1


My first mechanic post. But were struggleing here many comments no answers.

Maybe it doesn't apply to this vehicle. Dieseling run after shut off is sometimes caused by the vacuum switch on the air intake not switching to cold air when the vehicle is warmed up. The switch is just a themastat that opens and closes a vacuum powered valve that controls where the air to the carb comes from. The air intake switch (gm vehicles) is typically on the snorkel part of the air cleaner. What happens when the vehicle is dieseling is the vehicle is pulling hot air from around the exhaust and this very hot air when mixed with fuel is very easy to ignite causing the dieseling. Just adjust the switch so the carb is getting outside air and not the air from around the exhaust manifold. There should be an adjustment screw at least all my older GM vehicles had an adjustment screw for the air intake switch.

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