[71SportRagtop]

I am NOT a Holley guru at all. This is my first Holley carb ever. It's a 750 double pumper that came with the 428 I installed into my Firebird this winter. It was supposedly custom tuned for my motor. When the motor was installed, the carb fired right up and ran great. After initial break in of the cam, I set the timing and adjusted the carb to get 11" of vacuum at idle which I was OK with due to the aggressive nature of my cam. Since then, the car has run REALLY strong.

Until now:

I was away the weekend before the 4th of July and also the weekend of the 4th. So my car had not been run in 2 weeks. Last weekend I fired it up just to keep the juices flowing. I started it as normal, 2 quick pumps of the gas pedal and turn the key. It immediately popped through the carb. I let off the key and waited a second and fired it up. Since my car has no choke, I usually bring the RPM's up to about 1100 to bring the car up to operating temperature. While it was warming up, I heard two more slight pops but after coming up to temp the car seemed fine. So I warmed it up and ran it around the block and everything seemed fine.

Yesterday I fired it up to back it out of the garage to clean it for a cruise night. It fired up but something wasn't quite right. So I got it out of the garage and brought it up to operating temp. I then checked and reset the timing and put my vacuum gauge on the carb to check the idle mixture screws. At operating temp, I only had 5" of vacuum which is a lot less than I had when I first tuned the motor. I had the car idling fine but while driving, something doesn't seem right. It feels like there's a vacuum leak. The wind out of gears isn't smooth like it was before.

The way I adjust my carb is to turn the idle mixture screws all the way in and then back off 1-1/2 turns as a base line. Then once at operating temp, I put the vacuum gauge on and play with the screws until I get max vacuum. That's the extent of my carb adjusting skills. When I turned my mixture screws in, the motor seemed to rev up slightly instead of stumbling like normal.

Any thoughts on how to check my power valve? I'm a total rookie when it comes to Holley carbs.

[PunchT37]

Originally Posted by 71SportRagtop

When I turned my mixture screws in, the motor seemed to rev up slightly instead of stumbling like normal.

Any thoughts on how to check my power valve? I'm a total rookie when it comes to Holley carbs.

Sounds like it`s busted to me.

They`re cheap enough. Head down to the store and grab another. Get a bowl and metering plate gasket in case you ruin the ones you have now. Make sure you check the rating.

[shaker455]

Older carbs do not have powervalve blow out protection.

Maybe post a pic of yours and will see if it's an older model.

You can remove the PV and do a simple test but your going to need gaskets.

[Tom Vaught]

If you had a Blown Power Valve, the normal symptom would be the engine would have been running very rich on your drive and the plugs would be soaked with liquid fuel.

Depending on the ignition, and the fact that the car sat for two weeks the the pump shot could have tried to start the car but the bowls could have been low on fuel. This would cause more air in the passages vs a normal condition and you would get a lean pop, or a couple of lean pops as the bowls were trying to fill.

While it is possible that the power valve was harmed by the "pops", depending on the age of the power valve and the gas used today. More likely, the pops created a vacuum leak that you compensated for with your mixture screw carb adjustment, and was able to drive the car. Now the carb calibration is a bit rich and when you turned in the screws the idle got better because the idle was slightly leaner.

The engine is telling you fix the vacuum leak (gaskets) and tune the carb again to get the 11" of vacuum at idle you had before.

The power valve may not be the issue at all.

Tom Vaught

[71SportRagtop]

Originally Posted by Tom Vaught

If you had a Blown Power Valve, the normal symptom would be the engine would have been running very rich on your drive and the plugs would be soaked with liquid fuel.

Depending on the ignition, and the fact that the car sat for two weeks the the pump shot could have tried to start the car but the bowls could have been low on fuel. This would cause more air in the passages vs a normal condition and you would get a lean pop, or a couple of lean pops as the bowls were trying to fill.

While it is possible that the power valve was harmed by the "pops", depending on the age of the power valve and the gas used today. More likely, the pops created a vacuum leak that you compensated for with your mixture screw carb adjustment, and was able to drive the car. Now the carb calibration is a bit rich and when you turned in the screws the idle got better because the idle was slightly leaner.

The engine is telling you fix the vacuum leak (gaskets) and tune the carb again to get the 11" of vacuum at idle you had before.

The power valve may not be the issue at all.

Tom Vaught

That sounds like my exact issue. As stated the car popped a few times after not running for 2 weeks but cleaned up at operating temp. Mt first run around the block the car had tons of power. Now it doesn't feel right. Less power and a slight stumble to doesn't seem to smooth out on acceleration.

So what gaskets are you thinking need to be replaced? The carb to intake gasket? The bowl gaskets? I can't imagine that my intake gaskets are blown out?

[shaker455]

If your gonna test for a leak I would use a bottle of soapy water in a spray bottle.

Put it down heavy around the carb & see if the idle drops a bit.

You might need a piece of cardboard to block the fans airflow where your spraying.

[gtofreek]

When I had bad power valves, the engine would flood out randomly. I would have to hold the gas pedal to the floor and crank it for 30 seconds or so to get it to fire up again.

If you idle speed screw is screwed out to far[throttle plates open too far in order to compensate for a big cam with low vacuum], then the transition slot is uncovered, causing you to lose all adjustability on your idle mixture screws. Typically, small holes are drilled in the butterfly valves to give enough air to idle without having to crank the butterflies open.

[Tom Vaught]

A couple of possibilities really when you have a backfire: Intake Gaskets, or a stuck PCV valve due to the backfire condition in the intake and crap in the PCV wedged the valve. If the velve is wedged open then the engine sees 4 additional cfm at idle and the whole idle cfm of the engine is only 16-18 cfm so a 22% to 25% change in idle air flow.

Bowl gaskets would bleed fuel all over the intake and a metering block gasket would not run correctly during a drive around the block.

ps As GTOFREEK said, you should not be messing with the idle adjustment every time you fire up the car. The Primary Blades should be set at the right location on the transfer slot and left alone. Pretty much the same deal on the idle mixture screws. If your Holley is a spread bore carb you will be chasing the idle speed and Vacuum constantly as that carb IS affected by atmospheric condition changes (idle circuit).

TV

[TedRamAirII]

If the idle mixture screws are all the way in, and it still runs, the power valve is blown. I think Holley carbs after 1992 have a power valve protection check ball installed at the factory.The Power Valve has a diaphragm so you should be able to suck on it with your puckered lips and see if its good. if you can suck air through it, its bad. Choose a PV with 2-3" less vacuum than at idle. like if its 12" at idle, choose a 9.0 valve. I forgot everything I used to know about Holley, or thought I knew.