Holley Idles Too High Even Though Adj Screw Backed Out

[Jack O.]

I have a Holley 4150 carb on a '71 LS6 intake on my '69 L71 until I get the original 3x2 setup rebuilt. Once the engine warms up it idles at about 1100-1200 even though the idle screw is backed completely out and is not engaging. I've completely removed the choke setup including the fast idle cam to rule that out and have looked down the carb to verify the primary and secondary butterfly valves are not open. I even disconnected the secondary's from the linkage to be sure it wasn't preventing the throttle from fully returning. Applying light pressure to the linkage to ensure it's returned as far as it can results in very little to no reduction in idle.

So, what are the most likely other culprits? Vacuum leak? I've checked all connections and checked around intake to head and intake to carb for leaks with carb cleaner in an attempt to find leakage. I'm pretty sure it's not leaking at the carb/intake. Timing too advanced? I do have it advanced quite a bit to try to get it to idle smoother. Any other ideas or likely causes?

Thanks!

Jack

[Timothy B.]

Re: Holley Idles Too High Even Though Adj Screw Backed Out

Jack,

One thing to double check is the secondary stop screw, you need to remove the carburetor to check this. On a vacuum secondary carburetor, the screw needs to be set to where the blades just don't stick in the throttle body so the screw just touches the secondary link. To properly set the carburetor throttle plates, adjust the primary blades so the transfer slot is just at the lower edge of the blade even to the point it's just bearly visable. Next, get a 8-32 set screw 1/2" long and replace the secondary stop screw with the set screw so you can adjust the screw with the carburetor on the engine. Set the idle speed using this screw and don't touch the primary screw then adjust the emulsion screws for best idle.

One other thing is the PCV valve as this will allow air to the engine at idle speed.

[Terry D.]

Re: Holley Idles Too High Even Though Adj Screw Backed Out

How far advanced is it? Unless you have a really radical cam advancing the timing to get it to idle smoother is not the correct fix.

[Jack O.]

Re: Holley Idles Too High Even Though Adj Screw Backed Out

I'd say 20 to 24 BTDC. I set it to 14 BTDC and sure enough the idle dropped enough that I had to engage the screw to increase it.

When I bought the car it had the original L71 427 in it but had 074 aluminum heads, a 6-71 blower with 2 AFB carbs and so the cam probably is fairly radical but I can't say for sure. I pulled the blower and am running the single Holley but because of the blower I know the compression is very low. I'm just trying to keep it running as good as possible until I get the time and will to rebuild the engine which is what it really needs. It runs really good at higher RPMs but not so well at lower RPMs and gets horrible gas mileage - and I mean worse than even an L71 Corvette should be getting.

Jack

[Duke W.]

Re: Holley Idles Too High Even Though Adj Screw Backed Out

Does the carb provide full time or ported vacuum advance? If full time and the VAC is the original 201 15, it likely doesn't meet the Two-In Rule. You can set everything up per my 2012 San Diego National Convention, or just deal with it until the 3 x 2 carbs come back. While they are off apart/apart you should convert the center carb internally to full time vacuum advance. The OE cam needs a 12" B26 VAC. The installed cam might need am 8" B28. Just measure manifold vacuum and apply the Two-Inch Rule.

Duke

[Stuart F.]

Re: Holley Idles Too High Even Though Adj Screw Backed Out

The vacuum advance line on my son's 69 L-89 "T's" into the choke pull off vacuum line at the center carburetor for full time vacuum. Now, perhaps the L-89 is different than the L-71 in this regard, but the connection looks to be completely factory made and I do not see any other vacuum ports available on that carb.

Stu Fox

[Duke W.]

Re: Holley Idles Too High Even Though Adj Screw Backed Out

That's the quick and easy way, but it's detectable in judging. It's easy to undetectably modify the carb. base when it's off the car. Plug one hole with lead shot or epoxy and drill another hole. Photos have been posted in the past.

Duke

[Jack O.]

Re: Holley Idles Too High Even Though Adj Screw Backed Out

Thanks Duke. Do you have a link or pointer to your 2012 setup you reference? Sounds like something I need to read. I'll have to check the things you mentioned. It's an aftermarket Holley and I believe I have it hooked into full time vacuum but sounds like even if I don't I should be able to try what you're suggesting by finding another vacuum source. For example, to test things out could I simply tee off the manifold vacuum? Right now it's only supplying the vacuum for the vacuum based components as I do not have PB.

[Jack O.]

Re: Holley Idles Too High Even Though Adj Screw Backed Out

I found Duke's document...should have looked before I asked

[Timothy B.]

Re: Holley Idles Too High Even Though Adj Screw Backed Out

Jack,

When the engine is warm how much vacuum does it make? What RPM does the engine want to idle at where it will not stall?

The camshaft is probably way to big for good low RPM manners, the solutions are usually lots of timing and carburetor tuning and it may still be hard to drive on the street.

[Steve D.]

Re: Holley Idles Too High Even Though Adj Screw Backed Out

Tim

For AFB/Edelbrock without adjustable secondary plates, is there an alternative way to adjust the idle speed without changing the proper setting of the primary plates?

Steve

[Timothy B.]

Re: Holley Idles Too High Even Though Adj Screw Backed Out

Steve,

You can experiment with PCV valves as the calibrated air leak at idle is different with different size engines and even use a fixed oriface PCV to regulate idle speed. I think that a certain point would be reached where you would need to do something different if the oriface size gets to big or to small.

In that case people have drilled small holes in the primary throttle blades to allow more air plus advance the timing a little more. This will allow the idle to speed up, keep in mind with any changes the emulsion screws need to be re-adjusted. In the other direction to slow the idle the reverse of the above holds plus if the throttle blades get closed and the transfer slots are covered the primary throttle blade can be filed to allow small transfer slot exposure so there is no stumble with throttle movement.

[Steve D.]

Re: Holley Idles Too High Even Though Adj Screw Backed Out

Tim

Thanks. Believe it or not, some of this is starting to make sense to me.

[Richard F.]

Re: Holley Idles Too High Even Though Adj Screw Backed Out

Is a blown power valve possibly causing some of this? Horrible mileage for example.

[Timothy B.]

Re: Holley Idles Too High Even Though Adj Screw Backed Out

Richard,

A blown P/V would allow manifold vacuum to suck fuel from the fuel bowl into the engine under the throttle plate and create a very rich mixture, the engine would have a difficult time staying running at idle speed.