[Jack Gifford]

The 455HO in the Wangers' Bandit Suburban wasn't running right, skipping under load. After chasing possible ignition faults, I've been looking for vacuum leaks. After ruling out all other sources of leaks, I discovered that blocking the PCV vacuum line eliminates the skipping. However a new PCV valve didn't help. I put a 'T' and a vacuum gauge in the line- blocking the PCV with a finger shows 20 in.Hg. at idle, but with the valve inserted into the valley cover, vacuum is only 5 In.Hg. To me, this seemed like a huge vacuum leak. But leaving the PCV in place and moving the gauge to a separate vacuum port on the carb shows 20 In.Hg. ???? Does the carb have an internal small orifice in the PCV vacuum port?
Separate question- I assume that Quadrajets have some sort of power valve, but haven't seen any mention of it in service literature. Do they have a power valve, or some other means of richening the mixture under load?

[77 TRASHCAN]

Quadrajets have a "power piston". The primary metering rods are attached to the top of the piston as ton with 2 small arms.
The power piston is in a bore that has an opening in the borlttom of it that opens the the intake plenum. There is a spring inside the bottom of the power Poston that regulates the power piston position, depending upon throttle position ( more or less vacuum in plenum).

The power piston can only be reached by removing the top of the carb. The power piston bore Cann get gummed up, making it stick in its bore...

I've never known of a PCV restriction in a Quadrajet.
Cliff?
Don't assume the new PCV is good....

[Joe's Garage]

When you consider that the PCV system is designed to be a fairly large, semi-controlled vacuum 'leak' that sucks in the oil vapors from the crankcase, it makes sense that a vacuum reading off THAT hose could be considerably lower than the actual manifold vacuum, because the PCV valve end of the hose is normally partially open to atmosphere except when you accelerate enough to drop the vacuum enough that the valve closes.

The fact that your engine's 'skip' goes away when you eliminate that 'leak' tells me your engine doesn't like something about the way the 'leak' is taking place.

Why the engine doesn't like it is the question. I don't have an answer to that.

Good luck!

[shaker455]

Originally Posted by Jack Gifford

The 455HO in the Wangers' Bandit Suburban wasn't running right, skipping under load. After chasing possible ignition faults, I've been looking for vacuum leaks. After ruling out all other sources of leaks, I discovered that blocking the PCV vacuum line eliminates the skipping. However a new PCV valve didn't help. I put a 'T' and a vacuum gauge in the line- blocking the PCV with a finger shows 20 in.Hg. at idle, but with the valve inserted into the valley cover, vacuum is only 5 In.Hg. To me, this seemed like a huge vacuum leak. But leaving the PCV in place and moving the gauge to a separate vacuum port on the carb shows 20 In.Hg. ???? Does the carb have an internal small orifice in the PCV vacuum port?
Separate question- I assume that Quadrajets have some sort of power valve, but haven't seen any mention of it in service literature. Do they have a power valve, or some other means of richening the mixture under load?

Jack, the under load skip maybe more than once source contributing, the carb primary throttle shaft maybe worn and allowing too much air in leaning it out besides a possible lean calibration. Is the PC plugged into the intake?
Jeff

[Cliff R]

Skipping under load? Is the problem mosty at light engine load or "normal" driving? Does it also "skip" at idle speed?

The PCV system really isn't a player in any of that as it's nothing more than a controlled vacuum leak. In other words we don't run a richer set-up when not using one vs having one in place.

What is the carburetor part number? What has been done to it?

Engine and engine specs?.......

[mgarblik]

All good questions above. I would add: Was everything fine and then all of a sudden one day this "skip" started? Did you change anything under the hood or do anything and then this problem began? I ask this because a small split vacuum line comes to mind.

[Jack Gifford]

Originally Posted by 77 TRASHCAN

... Quadrajets have a "power piston"...

Thank you.
With the carb on the engine, do I need to worry about any falling parts (balls, springs, etc.) as I remove the top?

mgarblik- I just got it out of 13 months in paint-jail, so my recollections are faded! As I recall, it developed gradually.

[77 TRASHCAN]

It's not worth trying disassemble on engine. The base gasket could be part of the problem, leaking vacuum..

I know nothing about your vehicle. I did see it broken down on I-40, over 20 years ago. I saw the Ponti- Carlo on trailer, reason I stopped. Robert was headed to west coast from Michigan. The 700R4 trans gave up. He had everything under control...

I'm assuming the Quadrajet has been on that engine. From when Jim had it installed. It likely needs throttle shaft bushings. At least...

I had an intermittent plug wire problem when my TA was just out of warranty. It has an occasional skip...

Cliff has all the parts and knowledge you will need. IF the carb is the problem. Given its age......

[zippy]

There are so many different PCV vacuum ratings. My 455HO used to use so much oil. Blow blue after a long hard pull. Was new build too, thought ring flutter, removed total seal rings, installed conventional, nope, sent Heads to SD Perf. to have rechecked (he did them initially too) Nope. It was wrong PCV valve for my engines vacuum it turns out.

[25stevem]

Jack you just posted a key bit of needed info!

Since it was in paint jail for 13 months I will bet you that brass power piston is now hanging up in its Bore due to fuel vapor residue which will not allow needed enrichment to the primary's the once the throttle is passed idle.

Most of the crap will be like a white built up powder.

You will need to remove both jets that the Rods go down into to full blow out the crap that you scrape/ remove from the Bore, no less what falls off the power piston.

[Formulas]

Paint jail .. . .. . Not knowing anything about your air cleaner set up did somebody stuff a rag in a snorkle, tape over something to keep dust out? that sort of thing

[SRR]

Is the fuel 13 months old?

[Jack Gifford]

Old thread, but.....
"If you think it's fuel related, it's probably ignition"...... which proved to be true. #6 plug wire was intermittently open; quadrajet is working fine. Duh....

[Half-Inch Stud]

Originally Posted by 25stevem

Jack you just posted a key bit of needed info!

Since it was in paint jail for 13 months I will bet you that brass power piston is now hanging up.............Most of the crap will be like a white built up powder.........

Paintshop fumes and dust caused my engine(s?) to run poor (longk ago) i had to replace the Sparkplugs, gasoline-clean the plugwires, wipe the dizzy cap, to run normal again.

[mgarblik]

Originally Posted by Jack Gifford

Old thread, but.....
"If you think it's fuel related, it's probably ignition"...... which proved to be true. #6 plug wire was intermittently open; quadrajet is working fine. Duh....

Absolutely agree with this statement on any engine with a distributor. Lots of things to wear out or breakdown. Modern ignition systems with coil on plug ignition, gold iridium spark plugs and the advanced diagnostics, are much easier to diagnose. Also much more reliable. Glad you got it all sorted out. Love that Suburban.

[Jack Gifford]

Mike- I still love the "Bandit", but access to the distributor is horrible, even on a stool and laying on the engine, after removing the shaker scoop and air cleaner of course!