Topic: Holley Idle Circuit Adjustment

[Angela]

Anyone have (or know of one in a book) a step-by-step process to adjust the idle circuit in a Holley 4150 for the '67 Mustang application? Based on my 67's behavior (described below),
I am suspicious of the idle circuit not being setup correctly. I'm not sure what steps to take to correct the problem. Help appreciated.

*Symptoms*: Car will start and idle quite well. Idles at ~800 rpm in park once the engine is warm. Putting the car into *any* gear will cause the car to immediately stall, unless you apply your foot to the accelerator. If you "nurse" the accelerator to keep the engine running while shifting the car into any gear, the car will then drive quite well at any speed. Coming to a stop, the car will again stall unless you shift into neutral. The car has an automatic transmission.

[preaction]

Did you go through the process of setting the choke properly, does the motor start and run at 1900 rpm when cold, and when warm idle should be 550 rpm.

[rockhouse66]

Though I doubt that modifying your Holley is needed to resolve your issue, there is a pretty interesting article in the current (Dec., 2015) edition of Hemmings Muscle Machines about making internal changes to affect the idle circuit.

[WT8095]

Before you make adjustments, make sure you don't have any vacuum leaks. Check the vac lines to the trans, brake booster, and distributor. Also make sure your vacuum advance unit is working properly, and hoses are routed properly.

How far out are your idle mix screws?

[67gtasanjose]

Before you make adjustments, make sure you don't have any vacuum leaks. Check the vac lines to the trans, brake booster, and distributor. Also make sure your vacuum advance unit is working properly, and hoses are routed properly.

How far out are your idle mix screws?

+1 FULL AGREEMENT on this suggestion.

Furthermore, I stand by my comment in your other thread on locating somebody who can tune it up correctly FOR YOU. Old Holley's can be temper-mental if not adjusted perfectly and pretty much, only OLD HOLLEY guys, know them best. Fuels and other conditions have changed so very much over the years, so "going by the book" usually will NOT result in a perfect operating machine on these old big blocks.

Although I understand your hesitation to take your car to somebody you do not know, I cannot help but believe there is NOT a good, reputable fuel-curve specialist in your area that can Dyno this beauty of yours into perfection WITHOUT even having to even drive it on the road. Sometimes the most minor adjustments in combination with other very minor adjustments, you get a car that not only performs as it was originally intended but often you get a car somewhat reasonable on fuel usage.

I consider myself as a good "Carb Guy". I grew up working on very many carbs through the mid 70's and all of the 80's into the 90's when virtually every car made no longer uses them. Good carb guys can be difficult to locate but they are out there.  it took me years and years to get GOOD at carburetors. There is no "Easy Answers" book out there to do a quick study that can teach you what takes years and years of experience to learn the ins and outs of on these old baby's. I do not own a Dyno but rest assured, even after I tweak my 67 289 4V to the best I can get it to, I am STILL taking it in to be dyno-checked next...I HIGHLY RECOMMEND THIS if you want the most satisfying driving car you can get. It is worth the money to pay the experts.

[Angela]

Wow, lots of questions...where do I start?

Well, the car idles around 1500 when cold, then drops to 1000 as it warms up and finally settles around 800 rpm when fully warmed up. Those rpm numbers are with the trans in park. I have no idea what the warm engine idle speed is with the trans in drive, since the engine stalls when I shift into any gear.

I am not sure how I check to ensure the vacuum advance is working.

I will have to check the idle screws and report what I find. I recall setting them to the values specified in the shop manual.

I don't believe I have any vacuum leaks. I have a good Lisle vacuum gauge and I believe I have good vacuum. If someone knows of a specific test I should use, please educate me. As I'm sure you can tell, I am not very educated in the area of these carbs, vacuum and related adjustments.

I appreciate the advice about asking a pro to help me with the carb. Believe me, I would love help. However, I have asked everyone I know and cannot find anyone who knows how to do this.

[67gtasanjose]

Here's an idea on how to take a quick test for a vacuum leak...
While the engine is running, hold your hand over the primary bores  of the carburetor (the ones with the choke plate) and with a cupping of your hand over the choke plate (my hand is large enough, small hands maybe not so much) try and choke the engine some and note any change in idle speed or quality. Does the engine pick up speed and seem to smooth out? Does it not make any change at all or run worse? These answers might be clues.

If your engine is warm already, sometimes you can do this with the choke plate too (once the choke heater has warmed enough and is no longer on the high-idle cam) Other times, I have folded a shop rag and laid it over the air intake (ONLY AT IDLE SPEED) to help determine if I might have a vacuum leak. As I said, the changes or NO CHANGE result in idle speed/quality help me to determine if the engine is running lean or running rich
which helps me to decide what might be going wrong. AN EXHAUST GAS ANALYZER TAKES ALL OF THE GUESS WORK OUT, but these suggestions I am talking about are old-school approaches in determining whether you have a rich or lean idle condition.

My guess is you are running lean, usually caused by a vacuum leak and if the carburetor has been built properly it is often at the base gaskets of these 390's, usually a wrong gasket or bad spacer plate but never rule out a brake booster (you can temporarily cap off any vacuum ports going to the booster or other items)

Like I said before, there are so many angles to start with, but to diagnose this, a person needs to know the basics of what might be wrong. The basic question: Is it RICH or is it LEAN.

What city & state do you live in? Maybe somebody here on the forum can come by and take a look.

[Angela]

Thanks for the advice; I will see if I can replicate your test procedure and report what I find.

Respectfully, I don't understand why people seem to believe my mixture is incorrect, simply because the engine stalls when I shift into a gear. Can someone educate me? The engine runs extremely well once the car is moving.... runs very smoothly and very strong.

Also, if I connect my lisle vacuum gauge, it indicates I have very good vacuum.... i.e. no indication of a leak. Hence I don't know why folks believe I must have a leak.

Again, not trying to be disrespectful, rather I am trying to learn.

[67gtasanjose]

Respectfully, I don't understand why people seem to believe my mixture is incorrect, simply because the engine stalls when I shift into a gear. Can someone educate me? The engine runs extremely well once the car is moving.... runs very smoothly and very strong.

...Also, if I connect my lisle vacuum gauge, it indicates I have very good vacuum.... i.e. no indication of a leak. Hence I don't know why folks believe I must have a leak.

A vacuum leak is not exactly detectable with use of a guage. A vacuum leak can be inside the carburetor too but "outside" sources of a vacuum leak MUST be ruled out first since they are more likely is all I am saying.

Experience is basically impossibly to teach, I don't think anybody feels disrespected about your requests. (at least not me)

What can be ruled out or ruled in with experience might take an experienced person only minutes or moments while under the hood, yet might not ever be capable of figuring out what is actually wrong through typing back & forth on a computer or over a phone call...THAT is the part that makes your request for information difficult.

An example: Nobody can teach my how to play a piano with a book either...some things require a certain "talent" beyond the simpler "understanding how it works" part of the learning curve.

[preaction]

What is your vacuum at idle ? Has the vacuum modulator on the transmission been known to be good?  Also I mentioned about the choke adjustment because I went through a very similar situation with a 390 GT engine with a automatic and it ran strong as yours but floundered at idle and at first in gear and the issue was the choke.

[WT8095]

Respectfully, I don't understand why people seem to believe my mixture is incorrect, simply because the engine stalls when I shift into a gear. Can someone educate me? The engine runs extremely well once the car is moving.... runs very smoothly and very strong.

Also, if I connect my lisle vacuum gauge, it indicates I have very good vacuum.... i.e. no indication of a leak. Hence I don't know why folks believe I must have a leak.

1) The carb has multiple fuel systems, each with their own mixture. There's the idle system, the main system, and the power system. The mixture needs to be correct on each of them, and the transition from one to the next must be correct to prevent stumbling or stalling. Incorrect mixture has been mentioned because it's a common problem, but it's too soon to declare it's what you're experiencing.

2) I have not seen anyone mention that you MUST have a vacuum leak. It's a common problem, and you will not be able to get your carb properly adjusted if you have a leak. What we're saying is that you need to make sure you don't have a leak before you go any further, or you will be wasting your time. Depending on location, a very small leak can have a big impact on how the engine runs. The worst is around the throttle shaft; at the mounting gasket would be significant too. A leak can cause a problem while still giving you a "normal" vacuum reading on a gauge. More importantly, let's say your gauge reads 17" of vacuum. What are you comparing that to? The engine may pull 18" without a leak, but unless you've taken readings previously, you won't know.

The classic way of looking for small leaks is to do a spray test. This involves spraying very small amounts of gas or atomized fluid near the suspected leak while the engine is running, and observing if the engine speed changes. It may increase, or it may decrease - either way indicates a leak nearby. The catch is, you're spraying something highly flammable on a running engine. DANGEROUS, even for those with experience. I usually use carb cleaner or starting fluid, but propane can be used too. Did I mention it's DANGEROUS?

[J_Speegle]

I usually use carb cleaner or starting fluid, but propane can be used too. Did I mention it's DANGEROUS?

Have used WD40 in the past for some of these investigations - lot less volatile

[Angela]

I haven't made any further progress on figuring out what is wrong with my 390 stalling (when put into gear) issue. However, to address the question some of you have raised about am I running rich or lean.... is anyone aware of a simple, relatively affordable rich/lean mixture meter? I think some manufacturers refer to these things as "exhaust gas analyzer". Those tools appear to be very expensive and I wouldn't need all the bells-n-whistles they offer.

I don't know any other way to tell if the 390 is running rich or lean, except for removing and checking the color of the plugs. In the case of a 390 in a vintage mustang, I'd rather go to the dentist daily than try and remove spark plugs.

[67gtasanjose]

...how to take a quick test for a vacuum leak...
While the engine is running, hold your hand over the primary bores  of the carburetor (the ones with the choke plate) and with a cupping of your hand over the choke plate (my hand is large enough, small hands maybe not so much) try and choke the engine some and note any change in idle speed or quality. Does the engine pick up speed and seem to smooth out? Does it not make any change at all or run worse? These answers might be clues.

If your engine is warm already, sometimes you can do this with the choke plate too (once the choke heater has warmed enough and is no longer on the high-idle cam) Other times, I have folded a shop rag and laid it over the air intake (ONLY AT IDLE SPEED) to help determine if I might have a vacuum leak. As I said, the changes or NO CHANGE result in idle speed/quality help me to determine if the engine is running lean or running rich
which helps me to decide what might be going wrong.

This usually works for me.

[Angela]

OK, so are you suggesting that IF the engine increases in RPM with your hand held of the choke plate, then the carb is set too LEAN?