I have always had a problem maintaining a reasonable idle speed. With the engine fully warmed up I will turn the idle speed screw to idle at say 1000 RPM, I am not too concerned with getting it down any further as I don't use a choke and if you slow it down too much it will kill when it is cold. Anyway after driving it a while the idle speed is up to 1600-2000 without touching anything. I have always had this problem with several different carburetors and a variety of cam shafts etc. I have tried shortening the return spring so much my foot hurts when I drive it to no avail. I can adjust the timing inside the car and that doesn't seem to make any difference. I generally run 28 degrees total advance plus vacuum on a Mopar Performance electronic distributor. Other than the erratic idle speeds the performance and gas milage are exemplery. Any ideas why the idle speed tends to increase? Idle quality is not the issue just why does it speed up for no apparent reason.

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If you didn't drive it there, it's not a street car.

Sure......a vacuum leak will do that.

How old is your motor? or since any changes to the intake gasket?
The carb hold down nuts could be loose as well leaking around the base or a cracked vacuum line.

Is this a stock motor with a stock cam?

What kind of a rig is this? Super Six? What year motor? 1920 or 1945 carb? etc...

On my stock 1980 SL6 barn find, it will idle down to 500 rpm in gear and never miss a beat for hours...sitting in traffic.

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Aggressive Ted

http://cid-32f1e50ddb40a03c.photos.live ... %20Swinger


74 Swinger, 9.5 comp 254/.435 lift cam, 904, ram air, electric fans, 2.5" HP2 & FM70 ex, 1920 Holley#56jet, 2.76 8 3/4 Sure-Grip, 26" tires, 25+MPG

I would try a different intake manifold just to take that out of the possibilities.Maybee when the manifold gets really heat soaked there is a porosity or hair line fracture ya dont see.sure sounds like a vacume leak.Mark

I too have had this quirk..........enough to drive me up a wall. I have always found it to be either a vacuum leak a bad idle adj. screw or a wacky choke pull off. If you are still running one. I think it's so hard to find because there are so many things that can make it happen.
I have had the same problem with Holley carbs on 273's
They have all been vacuum leaks! I usually take a can of something flammable and start spraying with the engine cold and warm. Well, not really flammable WD will do. So will atomized water.

I know this probably didn't help a bit cause I still don't know. Last time I scored another carburator and have been living happily since.
max

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1966 barracuda 360
1964 signet conv. 170 turbo
Bonneville or Bust 2012

If it isn't a vacuum leak (check as was suggested) it might also be a old, nasty, rusty distributor. I had problems with a rig once where the advance would, well, advance when driving down the road--there was so much rust and goo in the distributor that the advance would not retard quickly (once I got off the throttle). Now, if I stopped the engine and left it overnight the advance would end up back where it belong. Drove me CRAZY! Finally got a hint from the proverbial "old guy." Pulled the dizzy and cleaned it up and lubed it...problem solved.

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Slanted D150

Why not install a choke and then set the idle properly?
The "all in, right now" mechanical advance on the MP distributor makes it difficult to get a consistent idle speed -- sounds to me as if your initial 1000 rpm idle speed might be right on the edge of where the weakly-sprung mechanical advance in the MP distributor starts to dump in.

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Too many people who were born on third base actually believe they've hit a triple.

Dan:
[quote]The "all in, right now" mechanical advance on the MP distributor makes it difficult to get a consistent idle speed -- sounds to me as if your initial 1000 rpm idle speed might be right on the edge of where the weakly-sprung mechanical advance in the MP distributor starts to dump in.[/quote]

I have to agree with Dan. Take a look-see under “Engineâ€￾ > Sticky: “Timing Curve/ etc.â€￾ you will find several articles with a graph or two by others, and a trial & error process of how I was able to settle down an idle problem with guidance from Aggressive Ted.

Once I was able to keep the mechanical advance from kicking in before 1600 rpm, my idle stabilized, the idle mixture became tunable, and I was able to back off the idle stop screw which eliminated a nasty flat spot during throttle opening off of idle.

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67' Dart GT Convertible; the old Chrysler Corp.
82' LeBaron Convertible; the new Chrysler Corp
07' 300 C AWD; Now by Fiat, the old new Chrysler LLC

Check with carb spray at the gasket between the throttle body and the base plate if this is a Holley 2 barrel or 4 barrel carb, these do warp and cause intermittent leaks after years of use.

Did you respring the distributor for daily driving use or is it still set up for throwing the Seymour Special , down the track? Possibly the dizzy mechanism might need to be cleaned out and regreased if you've had it together for quite awhile, and some things is sticking.

-D.Idiot

I have got the 350 Holley all hooked up now but I haven't started it or driven it yet. I will do that this weekend, a vacuum leak that only opens up under certain conditions sounds like a possible explanation. I will try the carb spray test when the idle speeds up again which I am pretty sure it will. I don't think the problem is ignition advance related because I can advance the spark 10 degrees or retard it 5 degrees while it is idling and it doesn't seem to change the speed much. I don't think the idle speed is as sensitive to advance when it is over 1000 as when you bring it down to a minimal level. I have used several distributors through the years and have always had this same problem. Thanks for the ideas everyone.

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If you didn't drive it there, it's not a street car.

Different carbs,different distributers,,,,same manifold?Curious Mark

Exner Geek,
You may have a bigger vacuum leak than you think if your setting the timing at 750 rpm. It should drop down allot when you retard - 5 degrees. That will significantly reduce engine rpm on my motor. If it was between 5 degrees and 10 degrees advance, I could see your point.

My engine idles at 600 rpm in gear with everything on in a rain storm. It will idle at that speed for hours with out missing a beat. Every time I get stuck in a traffic jam, I prove that........which includes the electric fan kicking on as well. My engine is set a 10 degrees advance at 750 rpm.

Your right about it not being sensitive above a 1000 rpm, but not many folks see that rpm at idle unless they have a racing cam or a poorly tuned stock motor with major vacuum leaks. The idle mixture circuit is not functioning as much at that point because your drawing from the main jet due the throttle blades being open.

What intake manifold are you running and how many times have you had it off? What manifold gasket are you using?

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Aggressive Ted

http://cid-32f1e50ddb40a03c.photos.live ... %20Swinger


74 Swinger, 9.5 comp 254/.435 lift cam, 904, ram air, electric fans, 2.5" HP2 & FM70 ex, 1920 Holley#56jet, 2.76 8 3/4 Sure-Grip, 26" tires, 25+MPG

One more thought on vacuum leaks: I emptied two cans of carburetor cleaner over the course of many leak detection sessions and was unable to detect a massive vacuum leak at the base of my 390 Holley.

Not until I had removed the carburetor from the manifold, and placed a straight edge at several locations spanning the base did I figure out the base had quite the whoop in it. What would happen as I spread one side of the base, the other three sides would just pick up the sucking slack. In other words I could not spray enough carburetor cleaner on all four sides, at once, to get the engine to react to a new air-fuel mixture.

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67' Dart GT Convertible; the old Chrysler Corp.
82' LeBaron Convertible; the new Chrysler Corp
07' 300 C AWD; Now by Fiat, the old new Chrysler LLC

In answer to some questions Ted brought up, I am not really trying to get it to idle slow, I like to have it at 1000-1200RPM. I am not using a choke or any fast idle features of the carburetor so it has to run that fast hot for it not to kill when cold. I have always found things like chokes, parking brakes, gas gauges, speedometers, etc. to be more trouble than they are worth, just me I guess. It does have a racing cam but it is on the mild side 260 degrees and .450" lift.

The real problem is I can't keep it at 1000-1200 even when I set it there when fully warmed up. Pretty soon it will be idling at 1600-2000, some kind of an intermitant vacuum leak would seem to make sense so I will try to track it down when I start it up this weekend. The manifold is an Offenhauser with a 1" spacer and a Clifford 2BBL adapter on top. At other times I have run a 4 BBL with the same idle problems. I usually use the manifold gaskets that come in gasket sets, I have used Detroit, FelPro, Rol, and Corteco through the years. I had one of the thick green Cox Brothers gasket on it when I was using my big valve head. I haven't had the manifold off this year but when I do it always leaves a nice even impression on the gasket. I try to be careful to not over tighten the mainfold nuts. I run Clifford shorty headers and they also seem to seal very well, no soot on the block like I often see on the race car.

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If you didn't drive it there, it's not a street car.

Exner....,

That is a good reply with lots of detail. Sounds like your being careful however, that much variation is annoying. It might be in the stack up.......only careful checking like wjajr mentioned will confirm it.

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Aggressive Ted

http://cid-32f1e50ddb40a03c.photos.live ... %20Swinger


74 Swinger, 9.5 comp 254/.435 lift cam, 904, ram air, electric fans, 2.5" HP2 & FM70 ex, 1920 Holley#56jet, 2.76 8 3/4 Sure-Grip, 26" tires, 25+MPG

You should be.
That's wasteful and silly. Sorry.
That's even silier. Set it up right.
Yeah, I guess. Most of us don't randomly discard useful equipment from our cars, and -- whaddya know! -- they run correctly.

It's your car, so you get to make whatever bizarre choices you want to make, but this what you're doing kind of takes away most of your room to be complaining about how the engine runs.

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Too many people who were born on third base actually believe they've hit a triple.