Upon reaching the 1/4 mark in the fuel tank, the fuel pressure gauge would shake and gyrate between 0-4 psi until it ranged down to 0 and after a while the engine would die of fuel starvation. Until that point, the gauge would read 3-3.8 psi and was generally steady. Adding gas to the tank alleviated the symptoms. The bottom of the tank is roughly parallel to the top of the block. The fuel pump is mounted on the bulkhead with approximately 15 inches of head to the bottom of the pickup.
Although I had a theory or two, I decided to follow a diagnostic process based on what I've gleaned from this forum rather than getting my exercise by jumping to conclusions. Following are the steps and findings:
Currently the engine runs smoothly with less than 1/4 tank of fuel, fuel pressure steady at 3.4-3.8 psi. Upon shut down, fuel pressure drops to 1.2 psi in about a minute. After two hours the fuel pressure is .8 or so, and after four hours it has leaked down to 0 psi.
While I believe the fuel pump was getting weak, I also believe an air leak at the shut off valve caused the pump to overwork, introduced cavitation, and was most problematic when the fuel was lower in the tank because the pressure assist from the effect of gravity on a fuller tank is diminished.
So much for background. Here're the questions: Is the leak down I'm experiencing typical? If not, what could I have missed?
Thanks!
Although I had a theory or two, I decided to follow a diagnostic process based on what I've gleaned from this forum rather than getting my exercise by jumping to conclusions. Following are the steps and findings:
- Visually checked vent and pressurized with fuel fill cap removed--no obstruction
- Dipped tank with Gasoila Water Finding Paste on dipstick--gas only
- Drained Fram primary filter/water separator--gas only
- Removed primary filter element and inspected--appeared to be fine, but replaced primary filter and polishing filter with no joy
- Removed 10 year old Facet pump with 800+ hours and disassembled--check valve appeared fine, filter clean, cap gasket dry rotted, replaced with new Facet pump and no joy
- Removed, cleaned carb and checked float and needle valve, reinstalled with new gaskets on bowl and flange--no joy
- Checked fuel line, fittings and installed new clamps starting at the carb--found tiny amount of fuel residue on shut off valve at tank. Replaced with new valve. Joy!
Currently the engine runs smoothly with less than 1/4 tank of fuel, fuel pressure steady at 3.4-3.8 psi. Upon shut down, fuel pressure drops to 1.2 psi in about a minute. After two hours the fuel pressure is .8 or so, and after four hours it has leaked down to 0 psi.
While I believe the fuel pump was getting weak, I also believe an air leak at the shut off valve caused the pump to overwork, introduced cavitation, and was most problematic when the fuel was lower in the tank because the pressure assist from the effect of gravity on a fuller tank is diminished.
So much for background. Here're the questions: Is the leak down I'm experiencing typical? If not, what could I have missed?
Thanks!
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