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Fuel filter & check valve location for carbs


Stanley

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My new in-line fuel filters got here today. There's a 100 micron pre-filter going near the tank outlet, and a 40 micron filter that will be in the engine bay before the mechanical fuel pump. For now, the electric fuel pump is removed, the mechanical one seems to have plenty of pressure.

The glass filter that's in front of the mech fuel pump seems to have almost no gas in it when it's idling. My float bowls are set a little higher than stock, but it's still running lean at 3.5 turns down, so I'm not eliminating any suspects yet. I read in a tech article that any air bubbles that get into the fuel pump will emulsify into the fuel and cause a lean condition.

Yeah, I know most stuff I've read online says air in the filter is normal - just paint the glass filter black so you don't see the bubbles.

Supposedly there are a number of possible sources of that "air":

1. leaky hose connection between tank and filter, or crack in fuel pick-up tube in the tank.

2. Cavitation at restriction causes air bubbles (probably not enough flow in my setup for that.)

3. Sloshing in tank causes air-entrainment that results in bubbles hanging at high point in line, the filter.

4. Suction in line plus heat causes reduced boiling point in fuel, so it's fuel vapor not gas.

5. When float valve shuts down, there is no flow, and gravity siphons gas back toward the tank leaving air bubble.

Anyway, I want to set up my filters to reduce air in the system if possible.

One suggestion, from a filter manufacturer is to place the pre-filter as low as possible, so it will be gravity-filled from the tank.

Another suggestion, from a pro drag racer, is to install the (front) filter vertically with the outlet facing down. Another suggestion is to put a check valve inline. (I guess it would have to be after the pre-filter or the gravity-fill idea wouldn't work, since it takes some pressure to open.)

Any ideas? what would be specs for the check valve? The one for F.I. Z's probably wouldn't work since I'm only getting 3.9 psi with the mechanical pump.

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Tried a few little experiments today. Got a 2 ft. piece of 5/16" hose and a 2 ft. piece of 1/4" Tygon clear tubing. Hooked the hose from the metal fuel line to the clear filter, and hooked the Tygon from the filter to the mechanical fuel pump inlet. So I had a flexible setup to try the filter in different positions.

About 1/4 tank of gas.

With the filter slanted outlet up, a steady stream of small bubbles came out and collected at high point in the Tygon tube. Raising the filter up made more bubbles than at lower position. Only a little gas at the bottom (inlet side) of the filter. The bubble at the high point got bigger and bigger and the suction pulled it toward the pump until it all got sucked in.

With filter outlet slanted down, it quickly filled 1/2 full. A bubble started in the Tygon right under the outlet, and began to grow. It got bigger and finally got sucked to the high point in the Tygon, until it got sucked into the pump. The gas level in the filter went down towards the bottom (outlet).

Horizontal, the bubbles seemed to develop faster when positioned higher. But same thing, they grow, find the high point, and get sucked in the pump.

So some empirical evidence of something or other. I got nothing.

Bought a check valve today, wonder if it will do anything. If not, I wasted $22. On Monday I should get some clear 5/16" tube, so I'll be able to see bubbles on there way toward the filter if they're there. When I took of the Tygon and put it back together for the road, and tested it, there was more gas in the filter, 3/4 full instead of 1/8. Maybe I fixed it?

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Got my 5/16" clear yellow Tygon today (way better than the clear colorless Tygon, btw, it's thicker and has a much smaller bend radius, and it's rated "A" for gasoline). So hooked up the clear fuel filter between the hard line and the fuel pump with about 2 foot of clear tube on each side.

Seems like the filter is making all the bubbles, somehow. With the filter located horizontally, about the height of the outlet on the hard line, it sends out a steady stream of bubbles, though there are little or no bubbles coming out of the hard line (except at start-up). With the filter lower but horizontal, there's somewhat fewer bubbles.

With the filter low (resting on the oil filter), inclined about 30 to 45 degrees from horizontal, with the outlet down, the filter stayed full at idle, and there were no bubbles going into the fuel pump. Ran full for ten minutes. Perfect? Then I reached over and revved it up; the glass started to get empty and a big bubble started up the outlet line and grew until it got sucked up.

And the check valve I got doesn't work with suction on the outlet side. Maybe with the electric pump by the tank.

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Starting to think most of the air is getting into the system from leaks. The glass filter for example, has 6 places that can leak: the O-rings at each end of the glass, the threaded plastic hose barbs, and the hose to hose barb connections. If it was under pressure you could see a gas leak, but it's suction.

Thinking I'll use threaded fittings where possible, double clamps on the hose barbs where I can't, and keep the hoses short and situated so there's no high spots where big bubbles can develop.

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