Idle Not Consistent // Thoughts? Ideas?

[bigboi]

I have a 396BB w/ Holley Carb. Mechanical Secondaries. When I start my car the idle is about 700rpm. Once the car is driven for about 5-10 minutes the ide is at 1200rpm. I don't really like the idle at 1200rpm because you cannot hear the chunky cam loping when you are sitting at the stop light.



Any thoughts why the idle increases after the car is warmed up?

[WS6]

Make sure your choke is fully disengaging. The only other thing I can think of right now is that your idle is not set correctly. Try backing it down from 1200 once it gets there. This means you'll most likely need to choke the engine more or for longer to get it to warm up. Idle is only to be set once the engine is warmed. I'm thinking maybe once it is warm, idle is actually set for 1200.

[bigboi]

There is no choke on the carb. I have been playing with the idle screws (both sides are 1.25 turns out) and have got it to idle around 900 - 1,000 when the car is warm, but as you were saying, when you first start the car you have the feather the pedal to get it warmed up... Kinda a pain in the butt.

Is there any benefit to a chokeless carb? Wouldn't having an electric choke be a better idea for a street car?

Why would someone go with a chokeless carb? Just because?

Brian

Make sure your choke is fully disengaging. The only other thing I can think of right now is that your idle is not set correctly. Try backing it down from 1200 once it gets there. This means you'll most likely need to choke the engine more or for longer to get it to warm up. Idle is only to be set once the engine is warmed. I'm thinking maybe once it is warm, idle is actually set for 1200.

[WS6]

If you can install an electric choke it will make life easier. You can not set the idle with the idle curb screws. Those adjust the fuel flow at idle. The throttle stop controls the rpm itself. The throttle stop is what the throttle lever rests against. It should be a spring loaded screw.

Sounds to me like you've got it too rich if you're able to start a BB and not sit there with your foot on the pedal since you have no choke. If you go through Holley's website you may be able to find some info on setting the idle up.

Try to locate the throttle stop screw first and mess with it.

[streetk14]

I agree with Trey on this one. Make sure you are adjusting the idle speed screw and not the idle mixture screws. You will want to dial in the idle mixturer screws AFTER you have the idle speed problem figured out.

I used to run my carb'd motors without a choke here in Southern California, but it doesn't get too cold here. Even so, the motor would need to be manually held at a little higher rpm to keep it alive for a few minutes when starting it up early in the morning on cold days.

The idea of a choke is to richen the mixture when the engine is cold to help with cold-start and warmup. After a few minutes, the choke will open and lean out the motor. A good electric choke is probably a good idea for your car.

The benifit of having no choke or choke tower is a smoother carburetor inlet and increased airflow potential. If the carb is properly set up, the motor is going to be lean on cold-start with no choke. You really want to set your idle speed and mixture when the engine is warmed up. I say set it where it should be (800-900 rpm) with it warmed up, adjust the idle mixture with a vaccum gauge, and then see how it runs when firing it up cold.

Andy

[bigboi]

OK here is where I am at right now...

1. I got the car up to temp and killed the motor.
2. I turned the idle mixture screws 3 turns out.
3. Started the motor.
4. Turned each idle mixture screw in 1/4 turn until the engine rpm dropped.
5. Once the idle dropped I turned each screw 1/8 out.

The above information was taken from the Holley website. I am sure I could probably get a more exact tune by using a vacuum gauge but the above should work.

I then set the idle screw (the one loaded with a spring) to achieve 800-900 rpm.

I then adjusted the float levels so the fuel was just below the sight hole. When bumping the car with my body the fuel dripped out of the sight hole. According to my research this should be the correct way to do this.

Lastly I changed out the fuel pump (mechanical Holley) and fuel filter.

All is good with the idle...

BUT NOW HERE IS MY PROBLEM...

When running the car down the street at a consistent rpm (say in 3rd gear) I sometimes get a stumble. Also sometimes I get a stumble off the line. The stumble is light jerk feeling... and then after the stumble the car goes back to a consistent rumble... though... it may jerk again 1 minute down the road.

What do I need to do next to correct this? Distributor doesn't use vacuum. Brakes use an electric motor to build vacuum in a small canister. The only vacuum line I have runs from the driver side valve cover to the carbs back vacuum hole. Even when I block this hole off and just run breathers on the valve covers I still have the stumble.

Holley Tuning Pros?? Thoughts?? Suggestions??



Brian

[CarlC]

Mine is the same way. Totally normal for a no-choke carb. Your options are adding a choke or blipping the throttle until it's warm.

Chokes are often removed to help increase airflow. Others remove them to cut down on complexity. They are nice to have.

[WS6]

What's your timing set to? What RPM are you running when the stumbles occur? If low enough the idle mixture screws could be incorrect still and causing problems.

Set your idle speed first, then adjust your mixture screws. You're doing the mixture screws right though as far as listening for an RPM change. But you need to adjust them from the idle RPM point.

Alsos since you are mechanical secondary, make sure it's not opening too early or too fast. Is it a double pumper? The secondaries won't open too fast if it is.

[bigboi]

I will redo the idle mixture screws again since I have the idle set correctly now.

What should the timing be at on a BB chevy?

What's your timing set to? What RPM are you running when the stumbles occur? If low enough the idle mixture screws could be incorrect still and causing problems.

Set your idle speed first, then adjust your mixture screws. You're doing the mixture screws right though as far as listening for an RPM change. But you need to adjust them from the idle RPM point.

Alsos since you are mechanical secondary, make sure it's not opening too early or too fast. Is it a double pumper? The secondaries won't open too fast if it is.

[Shaker455]

Brian,
If you need help tuning your car PM or email...I can help

[WS6]

I have to look again but I want to say 10* advanced.