I put on a holley 600 vac sec on my 302 the problem I have is that when I start it it will start briefly and then shut down I have to step on the gas a little and keep it pressed and then my truck will turn on. What can cause this problem

 

Does it only do it when cold? Does it do it only when warm?.......or both?

 

Sounds like an idle speed adjustment issue. There are two idle speed adjustments on a holley carb. One for curb idle and one for cold idle. The curb idle screw is on the driver's side of the carb and works on the throttle linkage. The cold idle adjustment is on the passenger's side, is darn hard to get to, and works off the choke linkage.

Which to adjust depends on your anser to WarWagon's question.

 

It doesnt matter if the truck is war or cold it will still do it

 

It could be float adjusted too high or the idle mixture screws are not properly adjusted.

 

Sounds like a curb idle adjustment. What is your idle rpm?

A few other quesitons.

Auto or manual trans?
Manual or electric choke?
When you start the cold engine, are you setting the choke?

 

I have this theory that what we call the "Idle Mixture Screws" are not a mixture adjustment at all. I have a feeling that they are simply an idle air bleed and control the idle speed. But the mixture is controlled by the main jets.

I came to this conclusion from 2 things.

1. Documentation on the Holley web site

'Turning the screws clockwise will “lean” the idle system. Conversely, turning the screws counterclockwise will “richen” the idle system.'

If they are really adjusting the mixture, why the quotes? Seems like holley is bowing to convention here rather than giving them a technically accurate name.

Also it states of the screws:

'These control the volume of the pre-mixed air/fuel coming through the idle well.'

If the air and fuel is "pre-mixed" then the screws are not controlling the mixture. Just the amount of air and fuel entering the engine. Just the same as the curb idle screw.

2. On my holley, the screws don't act like mixture screws should. If they really controll mixture, you should be able to adjust from a lean condition to optimum to an over rich condition. The idle should in turn go from bad to optimum to bad again as you move in and out of optimum. Mine does not do that. I am able to turn the screws counter clockwise and the idle speed increases till the screws fall out.

Currently, I have my curb idle screw backed out so the throttle blades are completely closed. The idle bleed air screws are adjusted to obtain the correct idle speed. And the darn thing seems to work great. Also, this is the only way I can stall the engine if I turn the screws all the way in. If the curb idle is cracked open, the engine will not stall when the screws are closed.

 

You are talking about is what is known as a Holley 4160 with "reverse idle" Those carbs are marked with a tag and arrow by the adjustment screws in the metering plate. They are the opposite of a regular Holley 4160. These carb where built the meet pollution control engines.

 

Yep, these have a preset fuel amount and you are adjusting the airflow to the idle mixture. A standard Holley, you are adjusting the amount of fuel in the idle mixture. The air is regulated by the air bleeds on the top of the carb.

 

Alright, something happend the other day that is going to make me redo the tune on this Holley. The duraspark amplifier box died. When I was troubleshooting it the engine backfired through the carb. Since the throttle plates were completely closed the shock wave had nowhere to go but through the passages in the carb. This ended up pressurizing the fuel bowls which sent a geyser of fuel up from each of the two bowl vents. I had the air cleaner lid off. Has the throttle blade been cracked open, the backfire would have been allowed an easier vent back through the venturi and the fuel wouldn't have spit out of the bowl.

What I'm going to do is connect a vacuum guage to the ported vacuum connection and open the throttle blades a bit but stop before I get vacuum at the port. Then I'll fine tune the idle speed with the set screws on the side of the metering block.

Wanted to correct this before someone else tried it.