1966 L36 with Holley 4150 idle surge

[Jerry S.]

After searching the archives without success, thought I'd present my situation and get some feedback. After car warms up, the idle at stop lights seems to surge between 300 to 900 RPMs and comes close to stalling. By applying a little gas I can keep the idle more consistent so it doesn't stall. Spoke to a Holley Tech and his recommendation was to readjust the idle mixture screws to 1 1/2 to 1 3/4 out and see if that helps. Wondering if anyone else has had this phenomenon and what worked to correct the issue.

[Dean N.]

Re: 1966 L36 with Holley 4150 idle surge

jerry,
i have sent you a PM
Dean Nelson

[Joseph S.]

Re: 1966 L36 with Holley 4150 idle surge

Jerry
Your issue may not be the carb at all. Check your engine vacuum at idle (leaks, engine condition) and/or your distributor vacuum advance.
There have been many posts about the "2" rule" and I have personally worked a few idle surging issues that were rectified by applying a vacuum advance rated 2" above idle vacuum. The vacuum advance should full in at idle.

Joe

[Joseph S.]

Re: 1966 L36 with Holley 4150 idle surge

Jerry
Your issue may not be the carb at all. Check your engine vacuum at idle (leaks, engine condition) and/or your distributor vacuum advance.
There have been many posts about the "2" rule" and I have personally worked a few idle surging issues that were rectified by applying a vacuum advance rated 2" above idle vacuum. The vacuum advance should full in at idle.

Joe

[Duke W.]

Re: 1966 L36 with Holley 4150 idle surge

As above, but it could also be a high overlap aftermarket cam.

How long has idle instability been present?

A L-36/68 with an OE equivalent cam and manual transmission should idle fairly smooth at 600 @ 15" in neutral and a properly functioning OE VAC that I believe is stamped 360 12 meets the Two-Inch Rule, providing a maximum of 12 deg. crank advance at 12" and higher manifold vacuum.

So you need to provide the ID data on your VAC, measure and report the start and stop points in "Hg with a vacuum pump.

Find the lowest speed that the engine will idle stably and report that speed and manifold vacuum as xx" @ yyy engine speed. The idle speed/mixture adjustment procedure is in your CSM. Also check and report total idle advance (VAC connected) and whether it is stable or jumping around, and if so state the min/max range

If the VAC proves to be a problem either because it has failed or is not properly matched to the engine's idle behavior a different ten dollar VAC will likely cure the stability problem.

Duke

[Jerry S.]

Re: 1966 L36 with Holley 4150 idle surge

Thanks so much for all the valuable input. I probably should have stated that I am only a novice auto mechanic at best, but love to tinker and learn more about these amazing classics. So far I've been able to make minimal adjustments to the Idle mixture screws and that seems to have helped. Did this without the vacuum test but will do a test drive today and confirm with the vacuum test once i get my Amazon delivery. Also found a small leak on the bottom driver side front of the primary metering block that I was able to fix by tightening the phillips screw on the bottom plate.

[Jerry S.]

Re: 1966 L36 with Holley 4150 idle surge

So I have finally had time to run a vacuum gauge connected directly from my manifold. I'm getting good readings except for the fact that the needle jumps spastically between about 12 - 20 Hg. Thinking that about 16 Hg is where I need to be. The adjustments I made to the 2 idle mixture screws seems to have help with my idle surge and maybe just a little more tweaking to get it perfect. Will be interested to hear from Dean if the rebuild did what you wanted it to. I probably have a rebel in my future.

[Mark E.]

Re: 1966 L36 with Holley 4150 idle surge

Jerry,

If vacuum "jumps spastically between about 12 - 20 Hg" at idle, then there's an issue. It could be anything from a vacuum leak somewhere, an ignition miss, restricted idle air bypass or idle circuit, to a weak cylinder (due to poor compression, fouled plug, bad plug wire, stuck valve, leaking valve, broken valve spring, etc.) or restricted exhaust. I would start by checking the ignition, then for vacuum leaks.

Does vacuum vary almost randomly? Then it's likely something affecting all cylinders (e.g. vacuum leak, carb issue, exhaust).

Does vacuum vary with half-speed periodicity? This points to a weak cylinder or two. You can do a quick check for weak cylinders with an IR gun and/or cylinder balance analyzer if you have access to one. Then follow up with a compression or leak down test as needed.

There's an art to interpreting vacuum... check out Google and YouTube for some quick lessons.

[Duke W.]

Re: 1966 L36 with Holley 4150 idle surge

Jerry,


There's an art to interpreting vacuum... check out Google and YouTube for some quick lessons.

Google and YouTube are worthless. Their generic "information" and the color lines on some vacuum gages are totally misleading to the point of being misinformation that should be banned by Facebook and Twitter.

They will tell you that your PERFECT running 327/365 HP that pulls 10" Hg at a lopy 900 idle speed has a serious problem, which is pure bunk. I've listed typical idle behavior for virtually all OE engines through the C3 era and it ranges from 18-19" at a butter smooth 500 in neutral for base engines and optional engines with the base engine camshaft to 10" at a lopy 900 for 30-30 cam engines. Engines that were available with automatic transmissions should idle in Drive at the same RPM as a manual in neutral, but at a bit less vacuum due to converter load.

Valve overlap has a HUGE impact on idle behavior and vintage Corvette engines have overlap ranging low to WAAAAAAY too much.

I recall a guy on the CF who said he couldn't find what he was looking for on YouTube. I responded that what he needed was in the CSM, and he should buy one. He replied he "had one but didn't thing to look in it".

Duuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuuh!!!

And some guys wonder why I don't waste my time over there anymore. How did we ever manage fix anything without YouTube back in the 20th century?

Duke

[Dean N.]

Re: 1966 L36 with Holley 4150 idle surge

Jerry,
I got my Holley back from Custom Rebuilt Carburetors on N.J. and installed it today.
it works just fine. started it up and the cold idle is at 2100 to 2200 rpm just as it should be
let it get to 180* and took it off high idle and it went to 1000 RPM as they had set it.
started to turn down the idle screw ....900.....800....700 no surge......600 still fine no surge....
go for the big one.....500 RPM ....no surge and ran fine.
at 500 RPM it is real real low! I think it is to low so i put it back up to 600 RPM and left it.
after it sat for 1/2 hour I went to start it. did not step on the gas and turned it over, about two rev with the started and it fired right up.
it now runs like it is and new 327/300 HP car.

what they found in the rebuild was the meter block was bad and installed a new one, problem gone. they did not say what was wrong with it.

based on what it was doing (the surge) and it had been sitting for the last 6 years not ran, it took a total rebuild to solve this problem.
you might be well served to send it out to them and have a total rebuild. cost was $525.00 but now it runs as new and it looks brand new.

call me if you want to talk about it.

Dean Nelson......now mister 600 rpm!

[Jerry S.]

Re: 1966 L36 with Holley 4150 idle surge

Thanks Dean,
It sounds like you have solved your idle surge issue. I may look into sending my carburator for some work but would like to search my local area in Phoenix to see who is good with Holley rebuilds first. I may reach back out to you if I decide to use your N.J. shop.

[Timothy B.]

Re: 1966 L36 with Holley 4150 idle surge

Many times the primary metering block warps and does not sit flat to the main body and will not seal tight. Engine vacuum can suck air or fuel from places it's not supposed to causing these running issues.