The Stovebolt Site Forums The Shop Area Engine Shop fuel filter near empty/bubbles

55 Ist series 3600 235ci stock 6 volt etc.

Fuel in filter is 1/4 in. from bottom, mounted vertical. OK by wicks filters. Bubbles in filter are coming into filter from fuel line. Bubbles also in glass bowl of fuel pump. Coming in intake side. Is it sending unit leaking air inside tank????

How does the engine run? If it's not stumbling or surging or losing power on an uphill pull, close the hood and drive on. Some people also lie awake at night and wonder why the water taps in the kitchen sink aren't dripping.
Jerry

What your describing is semi-normal. As Jerry said, if it runs ok just drive on.

Can u post a video on this site?

You can post a link to YouTube or other host but not directly to Stovebolt.

Thanks

While I have a fuel filter on my '38 it's not a see through filter BUT this reminds me of my old '70 Harley that I put a see through filter on. It was never full of fuel but the bike ran normally. I puzzled over it for a while but since it ran OK I never pursued the "why". It wasn't a problem so there really wasn't anything to "fix".

Thanks

I will bet that the air that is in the fuel filter is under pressure, which means that as soon as the float needle comes off the seat, the fuel pump and the residual pressure of that air will force fuel to go to the carburetor. This "trapped" air can't get out since it above the down pipe/filter element of the filter. Now if you have a leaky gasket where the glass bowl meets the filter housing, there will not be air in the filter, but there will be a fuel leak.
In other words, don't worry, be happy.
Carl

Video of the glass bowl fuel filter fuel pump in my recently sold "54 3104. Big air gap in the fuel filter. Air bubbles entering the fuel pump.



- Lonnie

Why is there thread tape on an inverted flare nut?
Jerry

So, I had this as well, and truck wasn't running right. The fix for not running right was actually wrong ( too small) fuel orfice in carb. However, in first pursuing the bubbles coming into the fuel filter I found a just barely leaky fuel line. It was fine when driving, stored outside and even during warm days as it would evaporate. I found it one cooler morning when parked inside. A just barely damp spot on the fuel line.
Easy enough to check...

Watched your vid. The fuel pump shouldn't have air coming in. You have a small air leak between the tank and the pump. It probably isn't big enough to affect driveability yet. It may be pumping air into the fuel filter at the carburetor. That bowl should have more fuel in it (not full, just more. Maybe 2/3 full).
I will give this recommendation. Lose the Teflon tape at the fuel pump. The flare fitting absolutely does not call for it, and clean NPT fittings don't need it either. If correcting that issue does not stop the air leak, check your fittings between the tank sending unit and the line going to the pump.
I repetitively tighten and loosen the flared fittings and observe how the wrench gradually moves a little further around each time it is tightened. Once it doesn't progress any further around, you are done.
Doing it this way avoids messing up the threads and the hex on the fitting from too much torque being applied.
What's going while doing it this way, is that, as you are tightening, you are forming the flare to the angle of the inverted flare seat. Doing it progressively (tightening/loosening several times) prevents you from galling the threads.
Carl

Guilty as charged. I used Teflon tape because I didn't know better.

The fuel level is sometimes higher in the filter bowl but not full. Often at level in the video at idle.

I installed the Durex Moraine fuel filter, converted the flat top fuel pump to glass top, bent and flared new lines from the pump to the carb, and adjusted the existing line from tank to pump eight years ago. Connections didn't leak. Don't know if there was an air leak between the tank and the pump. New owner hasn't reported any problems either.

- Lonnie

A minor leak in the line from the sending unit at the top of the tank will not leak fuel, but it will suck air. A bigger leak will prevent the fuel pump from pulling fuel at all.

Air is vented into the tank via the vented fuel cap. Air doing what air does, enters to fill the area that gas once occupied. It also aides in the fuel siphoning to the fuel pump.

Since fuel is heavier than air there should be no air getting below the fuel in the tank if it's a bottom feed tank. If it's a top feed sender/fuel pickup air can be introduced at several points in the pickup tube. Since the fuel pickup/sender is encased in the tank there wouldn't be any evidence of a leak.

I don't know if his tank is in the cab on a 3600 or if it has a top feed/sender but if it is and does, that could be a possibility.

The tank is behind the seat in the cab, original 3104 location. Fuel pick up tube exits at top of tank.

- Lonnie