72 Holley carb adjustment

[Gary Schisler]

Woo-hoo! 75 degrees in Pittsburgh so I break the 72 LT-1 out of storage and I am getting a fair amount of popping out the tailpipes going down hill while idling. In addition, if I really get into it during acceleration and then upshift, I get a fairly loud pop out the tail pipe. This is more than I have noticed before. Here's the situation:

1. original engine tuned to factory specs and never rebuilt (37000 miles)
2. original Holley rebuilt/refinished by Jerry MacNeish last year (beautiful by the way)
3. original A.I.R. system hooked up and working
4. straight vacuum running from the carb to the original vacuum can and bypassing the TCS system.

This does this while the engine is warm or cold. The carb float level is fine, timing and dwell are spot on so where does one go when trying to figure out how to get rid of some of the "after"firing?

In addition, prior to starting the engine when cold, I push the gas pedal to set the carb to high idle and it doesn't catch. Once the engine has been idling for 30-45 seconds, it will set and idle at the proper high idle. Is this feature adjustable?

Thank you in advance.
Gary

[Clem Z.]

a air leak in the exhaust will also cause this *NM*

[William C.]

Re: a air leak in the exhaust will also cause this

Check the AIR diverter valve, if not functioning properly, it will cause the exact symptom of popping you mention.

[Gary Schisler]

Hmmm, hopefully you hit the nail

on the head. When I bypassed my TCS system, I went to full unregulated vacuum off of the front of the carb to the vacuum advance. This required disconnecting the small 1/4" rubber hose that T's with the hose coming off of the AIR pump and going to the AIR tubes in the exhaust manifold. In taking a long hard look at my carb, I saw this tube hanging free and open to the atmosphere. I assumed that having this small tube open wouldn't matter since it wasn't getting vacuum. I will plug this rubber hose with a golf tee and see what happens.

Bill, you mentioned checking the diverter valve. Other than watching the AIR pump pulley spin around, is there a procedure for checking this?

For you AIR experts, should I completely remove the belt from the AIR pump and then only reconnect it for judging/showing, just as I do with my TCS system? Suggestions on what to do with the AIR system are accepted gladly since I am, for the near future, just driving my car and not showing it.

The next question deals with adjusting the high idle setting on the carb. According the GM Service Manual, with the fast idle lever on high step of the cam, engine warm, I should "bend fast idle lever as required." Great! Any help on what "as required" means, if the carb won't initially set up on the fast idle can just by depressing the gas pedal with a cold engine?

Thank you Bill and Clem. I will do these checks tomorrow and report back.

Gary

[Jim T.]

Re: 72 Holley carb adjustment

This test is very easy. You did not mention it, but did you fill the front fuel bowl through the vertical vent with fresh gasoline prior to starting your 72 LT-1? If you did not you most likely encountered excessive cranking of the engine to get it started. Anyway if you had any hint of a backfire during the engine starting process, this could of ruptured the Holley's power valve. To test the power valve, remove the air cleaner top, start the car and put a finger over the front vertical vent on the Holley. If the engine starts to die right away, or dies the power valve needs replacing. If the power valve is functioning the engine will not die quickly, but will die eventually with the vent closed off with a finger. Power valves are so easy to replace.

[Robert E.]

Re: Hmmm, hopefully you hit the nail

Gary,

Last summer I ran into the same problem with after firing with my '71 LT-1. What I did to diagnose the AIR pump and the diverter was to remove the belt that runs it and take it for a drive. Mine stopped popping immediately.

I sent my pump to Bill Hodel to have it rebuilt and at the same time, I had it made inoperable by having him remove the paddles within it.

Give this a try.

Robert
41801

[Clem Z.]

after you make the pump inoperable you must

seal off where the "spider" goes into the exhaust manifold or the hot exhaust gas will burn thru the "spiders" thin tubing since there is no air flowing thru it to cool the tubing. i used the correct size expansion plugs laid in the manifold holes before inserting the "spider"

[Clem Z.]

you may fail your NCRS test because the

noise the comes out of the diverter valve when you gas the engine and back off quickly will not be there with out the pump putting out air.

[Gary Schisler]

Re: Hmmm, hopefully you hit the nail

Robert, did you have to alter any of the tune-up parameters once your AIR system was neutered? Secondly, is there any visible difference now that the pump is a "show only" pump and no longer functional?

Thank you,
Gary

[Robert E.]

Re: Hmmm, hopefully you hit the nail

Hi Gary,

I'm certainly no expert with regards with anything to do with these cars, but having the pump neutered, it has had no affect on performance or tuning, to my knowledge anyways. I had read a post from John Hinckley, and he suggested the procedure. Worked for him and his '69 z/28, that's good enough for me.

Robert
41801

[John H.]

Re: Hmmm, hopefully you hit the nail

Gary -

If the diverter valve diaphragm has failed (as most of them have), or if the vacuum signal hose to it is disconnected, the spool in the valve defaults to "open", so there is constant output of air to the manifolds - this results in "popping" on overrun.

Best solution is to "gut" the pump (remove the vanes), so it essentially becomes just an idler pulley (and that removes the common high-mileage failure mode, which is vane seizure). Also remove the air manifolds from the exhaust manifolds, remove the stainless tubes, install a #10 x 1/2" flat head machine screw in each hole (to plug the hole), and re-install the air manifolds. This "neuters" the system with no visual clues.

There are two adjustments for the fast idle. The adjustment of the rod to the remote choke thermostat determines how high on the fast idle cam the lever goes when you "set" the choke at cold start, and the end of the lever itself determines the rpm at each step on the cam. There's a screwdriver slot in the end of the fast idle lever - bend it clockwise to increase, counter-clockwise to decrease. This is outlined in the 6M section of the Chassis Overhaul Manual.

[Gary Schisler]

Re: Hmmm, hopefully you hit the nail

Thanks John. I wasn't sure which way to bend the fast idle lever. The Service Manual says as required - hey thanks GM! I will look in the Chassis Overhaul Manual on this.

To all who responded on the AIR system, I talked with Bill Hodel at length and he mentioned the test for the diverter valve as well as the neutering function. He commented that removing the vanes is a measure that he suggests only for those AIR pumps that have failed diverter valves and you don't want to spend the $400+ to fix it. He said if everything was working, then don't gut it. He also said that Jerry MacNeish of Camaro High Performance sends all of his AIR pumps up to be fixed or neutered and then cosmetically restored. I will check out my pump after I fix the potential areas as identified by all of you.

Gary