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small engine fuel filter

C

Chris Jones

Member
I've seen this on a few small engines before--the fuel filter (little clear plastic inline type) is almost completely full of air yet things still run fine. My current engine has a large tank beside the engine and a vacuum operated fuel pump separate from the tank and engine. The pump has a vacuum line from the engine to it as well as a fuel in an fuel out lines. I have the filter on the suction side and have clamps on all the lines. It's a new filter and lines. I used a small funnel to completely fill the filter and lines with gas. I managed to get all but a couple little bubbles out of the filter. The lines were full. Now I look at it and I can see fuel in the filter on the outlet side of the paper element in there but there appears to be none on the input side. There is plenty in the tank and the engine runs fine for as long as I want it to. Unfortunately I can't watch the filter with the engine over idle but at idle I can't see fuel coming into it. What causes this and is this normal or a problem or what. I'm considering moving the filter to the output side of the pump. What do y'all recommend.
 
I have been told that a fuel pump does better pushing fuel through a filter than drawing it through one. So the filter should be on the outlet side of pump.
 
Simply turn the filter so that one end is facing up towards the pump this will let the air escape.
On its side you just have a place where air can't get out.
Walt
 
It is probably fuel vapor, not air. ignore it is fine. or position the filter to bubble it back into the tank. Jim
 
That empty space in your filter is nothing to be concerned about. Just look at your filter as an addition to your fuel tank, the engine runs when it your tank is half empty and it's on the suction side of the pump. Sometimes what we think isn't always the way things are.
 

It is normal for the filter to run empty. It has to do with capillary action due to the small tubing and filter inlet. For filter inlet sizes larger than about 3/8" to 1/2" I.D. the filter should run full if positioned vertical.

Filters of any size will tend to collect fuel vapor when placed in a horizontal section of the line. However , when installed on the outlet side of a pump they should fill whenever the pump pressure is greater than the vapor pressure of the fuel.

As a general rule of thumb; filters should always be placed after the pump. Free flowing screens are aceptable of the inlet side of the pump.
 
Thank you everyone for the input.

The pump pulls the fuel out of the top of the tank, thus there is no way to orient the filter so the vapor/air will move back to the tank on it's own. I really hadn't thought about it being fuel vapor--I guess the low pressure caused by the suction in the line causes it to vaporize. Since the manufacturer had the filter between the tank and pump and it is working I'll leave it there. It just looks so weird. You can see the fuel in the filter on the outlet side of the paper and tell that's basically full but there is 0 liquid fuel on the inlet side of the paper element. The setup is a bit funny in that it uses 1/4 fuel line from the tank to the filter and 5/16 from the filter to the pump. Then 1/4 from the pump to the carb. Since that was how I found it that's what I put back there.

The filter on it when I got it was so bad it might as well have been made of black plastic--so I don't know if it always worked like this or not.
 
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