General Tech New—TR4 Fuel Pump Bowl Air Bubbling Issue

[KVH]

Here I am again on fuel pumps, but this time armed with some information from the great Oracle of Artificial Intelligence, “Grok.”

Please take a look at my fuel bowl. Pic below. The screen has apparently fallen into the gas bowl. Grok says this not only allows debris into my fuel system, but somehow is likely the root cause of “bubbling” inside my fuel bowl. The gas isn’t hot and I have no vapor lock issue. Instead, while the car starts and runs just perfectly, great in fact.

But I see constant bubbling and “air shots” inside the glass bowl. Grok says I must fix that problem because it’s either caused by an air leak or by internal pump problems. Grok says that the screen is designed to control the input of gas to the bowl and pump, and that the failed screen is not only contributing to the bubbling but wearing hard on my fuel pump which may soon fail.

Grok also “swears” that the bubbling has nothing to do with the fuel pump pressure regulator I added to my system. Remember, my car runs great. Never better.

Can any of you weigh in here? Is that failed screen really causing bubbling?

(PS I find no air leaks no matter how carefully I check. Though I’m wondering if air is entering above the bowl seal. I doubt it since there’s no gas leak)

Here’s what Grok says—is he right:

Yes, a dislodged filter screen inside the glass fuel bowl of your TR4A’s mechanical fuel pump can absolutely cause air bubbles while the engine is running — even with no external leaks.

Here’s why:

1. The screen’s normal job

• The fine-mesh bronze or brass screen is pressed into the pump body at the inlet port (where the bowl screws on).

• It filters debris and acts as a seal surface for the bowl’s cork (or rubber) gasket.

• The gasket compresses against the screen’s rim, creating an airtight seal.

2. What happens when it falls into the bowl

• The gasket now seals against the pump body’s rough casting instead of the screen’s smooth rim.

• Microscopic gaps form → air is sucked in on the pump’s intake stroke.

• This air enters the fuel stream inside the pump, appearing as bubbles in the glass bowl.

• The pump still pulls fuel (no external leak), but the air ingestion starves the carburetors intermittently → hesitation, surging, or hot-start issues.

Thanks all.

 

[CJD]

So, why don't you fix the screen??

 

[CJD]

Oh...also...I have run a lot of very old vehicles, including tractors, that have often had air in filter bowl, but they ran for decades and we never thought about it.

 

[charleyf]

Here I am again on fuel pumps, but this time armed with some information from the great Oracle of Artificial Intelligence, “Grok.”

Please take a look at my fuel bowl. Pic below. The screen has apparently fallen into the gas bowl. Grok says this not only allows debris into my fuel system, but somehow is likely the root cause of “bubbling” inside my fuel bowl. The gas isn’t hot and I have no vapor lock issue. Instead, while the car starts and runs just perfectly, great in fact.

But I see constant bubbling and “air shots” inside the glass bowl. Grok says I must fix that problem because it’s either caused by an air leak or by internal pump problems. Grok says that the screen is designed to control the input of gas to the bowl and pump, and that the failed screen is not only contributing to the bubbling but wearing hard on my fuel pump which may soon fail.

Grok also “swears” that the bubbling has nothing to do with the fuel pump pressure regulator I added to my system. Remember, my car runs great. Never better.

Can any of you weigh in here? Is that failed screen really causing bubbling?

(PS I find no air leaks no matter how carefully I check. Though I’m wondering if air is entering above the bowl seal. I doubt it since there’s no gas leak)

Here’s what Grok says—is he right:

Yes, a dislodged filter screen inside the glass fuel bowl of your TR4A’s mechanical fuel pump can absolutely cause air bubbles while the engine is running — even with no external leaks.

Here’s why:

1. The screen’s normal job

• The fine-mesh bronze or brass screen is pressed into the pump body at the inlet port (where the bowl screws on).

• It filters debris and acts as a seal surface for the bowl’s cork (or rubber) gasket.

• The gasket compresses against the screen’s rim, creating an airtight seal.

2. What happens when it falls into the bowl

• The gasket now seals against the pump body’s rough casting instead of the screen’s smooth rim.

• Microscopic gaps form → air is sucked in on the pump’s intake stroke.

• This air enters the fuel stream inside the pump, appearing as bubbles in the glass bowl.

• The pump still pulls fuel (no external leak), but the air ingestion starves the carburetors intermittently → hesitation, surging, or hot-start issues.

Thanks all. View attachment 106389

If you were to really look at a TR3/4 fuel pump, you would realize that the fuel comes into the inlet to the center above the fuel bowl and drops down into the fuel bowl. It does not go through the screen to get into the bowl. The screen has a round hole in the center which fits around the inlet. The fuel then fills/ overfills the bowl and now goes up through the screen and into the pump side. But this only does such when the bowl is full and being fed with additional fuel.
Grok is wrong.
Charley

 

[KVH]

I think Grok is saying the bowl seal along the top rim is the problem, apparently positing that air can leak but gas won’t. Sounds wrong to me. It also sounds like a made-up theorized explanation.

I don’t mind at all squeezing off the supply, removing the bowl for a better look and installing a new screen. I’m wondering for what real end, though protecting against debris and sediment should be reason enough.

BTW, I only quoted a fraction of the crazy long tome I received from Grok about fuel pumps and filter screens, and how air bubbles are caused.

Grok is overall amazing, but I think the number of times he, she, or it (or them) has been dead wrong would shock most folks. Sure wish I’d bought Envidia 5 years ago.

 

[charleyf]

I think Grok is saying the bowl seal along the top rim is the problem, apparently positing that air can leak but gas won’t. Sounds wrong to me. It also sounds like a made-up theorized explanation.

I don’t mind at all squeezing off the supply, removing the bowl for a better look and installing a new screen. I’m wondering for what real end, though protecting against debris and sediment should be reason enough.

BTW, I only quoted a fraction of the crazy long tome I received from Grok about fuel pumps and filter screens, and how air bubbles are caused.

Grok is overall amazing, but I think the number of times he, she, or it (or them) has been dead wrong would shock most folks. Sure wish I’d bought Envidia 5 years ago.

On the TRs the tank is above the fuel pump ( unless you are parking on a steep hill) so fuel will leak from any gap. Like the gasket or around the intake ferrule. Especially if the engine is not running.
Charley

 

[poolboy]

Is that a plastic screen ?
Some of the reproduction pumps do...They warp and some dissolve.
I caught this one before anything really bad happened and replaced it with the original brass screen.

 

[KVH]

WOW. Unbelievable. I’ll take mine out later to see what happened. As for my bubbling in the fuel bowl, I’ll just wait and see. But I’m not seeing any leaks.

 

[charleyf]

Thinking about the entire fuel system regarding the bubbles. The system starting at the tank or specifically the outlet to the tank and moving to the pump. It is a closed( probably not the best word) system as long as you have fuel in the tank. Then on to the fuel bowls. This is the first place that the system is vented via the overflow tube( H6 carbs). Any air / bubbles in the system would be vented off via the overflow tubes. So you should not be concerned with engine performance regarding bubbles.
I have a fuel filter located just before the front carb. That filter always has air in it. This I believe is a normal result of the venting from the float bowls when the fuel pump is not operating. As the filter is the highest point in the supply line. It took me awhile to determine the source of the air getting into the supply at the filter.
Charley