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Fuel Filter Placement

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glemon

glemon

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On a TR with a mechanical pump. Before the pump or after the pump. Maybe I am overthinking it, but my concern with pre-pump is that pumps are supposed to be better at pushing than pulling, which would suggest post pump, but I would rather the filter catch some bit of debris than have it get caught in the pump.

Then again the car didn't even have one to start.

I was just monkeying around with it, it works both ways, but it is one of those clear filters and it freaks me out that it does not fill with fuel but fuel nonetheless was flowing through it, I think this is kind of weird but I guess it works. But how doea it push, or even more perplexing pull fuel through the filter a bunch of air seems to be at the exit end. Am I overthinking it, does it just work and don't worry about it?
 
I always had one before the fuel pump when I was using the stock mechanical pump.

On the TR4/4A, the pump is generally gravity fed anyway. I haven't exactly measured to know if the last part of the tank is above the pump or not, but certainly the first 1/2 to 2/3 is.
 
If it's gravity fed, why is it called a pump?
 
DNK said:
If it's gravity fed, why is it called a pump?
Click to expand...

The input side is gravity fed (at least most of the time). It still needs to pump the fuel up to the carbs.
 
DNK said:
If it's gravity fed, why is it called a pump?
Click to expand...
Because it pumps.

pump 1 (pmp)
n.
1. A machine or device for raising, compressing, or transferring fluids.
 
Put a coarse filter (like the glass see-through type) before the pump. Put a finer, translucent, paper element filter between the pump and carbs. The coarse filter will catch boulders so they don't enter the pump and cause damage. The fine filter will catch light, sooty rust and deposits that have passed through the pump.

As you said, pumps work better as pushers than pullers. That's why the coarse filter goes on the suction side and the fine filter on the pressure side.

FWIW, the later Spitfires had a filter in between the tank and the pump. On almost every one I have seen, the owner has added a disposable filter just before the carbs.

As for the air in the filter, yes... you are over thinking this. The air you see is there all the time. The air is not a long serpentine bubble that goes from the filter up to the pump... it is just what you see trapped in the filter. If it really bothers you, start the engine and orient the filter so it is vertical. Let the engine run that way for a while and most of the air will be removed.
 
glemon said:
but I would rather the filter catch some bit of debris than have it get caught in the pump.
Click to expand...
The stock pump is supposed to have a filter screen inside, which should catch anything large enough to interfere with the pump operation.

The problem with putting a restrictive filter on the inlet side is that it increases the chances of vapor lock (caused by the fuel boiling at the reduced pressure). But lots of people do, and vapor lock doesn't seem to be a huge problem.
 
I was aware that there is a screen filter in the pump, but this all started when I was autocrossing the TR250 yesterday (quite fun by the way) and it lost power on the last run, then had to limp home by priming the pump to build up pressure after it would stall out after a few blocks, luckily was close to home. Turns out I had a clogged filter, which I had installed in line before the pump. It was one of those small micro screen types (probably desinged to save fuel injectors on modern cars with hemetically sealed fuel systems, not relatively coarse and crude carbs), so I decided to replace it with a larger paper filter clear one, I thought if I had another clog I would rather have it in the filter than in the pump for ease of fixing.

I am not sure what was in it, perhaps a stray bit of the gas tank sealer that was floating around or something that all the excitement of the aurocross worked loose. There did not seem to be a lot of gunk/debris in the system. Anyhow after a couple of back and forths with pre pump, pre-carb and am now back to pre-pump, I hit the hand primer and gas literally shot out at the carb end with the hose off so I think I will call it good.

Sure the wife will ask why the garage smells like gas in the morning.
 
Non-LBC Content to Follow:
I had a Toyota that died on me on a long winter trip. It turned out to be a completely blocked, metal housing paper filter before the fuel pump. Ever since that snowy night I have used translucent filters so I can see what is going on.

Fast forward about 15 years and I had just put our GT6 back on the road. It had a Carter P60504 pump and I put a translucent paper filter before the pump (under the car). The filter quickly plugged with debris(much quicker than I imagined possible), starved the Carter pump of fuel, which caused the pump to quickly overheat and die.

Now I only use the coarse, see-through filters before the pump and I have a shut-off valve inline so I can drain the line and change the filter quickly/cleanly.
 
I have a Bosch metal filter in line before the pump on my TR(4A!). I don't have the number handy, but any brand will do. And I have changed it out every two years or so. The one downside is the mess and the best way to get to it is from under the car. Granted, the tank went through the Gas Renu process and there are new lines all the way from the tank to the carbs. It definitely makes sense to have a fine filter after the pump and a lot of cars in the TR books have them, especially for the SU-equipped cars, but (famous last words alert) I chose not to do that.
 
The cutoff I have on the GT6 is rather compact. I don't think it was anything special, just something the (then) local parts store had.

The valve is a brass "elbow" shape with Buna-N seals. The valve had male threads which fit the bottom of the GT6 fuel tank and I put a barbed fitting in the valve's female threaded port. That allowed me to hard-mount the valve to the bottom of the tank and then use flexible hose from the valve to the filter. The filter has flexible hose over to the rear mounted Facet pump (which replaced the Carter after it failed).

When I installed the valve I was concerned that its internal passages would be too small and restrict fuel flow. However, to date I have had no fuel delivery problems.
 
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