Possible causes for Popping when let of Gas

[Nick M.]

Re: Possible causes for Popping when let of Gas

Doug
I havea 69 that is popping upon deceleration but am not yet familiar enough with the various components under the hood to even be able to identify the parts that you reference. For instance, where do I find the diverter valve?

[Jack H.]

Re: Possible causes for Popping when let of Gas

You might want to consider buying a copy of the 1969 Chassis Service Manual for your support library as it's chock full of handy information. Here's a figure from my '71 CSM in the emission section showing and explaining the function of the AIR system's diverter valve...

[Nick M.]

Re: Possible causes for Popping when let of Gas

Jack

Thanks for the photo and the suggested reading material. Does the diverter valve also contribute to the smell of gasoline in my garage after shutdown or would that be more of an adjustment for fuel mixture?

[Nick M.]

Re: Possible causes for Popping when let of Gas

Jack

Do you know how I can quickly test whether or not the diverter valve is functioning correctly or do I need to remove it and open it up for inspection?

[Jack H.]

Re: Possible causes for Popping when let of Gas

The diverter valve exists to route air from the AIR pump either to the exhaust manifold or 'vent' it to the atmosphere when extra air in the exhaust isn't needed or wanted. The CSM book will give you a thumbnail on theory of operation....

The smell of gas once you shut down your car's warm engine and close the garage door has been covered quite a few times (search the archives). In a nutshell, it's typically the result of unwarranted owner expectations....

These cars have carbs vs. fuel injection. For a carb to work properly, raw gas stored in the fuel bowl HAS to be vented to the atmosphere to equalize pressure to achieve the correct air/fuel mixture. Modern cars are fuel injected and gas is under pressure all the way from the tank to the injector tips, so there's nothing to vaporize when the engine is shut down hot...

One (of many) contributing factors to the odor issue can be the result of a 'cockpit' problem... The practice of 'topping up' the fuel tank before you drive home and store the car can result in insufficient expansion space in the gas tank giving you two sources of fuel odor!

[Jim T.]

Re: Possible causes for Popping when let of Gas

Paul Cook, with gas dripping into the secondaries for 5 minutes after you shut it off, could be your float in the secondaries fuel bowl. I do not know what type of float is in your LT-1's Holley secondary fuel bowl.
I have a Holley 4777 double pumper that was leaking fuel into the primarys after being driven.
Removed the fuel bowl and discovered that the plastic/nylon fuel float had developed a leak and had a lot of fuel in it, enough to keep the needle/seat valve open and allow more gas to come into the fuel bowl after shut down.
Looked at this removed plastic/nylon fuel float that I removed two years ago and it had just a very small amount of fuel left in it. I was curious as to how long it would take for the fuel to evaporate from it. Don't remember exactly, but it was half or just better than half full of fuel.
Installed Holley's metal floats in the primary and also replaced the plastic/nylon float in the secondary fuel bowl.
Replacing the fuel float in the primary bowl stopped the fuel leakage and smelling gas fumes in the garage after shut down.
Another reason for gas dripping in the secondaries after shut down is trash in the needle and seat not allowing the needle to shut off the gas flow into the fuel bowl.
If gas is always leaking/seeping into your secondaries, it could be contributing to the popping in the exhaust when you let off the gas.