57 idling issues. dies at below 2000

[PaxAmericana]

I recently rebuild the Holley carburetor (600 CFM 4160, hot air choke, vacuum secondary) on my '57 t-bird. The car was non-operative before this. Once I installed it, and attempted to start it, it was able to idle in neutral at 2000 rpm. If I decrease the idle speed via the idle speed curb screw below 2000 RPM, the engine will slow down, until it is 1000RPM at which point it nose dives to 0. I have not tried to run the car in drive. I haven't done too much, as I don't know where to start.

Here is some other useful information. I got a new fuel tank, and immediately stuck it on. I got a new pump, and a new fuel filter. All of my attempts to start the car has been without the air filter on. I don't have hose clamps on the rubber fuel lines that run short distances between the metal lines and the pump, and between the pump, filter, and carburetor.

 

[jack-in-sac]

This may or may not have any influence on your problem; Do you have a pcv valve? If they become clogged the idle speed must be high or the engine will stall. If you idle mus be very high have you verified that you do not have any vacuum leaks.?
Good luck, I am sure others will weigh in with helpful suggestions.

 

[PaxAmericana]

I don't think my setup has a PCV valve. Was that standard for this car, or a modification done later? I have not tested for vacuum leaks yet. My plan to test is to spray carb cleaner at the two throttle body gaskets and see if it changes the engine function.

 

[Douglemmo]

Hi,

You did not say anything about adjusting the idle air adjusting screws at the base of the carburetor. There are 2 of these screws, one for each primary venturi at the front of the carb. Screw both of these screws LIGHTLY in until each one is closed. Then back each one out 1 1/2 to 2 turns. then start the car. Use the idle speed adjusting screw to bring the idle down to maybe 800 rpm or whatever speed you can get to before it stalls. Now, using either a vacuum gage or tachometer, adjust first one of the air adjusting screws in or out to maximize the vacuum or rpm and then the other. It is now a matter of adjusting the rich/lean idle adjusting screws and the idle screw to get the engine to run smoothly at a reasonable rpm. If you search the internet you will be able to find a more detailed Holley adjusting procedure.
Good Luck,

 

[jack-in-sac]

I don't think my setup has a PCV valve. Was that standard for this car, or a modification done later? I have not tested for vacuum leaks yet. My plan to test is to spray carb cleaner at the two throttle body gaskets and see if it changes the engine function.

PCV was a later change mandated in some jurisdictions. As built in 1957 and 55 and 56 as well, the engine had a road draft tube cominf from the rear of the valley cover. It does wonders for rust prevention often coating the bottom of the car with a film of oil.

 

[doug7740]

Hi,

You did not say anything about adjusting the idle air adjusting screws at the base of the carburetor. There are 2 of these screws, one for each primary venturi at the front of the carb. Screw both of these screws LIGHTLY in until each one is closed. Then back each one out 1 1/2 to 2 turns. then start the car. Use the idle speed adjusting screw to bring the idle down to maybe 800 rpm or whatever speed you can get to before it stalls. Now, using either a vacuum gage or tachometer, adjust first one of the air adjusting screws in or out to maximize the vacuum or rpm and then the other. It is now a matter of adjusting the rich/lean idle adjusting screws and the idle screw to get the engine to run smoothly at a reasonable rpm. If you search the internet you will be able to find a more detailed Holley adjusting procedure.
Good Luck,

Use these simple tips to adjust the idle mixture screws on Holley carburetors to eliminate hesitation, rough idle, backfire, stalling and more...

Holley carburetors have idle mixture screws that control the air/fuel ratio at idle. Improperly set idle mixture screws are often the reason for carburetor symptoms such as hesitation, rough idle, backfire, stalling, hard starting, engine surge, and more. Fortunately, it's easy to adjust the idle mixture with a simple screwdriver and a quality vacuum gauge. The first step is to ensure that there are no vacuum leaks, so check all the hoses leading to and from the carburetor and intake manifold for cracks.

There are two idle mixture screws on the primary metering block of the carburetor. You will need to adjust both idle mixture screws for proper operation.

Connect a vacuum gauge to a manifold vacuum port and follow these procedures to obtain the highest amount of vacuum reading possible.

Step 1) With the engine off, start by turning one of the idle mixture screws located on the primary metering block, all the way closed (clockwise), until it lightly seats. The primary metering block is the one closest to the front of the engine.

Step 2) Then turn the idle mixture screws counter clockwise one and one half turn. Do the same for both idle mixture screws.

Step 3) Start the engine. If it has a difficult time idling adjust the curb idle speed screw (clockwise) so that the engine won't shut off.

Step 4) Let the engine warm up and make sure that the carburetor's choke is off.

Step 5) On a vehicle with an automatic transmission, have someone sit in the car and set the parking brake. Then have them put the transmission in drive while they are depressing the brake pedal. Do not touch or rev the accelerator pedal. On a manual transmission car, the idle speed adjustment can be set with the transmission shifted into neutral and the parking brake is set.

Step 6) Adjust each idle mixture screw by turning them clockwise and counter clockwise, until the vacuum gauge reads the highest possible vacuum reading. All the idle mixture screws should be adjusted the same amount.

Step 7) Once the highest vacuum reading is achieved, you can adjust the curb idle speed screw so that the engine idles at the proper engine RPM.

Once properly adjusted, you'll notice better fuel economy, more power and quicker throttle response.

doug7740
!955 Thunderbird Blue

 

[PaxAmericana]

I have done some work to find the problem. I do not have a PCV valve, so that isn't the problem. I capped all vacuum sources from the carburetor and manifold. I sprayed carb cleaner to the outside of the manifold gasket, the carburetor throttle body gaskets, and a vacuum t-joint installed on the back side. None of the cleaner had an effect on engine performance. But just by capping the port where the power brake booster went, I lowered its idle; however it still remained unsteady, and risked stopping if we let it sit for a few seconds without throttle action.

Then I played with my 2 idle mixture needles, the ones located on the primary metering block. Nothing changed by pushing them all the way in. And the engine rpm became more stable at lower rpm by backing them out 7 turns. That's quite far, anything wrong with that? What did I do to the mixture by backing them that far out (leaner/richer?). I couldn't test the manifold vacuum as I couldn't set the car in drive without it pushing forward.

I also did the trick where you smother the top of the carburetor with a shirt, the engine died within one second.

 

[ron56]

I have done some work to find the problem. I do not have a PCV valve, so that isn't the problem. I capped all vacuum sources from the carburetor and manifold. I sprayed carb cleaner to the outside of the manifold gasket, the carburetor throttle body gaskets, and a vacuum t-joint installed on the back side. None of the cleaner had an effect on engine performance. But just by capping the port where the power brake booster went, I lowered its idle; however it still remained unsteady, and risked stopping if we let it sit for a few seconds without throttle action.

Then I played with my 2 idle mixture needles, the ones located on the primary metering block. Nothing changed by pushing them all the way in. And the engine rpm became more stable at lower rpm by backing them out 7 turns. That's quite far, anything wrong with that? What did I do to the mixture by backing them that far out (leaner/richer?). I couldn't test the manifold vacuum as I couldn't set the car in drive without it pushing forward.

I also did the trick where you smother the top of the carburetor with a shirt, the engine died within one second.

 

[ron56]

What you are describing sounds to me like you either have a very low float bowl adjustment or you need to remove the meeting block and carefully check the idle fuel jets and passages. Sounds like the idle circuit isn't picking up fuel from the bowl. Maybe try taking a piece of small wire and try to "rod" out each hole in the metering block more to the fuel side because the air bleeds would have the opposite effect.

 

[Bigdog1949]

This may or may not have any influence on your problem; Do you have a pcv valve? If they become clogged the idle speed must be high or the engine will stall. If you idle mus be very high have you verified that you do not have any vacuum leaks.?
Good luck, I am sure others will weigh in with helpful suggestions.

Did you change the float level ?

 

[PaxAmericana]

I have found the solution to my problem! I can now have the engine idle at 550 rpm.

as ron56 said, it was the idle passages. After pulling the carburetor apart last night, I didn't notice any blockage in the idle passageways. But what I did notice was that I used a gasket between the throttle body and the main body that was too small. The idle passages were not sealed by the gasket and could leak out through the mating faces into the barrels.

Lesson learned: make sure to use the right gasket in your Holley rebuild kit, mine came with three different ones.

 

[jack-in-sac]

I have found the solution to my problem! I can now have the engine idle at 550 rpm.

as ron56 said, it was the idle passages. After pulling the carburetor apart last night, I didn't notice any blockage in the idle passageways. But what I did notice was that I used a gasket between the throttle body and the main body that was too small. The idle passages were not sealed by the gasket and could leak out through the mating faces into the barrels.

Lesson learned: make sure to use the right gasket in your Holley rebuild kit, mine came with three different ones.

Glad to hear you have been successful.

 

[bonneville]

Trouble getting the engine to idle below about 1000 rpm. I replaced the fuel tank and gas lines up to the fuel pump. Replaced the carb. installed new fuel filter. I noticed that the filter turned rust colored and found large particles of sediment in the. Bottom of the bowl. I am replacing the fuel pump and filter. My question: Can I clear out the carburetor by removing the idle adjusting screws and blowing the passages with air or should I remove the carb and disassemble for a complete cleaning?

 

[Douglemmo]

Hi,
You can try it - what have you got to loose? You could also try carb cleaner spray. It might work.