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Tom Vaught 11-15-2017 11:54 PM

I am going to try to list a number of solutions to try when working on a Holley Carb.

I will start with the initial engine start or lack of engine start with the carb installed on the engine.

1) Engine will not start:

a) Prime the Carb with fuel
(easiest way to do this is (with a squirt bottle of fuel) by squirting fuel down the vert tubes on the front and rear bowls).

b) Check the Float Level. Using a normal Holley fuel bowl, the fuel should just be BELOW the Hole threads on the fuel bowl with the plug removed.

c) Check the ignition system for spark to the plugs.

2) Backfires thru the Carb

a) Check the Ignition Timing

b) Turn OUT the Mixture Screws

c) RAISE the fuel Level in the Bowls

d) Adjust the Throttle Plates for a "square opening below the throttle blades". Readjust as required.

e) When all else fails, go up two jet sizes on the Primary Main Jets.

3) If the engine has "backfires" thru the Exhaust System

a) Check for Proper Ignition Timing

b) Turn in the Mixture Screws 1/4th of a turn and see if backfires go away.

c) Lower the Float Levels in the Fuel Bowls

d) Adjust the Throttle Plates for a "square opening below the throttle blades". Readjust as required.

e) Lower the size of the Main jets on the Primary side of the carb.

4) If the Fuel Level in the Bowls cannot be set properly.

a) REDUCE the Fuel Pressure to the Carb

b) Remove the Needle & Seat Assemblies and check for grit/foreign material stuck on the needle or seat.

c) Move the Floats thru full travel and check for binding or the Umbrella seal for the Accelerator Pump has not been trimmed to shortest height
(Allows fuel travel of the float).

5) Fuel Shoots out of the Vent Tube on the front or rear bowl.

a) Lower the fuel level in the Fuel Bowl.

b) Remove the Needle & Seat Assemblies and check for grit/foreign material stuck on the needle or seat.

c) Lower the fuel pressure to the carb

d) Move the Floats thru full travel and check for binding or the Umbrella seal for the Accelerator Pump has not been trimmed to shortest height
(Allows fuel travel of the float).

That concludes the initial fire-up checks on the carb. More on the next steps (Engine actually starts and runs) on another post.

Tom V.

Squidward 11-16-2017 12:20 AM

Nice, thanks! Waiting on part 2!

4zpeed 11-16-2017 12:30 AM

Jesus jumpin crimininies... CLIFFFFFF !!! I need a friggen Quadrajet !!!

Just kidding Tom, thanks much, subscribed, "Waiting on part 2".

Edit -:popcorn:


Tom Vaught 11-16-2017 08:15 AM

More Solutions: (The Next Several "Hints" will have a lot of duplication because the same "Items" can cause the Issue with the Holley carb.

6) Idle Mixture Screws 'Do Nothing'

a) Reset the Throttle Plates in the Primary bores
(Adjust the Throttle Plates for a "square opening below the throttle blades". Readjust as required.)


c) Lower the Float Levels

d) Reduce the Fuel Pressure to the Needle and Seat

7) Fuel Leaks from the Throttle Shafts/Carb Base

a) Lower the Float Levels

b) Reduce the Fuel Pressure to the Needle and Seat

c) CLEAN or REPLACE the Needle and Seat Parts (Primary & Secondary)

d) CLEAN THE AIR BLEEDS (Fine wire and carb cleaner for a rinse)

e) Reset the Throttle Plates in the Primary bores
(see above for setting).

8) Fuel Dripping from the carb 'Boosters'

a) Lower the Float Level in the fuel bowl

b) Reduce the Fuel Pressure to the Needle and Seat

c) Reset the Throttle Plates in the Primary bores


a) Check Choke 'Fast Idle Cam' to see if holding the throttle blades open.

b) Reset the Throttle Plates in the Primary bores

c) CHECK the INITIAL IGNITION TIMING Distributor may be in wrong position or weights hanging up/too light.

d) Check THROTTLE LINKAGE for improper adjustment of Carb Linkage and Trans Linkage

e) Check 'Dash Pot' (if installed) for sticking

f) Check for VACUUM LEAKS

10) Engine Dies below 1200 RPM

a) Check IDLE SPEED SCREW for setting to high

b) Check Throttle Blade Position

c) Check/Lower the the floats in the bowls

d) Check Ignition Timing

e) Check Idle Mixture Screws for One and a Half
turns out from seated on '2 corner' idle circuit carb

f) Check Idle Mixture Screws for 3/4 turns out from seated on '4 corner' idle circuit carb

11) Engine refuses to stay running


b) Reset Primary Throttle Blades for square window at Transfer slot (Blades may be too low)

c) Open up IDLE EZE screw farther

d) Check Idle Mixture screws (see above)

e) Lower Fuel Levels in Bowls

f) Verify Ignition Timing

12) Spark Plugs show excessive RICH Condition

a) Check Throttle Blade Position

b) Check Mixture Screws for 'too many turns out'

c) Lower fuel Level in fuel bowls

d) Reduce Fuel Pressure

e) Check for idle air Bleeds plugged (run fine wire thru them and then spray with carb cleaner - wear face shield or goggles)

f) Bump up the ifnition Timing a couple of degrees.

(More Tomorrow)

Tom V.

1968GTO421 11-16-2017 11:23 AM

Thanks, Tom, for writing this thread. Despite using Holleys for 40 years, there is always info to learn and re-learn. Appreciate it!

carbking 11-16-2017 07:56 PM


And while you have directed it toward Holley users, would guess at least 90 percent of your comments are also applicable to other makes.


Tom Vaught 11-16-2017 10:36 PM

Will post more on the Holley Tips tomorrow.
Most of the info will not be on how to tune a Holley or the specific circuits and what they do but on fixing issues with your Pontiac combination.

Tom V.

Tom Vaught 11-17-2017 09:14 AM

Good Morning.

I was thinking about Del Forest (A WW-II Mechanics Instructor and My Uncle's Chief Mechanic). Del was a smart guy and a very good instructor.

One of the first things that he taught me was to warm the engine up and VERIFY THE IGNITION TIMING IS CORRECT before messing with the carburetor. He also was big on VISUAL INSPECTIONS of the engine vacuum lines before messing with the Carburetor. He had inspection mirrors so he could look at all of the connections before he removed any of them.

He would find a lot of times that a hose was disconnected, hooked up to the wrong port on the carb, or was bleeding signal from another vacuum device. (Example PCV LINE teed into the Brake Booster Line). This would cause both the Brake system and the Carburetor systems to act strange.

He could build FLAWLESS Rochester Q-Jets that ran perfectly (even on the early carbs).

All that being said. He would many times find an issue where the actual carb on the engine was not the issue. Fuel Pump issues, wrong gas cap on the fuel tank, damaged fuel line going to the fuel pump/engine. He was big on getting the vehicle in the air (Hoist) and visually inspecting the entire fuel delivery system BEFORE messing with the carb. He also typically did a "pre-repair" test drive to see how the vehicle actually drove vs what the customer (or his friendly neighbor) thought the problem was.

He had no issues with adapting to the 70s emissions requirements and plumbing because he was a "Systems" type guy even when teaching in WW-II classrooms.

So my point for today is don't just start pulling off hoses, changing parts, blaming carburetors, before you actually know what the real problem is. Swapping Parts over and over is the last thing you want to do.

Have a great day and a great week-end.

Tom V.

Sprocket 11-17-2017 04:26 PM

Thanks, the last post there is super advice.

Tom Vaught 11-18-2017 06:36 PM

When I first began to learn about Carburetors, Holley Carburetors, from the people who actually designed them in Warren Michigan I was fortunate to see in operation how the carbs were tested for Air Flow and Fuel flow (using Stoddard Solvent). Even though Holley had a 3000+ cfm Air Flow Bench (first built to flow the massive Carburetors used on the Supercharged Aircraft engines built during World War II), Holley preferred to use 8 Flow Benchs that were hooked up to a 8 sliding drums with a precise Constant Volume, Constant Pressure, Constant Temperature design.

The Technician raised the known volume drums by filling them with temperature controlled air until they were fully filled to a precise pressure.

At that point, the carburetor was already installed in a enclosed air chamber and had fuel (stoddards solvent) flowing into the carb fuel bowls. The throttle blades had been set by the Technician for the .020" window in the transfer slot and the choke was fully open.

So then the Technician released the upper drum and for 1 minute it dropped slowly downward and then he read the idle air flow in one minute.

Typically, for most Holley carbs, it was in the 16 to 19 cfm range on the Primary Blades. The Secondary Blades were barely cracked (not touching the walls of the carb base plate). Usually this .020" Throttle Blade setting also matched up with approximately 1-1/2 turns of the Primary Throttle Adjustment Screw.

So the drum was raised several times and the test repeated then the Technician moved on the the next checking point on his test sheet.

After multiple checks, at different Throttle Blade settings, the Technician would have data which could be plotted on an 'airflow vs fuel flow' graph. They did this work for both initial designs for new carbs and for verifying carb production build quality control. Samples of each carb series produced in that month were sent to Warren Michigan for verification of the flow numbers. Holley also had smaller flow stands at the plants for spot checks during production. I observed hundreds of tests but never actually performed a test. That was the Technician's job.

At the end of the day the Carb specs for a given carb model would be checked against a Master Build Carb Sheet with the flow numbers of the Master Carb vs the 'Tested Carb'. Master Carbs were considered to be Perfect Carbs in all circuits of the carb design.

So when you bought a 3310-2 Holley Carb or a 4779-2 Holley Double Pumper Carb, or a 4412-2 Holley 2 BBL you knew exactly what you were getting. In many cases you did not need a whole box of Holley carb jets to dial the thing in for your application because 4 set sizes larger or smaller than what was installed in the carb (stock) typically covered all installations.

All that being said, Holley also had a Test Track to test the carbs on given production vehicles.

They had a Engineering Building with Hundreds of Engineers working in the buildings. They had Engine Dynometers and Chassis Dynomometers as well as two Emissions and F.E. Chassis Dynometers.

Then you had the Calibration Engineers who worked with the OEM people (In those days: Ford, GM and Chrysler) doing constant test drives on vehicles (with the OEM Engineer) using Horiba Air/Fuel Measurement tools costing thousands of dollars (at the time). I still have one of those old test set-ups.

From the web: "HORIBA Automotive Test Systems is a leading supplier in the fields of engine test systems, driveline test systems, brake test systems, wind tunnel balances and emissions test systems. More than just the world's leading supplier of emissions testing systems, HORIBA ATS is able to provide total solutions to its customers, with full turnkey capability."

So everyone who wishes to make their Holley Carburetor better vs the stock piece out of the box (old OEM calibrations) better be on their game.

Have a great weekend!

Tom V.

Tom Vaught 11-19-2017 12:34 PM

Today I would like to post some information about the different Holley carburetor systems and how they inter-relate to each other.

A Holley 4 BBL carburetor has two idle circuits (minimum) in each carburetor (one on each side of the primary metering block).
To get fuel (and air for one of those circuits) the following has to happen: Fuel flows from the fuel pump to the Primary Float Bowl thru the Primary Float Bowl Needle and Seat. The Needle and Seat sets the fuel level in the Primary Float Bowl. That fuel level setting is important and should be one of the first things checked if the engine will actually run at idle.

Then the fuel goes thru a Idle Feed Restriction (IFR), with an orifice typically in the .032" to .039" size. This restriction can be enlarged slightly but typically cannot be replaced easily without drilling and installing a new IFR brass bushing. .032" for smaller 585-600 cfm carbs and .038-.039" for 850 cfm carbs.
The IFRs are mounted at the bottom of the main well and idle well channels in most older Holley carb metering blocks.
Fuel goes thru the main jet, into the main well, thru the IFR, and then fills the Idle Well Channel.
When the idle circuit is active, the idle fuel is drawn upward past the idle air bleed and then turns downward as an air and fuel mixture toward the bottom of the Transfer Slot Passage. The Throttle Blade is opened only .020" (if set correctly) so very little idle fuel can pass thru that square opening. Most of the fuel takes a separate route past the Idle Mixture Screw and down the Idle Discharge Passage.
Usually with a 'two idle circuit' carb, the Idle Mixture Screw ends up being about one and 1/2 turns out from being seated.
This idle system seemed to work great for many years.

Some modifiers decided to make the idle fuel distribution more equal at idle and put a Primary Metering Block on the Secondary side of the carb. They duplicated the Idle Discharge Passage on the Primary side of the Carb on the Secondary Side of the carb. This worked well as a modification but Holley was already supplying a small amount if Idle Fuel thru a couple of fixed small holes from the Transfer Slot Circuit to the intake. This original circuit was designed to keep fuel in the secondary bowl from going "Sour" when the driver never used the Secondary barrels of the carb. The tiny hole constantly removed a slight amount of fuel from the secondary fuel bowl and added it to the idle mixture in the intake manifold.
So a 3310-1 carb in reality is a "4 corner idle" carb, except that you can't adjust the idle mixture on the secondary side without drilling out the idle discharge passage holes.

So about 1978, 'young' Holley Engineers decided that they needed to have idle circuits on all 4 corners ADJUSTABLE. So they tried to do the mods like the smart 'Modifiers' and drill the proper circuits. I saw a couple of their carbs with holes drilled right into the main venturi bores of the carb. OOPS! But they figured it out.
So about Carb list number 4779-6 (-6 being the revision number of the carb) the 750 cfm Double Pumper became a 4 Idle Circuit carb (that was ADJUSTABLE. So now the "Turns Out" of the Idle Mixture Screws went from 1-1/2 "Turns Out" to 3/4 "Turns Out" as the idle fuel flow requirement stayed the same but the Idle Mixture instead of being divided by 2 (TWO Idle Mixture Circuits) went to 4 (FOUR Idle Mixture Circuits). Best deal is to TRY to set each idle circuit screw the same number of turns out because if you have one screw at 1.5 turns out then a different screw has to be barely turned out to have the proper idle mixture in the intake.

With a divided (dual plane) intake you can adjust the mixture so that the front right barrel is 1 turn out and the rear barrel is 1/2 turn out but the mixture blends together in the manifold so really you did nothing except play games with the idle screws. THIS IS ON A 4 CIRCUIT IDLE CARB. A normal two circuit idle carb on a dual plane intake has ONE idle screw controlling the mixture for those 4 cylinders and changes in or out of the screw DO make a difference.

With an Open Plenum intake the cylinder that is on the intake stroke grabs air and fuel from anyplace in the plenum it can find it.
Like a clothes washing machine with water dashing in all directions over and over at idle.

So my point is if you have a open plenum intake you really do not need a 4 corner idle circuit carb, a stock 3310 2 corner idle carb will work fine. It will have the low location Idle Feed Restrictions and will supply liquid fuel to the top of the metering block where the idle air bleed will add air in the proper manner. The newer "High Location" Idle Feed Restrictions are another "better idea" that in reality made the idle calibration more difficult to tune. "Shaker455", "Tuner" (from several carb boards), and I always convert carbs back to the low mount idle feed restriction location. And then the carb works like it was designed to work in the old 3310 carb days.

So that is a explanation of the idle circuit, how it works, what a 4 corner idle circuit is, how it works, and why you should have a Idle Feed Restriction and Idle Air Bleed in the locations they are in. By the way a 3310 carb has an Idle Air Bleed in the .070" hole range. a .002" drilling change WILL CHANGE the idle circuit air/fuel ratio.

Tom V.

4zpeed 11-19-2017 03:14 PM

While I'm privy to the fundamentals, it optimizes my understanding as you bust out the Crayola's, certainly helps richen the picture. :juggle:

I don't stay idle much but when I do, its always nice to come here and get the right mixture of info. :D

I'll stay tuned up, and in, for "the rest of the story". :popcorn:

Again, thanks much!

Tom Vaught 11-20-2017 08:25 AM

1 Attachment(s)
A good friend of mine, TUNER, from the Washington State area posts a lot about carbs on the different websites.

He has posted on Speedtalk several times.
A friend of his Mark Whitener (goes by 'jmarkaudio' and Tuner have helped a lot of people on the different boards. Like I help the Boost Guys on this board and as a Moderator on the Turbo Forums board.

So Tuner was having a discussion with some guys on Speedtalk and a lot of misinformation was being posted by people who did not really understand how Holley Carb Circuits work.
I will include a picture from the thread that shows the drillings in the typical carb mainbody and what they do and the circuit they control.

Today's discussion is about the transfer slot, the Transition Circuit, the idle circuit, and a little bit about the main (booster) circuit.

First off, one of the guys in the thread attached
thought that the Power Valve Circuit would affect the idle circuit.(Blown Power Valve deal).
As was mentioned in the thread, by Tuner and others, the Power Valve circuit feeds the Main Circuit with extra fuel for Maximum Power needed by the engine. The Main Circuit is used for the vehicle "CRUISE Mode" (Main Jets).

Normally the Primary's Blade Angle of the carbs throttle blades will travel about 22 degrees before the Main System starts flowing. The Throttle blades already have some angle to them so the total travel to WOT is not 90 degrees but about 82 degrees. So 22 degrees is roughly 1/4th of the blade travel (Throttle Opening) before the main circuit starts working completely. Before that point you are in the Transfer Slot circuit which is the discussion for today.

As was said, the Blade angle on the carb should have the Transition Slot covered except for about a .020"-.040" tall window for the slot to be exposed. Most of the fuel is being fed to the idle circuit and its discharge hole in the base of the carb. Between the Idle Feed Restriction and the Idle Air Bleed and the Idle Mixture Screw you can dial the air fuel ratio to a nice 13.5 to 14.5 air fuel ratio at idle. The engine on a street car should idle about 600-700 rpm smoothly and a engine with more power should idle about 1000 rpm (CLEANLY) with a bigger camshaft installed in the engine.

If you remove the Accelerator Pump Lever on the Primary side of the carb and carefully open the throttle blades you can track on your air fuel meter exactly when the main circuit starts to take over from the Transition Circuit. (What the rpm point is, what the air/fuel ratio is, what the air/fuel ratio is when the main circuit starts. These are all little tests that a Holley Carb Engineer does on an actual running engine.
Reinstall the Accelerator Pump lever and you can note what the air/fuel ratio is under a light "Tip-In" of the gas pedal is. Do this a bit and you can tune the Pump Shooters using a air/fuel meter.
Just enough fuel for clean transition between the idle circuit and the main circuit.

So back on the post on Speedtalk.
One of the people there thought that that if the power valve failed it would affect the idle of the engine. As was posted in the thread two completely different circuits. Tuner posted that you could totally remove the main jets (which are much larger vs the Power Valve Channel Restrictions and the engine would still idle fine.
I personally have not done that test as I already know that they are on different circuits.
But read the thread on the Speed Talk link and learn from some real good carb guys like Tuner, jmarkaudio, and others. Mark Whitener is very good with Dominator Carbs. Somewhere is a link to a carb business he runs on the side. Shaker455 also does a nice job on carbs.

So there are literally millions of thread posts out there on how a Holley carb works, some are fact and some are confused mistakes, but on the whole if you look long enough and talk to the right people you can get some good Holley information. That being said, and I posted this before in other threads, at one time Holley bought carbs thru secondary sources in the 70s of about 30 carb modifiers. Of those 30 modifiers, 3 actually made some slight improvements to the carbs for a given application. The other 27 modifiers stuff was for the most part, SMOKE & MIRRORS. That being said, this is not the 70s, we have the internet and some great people passing along accurate info of stuff that works on a carb system.

Have a great day.

Tom V.

steve25 11-20-2017 09:04 AM

The absolute last thing to confirm on ANY Carb before you bolt it on is if the Needle (s) are seating!

Flip the Carb upside down and blow air into it from your Mouth , if you hear air leaking into a fuel bowl then you have a Carb the will flood out on you as soon as the fuel pressure comes up.

Tom Vaught 11-20-2017 12:08 PM

Agree, Steve. Do not want fires from flooded Bowls.

Being an old timer, Before I bolt the fuel bowls on, I always check to make sure I cut the installation orange rubber stem off the accelerator pump check valve. It barely let any fuel into the fuel bowl. Everyone makes a mistake due to brain fade occasionally.

Tom V.

Tom Vaught 11-21-2017 01:33 PM

Kind of busy today. Ran across a Hot Rod article I had read at one point on Power Tour carb tuning.

Holley had one of their Technical Guys tune several participants engines during the trip.
Basically the tech adjusted the carb for best cruise and f.e. as well as improving HP of the engines.
Using a air/fuel meter, basic tools, and "carb sense".
The Link is here:
A Holley Air/Fuel Meter was used as well as a timing light and carb parts were replaced as required to improve the results.

The reason for this post and the link is to provide some documented actual road numbers for people on a "cruise mode" situation which is the topic for today plus have people identify basic carb/engine screw-ups on the different vehicles. Have fun. There are several owner screw-ups in the article.

Tom V.


Excellent read.

Tom Vaught 11-21-2017 11:20 PM

The owner issues I identified were:
a) Not enough timing on one vehicle for a clean combustion process.
b) Need to replace a metering block after a "carb expert' did his magic.
3) Throttle Blades in the wrong position causing issues with the engine wanting to die below 1000 rpm
4) Selecting the wrong power valve for the application (5.0" power valve changed to 6.5" then back to 5.0" valve after other changes made to the engine system.

There are several more but the point is even with a good air/fuel meter, years of experience, and in this case access to a wheel dynometer, the car still need to drive properly for that specific engine combination.

Lastly. F.E. was optimized for several combinations but some drivers would just like a great running engine and are willing to give up some F.E. for other attributes: (Horsepower, quarter mile times, aggressive driving.)

Have a nice evening and a great Thanksgiving, back after the Holiday on this one.
Next mode is Power Valve, what it does and where.

Tom V.

4zpeed 11-22-2017 11:08 AM

Just shows many factors come into play, right down to who's behind the wheel and preference. ;)

Like many others I prefer F.E. and H.P., in my youth two tunes a day were required to fill my obsession. :o

In Jacksonville NC there was a huge summer day/night atmospheric fluctuation, the little women and I would cruze the evenings. :cool:

A small adjustment for the cool night air would really make'er twist, car ran a lot better too! :D

Very nice Link, reminds me of going to class an getting to watch a movie :hooray:, have a great Holiday, an Thanks/for/giving.


Bruce Meyer 11-22-2017 12:04 PM

Great info Tom. This thread should be made sticky.

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