Possible causes for Popping when let of Gas

[Paul C.]

Car was running great and then all of a sudden it started popping but only when you let off the gas and the idle seems to drop when it gets hot--have 190F thermostat and that is as hot as it gets. Otrherwise car sun great and acclerates fine. Any suggestions as to possible cause. 71 350 with AIR system.

[Clem Z.]

Re: Possible causes for Popping when let of Gas

Car was running great and then all of a sudden it started popping but only when you let off the gas and the idle seems to drop when it gets hot--have 190F thermostat and that is as hot as it gets. Otrherwise car sun great and acclerates fine. Any suggestions as to possible cause. 71 350 with AIR system.

if the popping is out of the exhaust the diverter valve on the AIR could be bad. check for leaks in the system as air getting into the exhaust system will cause the exhaust to backfire

[Jim T.]

Re: Possible causes for Popping when let of Gas

Paul your LT-1's Holley carb most likely ruptured a power valve. Power valves do not last forever, all it takes is some back pressure through the carb to take one out. A simple test you can perform without taking the carb apart is to first remove the air cleaner top and then start your LT-1. With it running put a finger over the front verticle fuel bowl vent. If your LT-1 dies immediately, it has been my experience that the power valve is ruptured. If your LT-1 continues to run with the finger over the vent for awhile, not long though, then it is probably not the power valve. Power valves are easy to replace, I buy them in a Holley blister package from Auto Zone. New metering block and fuel bowl gaskets are available as well. You might be surprised of the sediment on the bottom of your fuel bowl if it has been awhile since you took it off.

[John H.]

Re: Possible causes for Popping when let of Gas

I'd vote for a failed diaphragm in the diverter valve - when they fail (and they all do), the valve defaults by design to the full open position, which dumps pump air into the exhaust manifolds full-time, which causes "popping" in the exhaust on over-run. In addition, the failed diaphragm becomes a significant vacuum leak, which affects idle, vacuum advance, and part-throttle operation. Plug or cap off the vacuum signal hose that goes to the diverter valve to cure the vacuum leak. Bill Hodel can fix the diverter valve for you, or you can just live with the "popping", or you can "gut" the pump and nobody will know the difference.

[Douglas L.]

Re: Possible causes for Popping when let of Gas

Paul,

I have a 67 with A.I.R. It was popping in the exhaust when letting off the accelerator driving me crazy! The cause is the fuel mixture is too rich at that specific point in time. The A.I.R. system is supposed to lean the mixture to prevent the popping. The diverter valve is what does that. On the 67 system, the smog pump is constantly pumping air into the exhaust manifolds. When you let off on the gas, a vacuum signal is sent to the diverter valve opening the valve and letting a burst of air into the base of the carburetor which leans the fuel mixture. The diverter valve then closes and the system reverts back to pumping air to the exhaust manifolds. The diverter valve on my car was not working right because it was clogged with carbon. I took the valve and cleaned it with lacquer thinner. It now works like new. End of popping. GM changed the way the system works on 68 and up motors. But operationally, they are close to the same.

[Robert E.]

Re: Possible causes for Popping when let of Gas

Paul, I too own a '71 LT-1 with AIR. Mine started to do this a couple of years ago. The simplest test is to remove the belt off the AIR pump as I did and the popping went away. Like John mentioned, I send the pump and diverter to Bill Hodel for a rebuild, and it came back working perfectly.

PS, if you decide to have it rebuilt, test your check valves for proper operation so you don't damage a newly rebuilt pump. Don't ask me how I know

Robert
41801

[Mark K.]

Re: Possible causes for Popping when let of Gas

Had the same thing happen on my '72 LT-1. Changed the diverter valve...still happened. Changed out the power valve...all good again

[Nick M.]

Re: Possible causes for Popping when let of Gas

My 69 tri power makes the same popping noise. Any idea what would cause a similar backfiring on a 427, 400hp tri power car?

[Clem Z.]

Re: Possible causes for Popping when let of Gas

My 69 tri power makes the same popping noise. Any idea what would cause a similar backfiring on a 427, 400hp tri power car?

there has been several theories posted you need to check out both.

[Paul C.]

Re: Possible causes for Popping when let of Gas

Disconnected the AIR pump belt and it is still popping. Put finger over the vertical tube on front on carb and nothing happened--it never died. Now when I drove it today I noticed gas dripping into the secondaries for 5 minutes after the engine was turned off. I already have set up to get my Holley restored and resbuilt in October--but may not make that long.

[Harvey S.]

Re: Possible causes for Popping when let of Gas

We had a 69 Z28, all original down to the A.I.R. Smog system. The car popped all the time when the gas was released. I had the Smog pump rebuilt non functionally, and when reinstalled on the car, the popping was gone.

BTW - What is a "Power Valve"?

[John H.]

Re: Possible causes for Popping when let of Gas

BTW - What is a "Power Valve"?

Harvey -

You'll find it screwed into the center of the primary metering block - it's held closed by manifold vacuum and opens under moderate throttle (when manifold vacuum drops below 6.5" Hg.) for additional enrichment. Some Holleys (like the 4053 on my '69 Z/28) have TWO of them - one in the primary metering block, and a second one in the secondary metering block.

[Paul C.]

Re: Possible causes for Popping when let of Gas

I noticed that when I start the car and drive it---there is no popping when I let me foot of the gas for the first couple of miles. Then it warms up and the popping starts. When you get on it the motor seems to run fine and no missing. Any suggestions?

[Douglas L.]

Re: Possible causes for Popping when let of Gas

Paul,

When the motor is cold and the gas mixture is rich, no problem. When the engine warms up, excessive fuel it getting to the exhaust valves and that causes the popping.

In the 69 AIR system, the air pump sends air continuously to the exhaust manifolds. When you let off, the diverter valve is supposed to close the air to the manifolds and dump it out the diverter valve.

What could be happening here could be either the diverter valve or the carb. If the diverter valve is not working right, it will keep sending air to the exhaust manifolds under deceleration which is like an igniter to the unburned fuel - thus the popping. If the carb is out of adjustment, it could be running too rich all together.

I've never tried this on a 69 setup, but I think you can test the diverter valve by hooking an inlet hose to the "in" side of the valve. Then hook a "mighty vac" vacuum pump to the vacuum hookup on the valve. Then blow on the hose to the valve and pull the mity vac pump. The vacuum signal should divert air to the outlet where the diverter valve muffler is. If is doesn't do this, that's probably the problem.

On a 67 system, I found the diverter valve not functioning correctly. I cleaned the valve with lacquer thinner and worked like new - end of popping.

Let me know what happens and if all this is way out in "left field", let me know that too because I ain't not expert. The problem I had was I couldn't find anyone who was!!

Good luck,

Doug Lee

[Joe L.]

Re: Possible causes for Popping when let of Gas

It's fairly easy to do a rough check on a diverter valve. You can't easily check if it's working to specification, but you can check for a failed diaphragm (which renders the valve totally inoperative). Just remove the valve from the pump. Apply a vacuum to the vacuum port while looking into the intake orifice. If the valve is operable, you should see the "piston" move as vacuum is applied. If nothing moves, the diaphragm is shot and the valve is is inoperative. Then, it's "Bill Hodel Time".