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TR2/3/3A Fuel bubbling at glass filter bowl

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HighAltitudeTR3

HighAltitudeTR3

Jedi Trainee
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So I went for a cruise today and the car is running great, I have never noticed this before and I’m not sure it’s normal (although it doesn’t seem normal).


I have bubbling in my glass fuel bowl (filter removed for inline filter). It’s hard to say what’s causing this...
 
I don't know at that elevation, but here in my garage (at about 30') (and regular 'summer' blend pump gas) I measured mine boiling at 135 degrees.
My fuel filter is in-line nearer the SUs, but glass. It bothered me that the fuel filter would appear empty after a drive in temperatures over 95. After the car sits for a while, the fuel would start to bubble back in. Mind you, the car ran fine, wasn't overheating or having percolation/vapor-lock issues but the clear filter always appeared empty when I got home.

One day I attached a temperature probe to the filter and drove the car until I could see no fuel. Then I came back home and measured the temperature of the filter housing, and waited/watched until it cooled enough to bubble back into the filter (appearing to fill it up), then I waited/watched until it stopped bubbling. It stopped bubbling (boiling) and remained liquid/visible at 135. -I watched a video where a guy heated a quart of pump gas until it boiled with a flame, and decided there had to be a safer way to measure that.

It had nothing to do with the problem I thought I was having at the time, but it was interesting. If/when your electric/aux radiator fan turns on, it will move 180+ degree air onto the fuel line...
 
What I saw in the video wasn't the fuel boiling. It was what it looks like when fuel is added to the glass bowl.

When the engine is running, fuel does not flow into the carburetor in a steady, continuous flow. Rather, it only flows when the float in the carburetor drops and opens the float chamber valve. Very similar to the toilet tank valve setup.
I don't know if you've looked into a toilet tank recently but if you do and you carefully let some of the water out of it by pulling up very slightly on the drain valve the water level will start to drop.
It can drop quite a long way before the inlet valve starts adding water to the tank.

The same thing happens in your carburetor. The float chamber has enough fuel in it to keep the inlet valve shut. As the engine runs, the fuel level drops to a lower float position which will open the valve.
When it does, the fuel pump will start pumping a little fuel thru the glass bowl your looking at and you will see the "bubbling".
When the fuel level in the carburetor raises enough, the float chamber valve will close the inlet valve and the fuel pump pressure will increase to the point that it stops pumping fuel. That's when the "bubbling" in the glass bowl will stop.

There is not a lot of difference between the levels of fuel in the carburetor float chamber between the open valve and the closed valve conditions. That's why the "bubbling" occurs for a few seconds and then stops for a few seconds and then starts again.

You probably already know this but that glass bowl is there to collect any water that has been pumped from your fuel tank.
Water, being heavier than fuel will sink to the bottom of the bowl, preventing it from reaching the carburetor and causing mischief.
 
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