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-   -   Fuel supply issue (mechanical pump / filter location) (https://www.offshoreonly.com/forums/general-q/367230-fuel-supply-issue-mechanical-pump-filter-location.html)

vintage chromoly 06-29-2020 08:53 PM

Fuel supply issue (mechanical pump / filter location)
 
Hello all. Iíve been trying to sort out a fuel supply issue.

The boat is a naturally aspirated, BBC with a Baker 6 vane fuel pump. I have an Aeromotive bypass regulator and the pump puts out 15PSI. I have the regulator Set to bypass all but 6 PSI.

Iím using a FST fuel filter with -12 ORB ports. The fuel tank is in the belly of the boat and has a 1/2Ē fuel pick up tube (no sock) and a 3/8Ē NPT to -10 AN adapter. -10 AN line to the filter inlet and -10 line to the Baker pump. The output of the pump is -8 and it feeds both bowls with a -8 ďTĒ on the rear bowl and feeding the bypass regulator.

The filter is roughly 6Ē above the top of the tank.

My issue is that as my fuel level in the tank drops, my fuel PSI becomes unstable. It has dropped to 3 PSI, leading me to idle the boat back to the dock. I thought maybe I had a ruptured diaphragm on the regulator and it was bypassing most of the fuel. Iíve disassembled it and itís fine. I disassembled the Baker pump and itís fine. Iíve removed the comp cams bronze tip pump pushrod and it shows perfect witness marks. Iíve checked all the lines for blockage and checked the pickup tube in the tank for a fault. All is well there.

Iím wondering if having the filter higher than the tank could be causing my issue. My thought is that, when full the pump has less vertical rise to siphon the fuel into the filter, and as the fuel level drops, Iím asking more of the pump on the vacuum side.

My other thought is that I should have the filter after the pump and before the carb. I called and talked to the rep at FST filters today and he says that the filter will flow in excess of 300 gallons an hour. Itís only 3 microns, but has a bunch of surface area. He thinks my issue is the location of the filter being 6Ē above the tank.

Iím wondering if I would be better off with the filter mounted on the front of the cylinder head, above the pump and near the carb feed. I would have a horizontal run from the tank to the pump, a 10Ē piece of -10 up to the filter and the -8 feeding the carb.

Any suggestions or thoughts would be greatly appreciated.

Rob

SB 06-29-2020 09:13 PM

Iíd bet small crack in fuel pick up or your return is foaming the fuel and at a certain level pick up is grabbing it.

Iíll look for holleyís vid of fuel foaming from returns. Will open every oneís eyes greatly.

mike tkach 06-29-2020 09:13 PM

try bypassing the filter and see if the problem goes away.

vintage chromoly 06-29-2020 09:22 PM


Originally Posted by SB (Post 4746051)
Iíd bet small crack in fuel pick up or your return is foaming the fuel and at a certain level pick up is grabbing it.

Iíll look for holleyís vid of fuel foaming from returns. Will open every oneís eyes greatly.

the pickup has been checked. No cracks. I thought the same thing. I can remove the fuel
line fitting from the tank and put a few pounds of pressure on the vent tube, and watch the fuel rise in the pickup tube without any air leakage.
my return dumps into the fuel fill near the fuel fill at the top of the fuel fill hose.

vintage chromoly 06-29-2020 09:24 PM


Originally Posted by mike tkach (Post 4746052)
try bypassing the filter and see if the problem goes away.

thatís tomorrowís test.
I fed the filter from a 5 gallon gas can and all was well. The only variation with the gas can was that it was approximately 2 feet above the filter.

SB 06-29-2020 09:51 PM

Here is one from Fuel Lab:

The one I am trying to find shows more on return placement snd effects than this. But this is still good vid.

zz28zz 06-29-2020 11:50 PM

Have you verified the fuel tank vent line isn't plugged?

vintage chromoly 06-30-2020 04:17 AM


Originally Posted by zz28zz (Post 4746070)
Have you verified the fuel tank vent line isn't plugged?

yes

AusScarab29 06-30-2020 06:17 AM

The vent hose doesnt drop below the vent outlet

SB 06-30-2020 07:24 AM

At 3psi do you have a drivability issue ?
You had a drive ability issue and then noticed gauge or you just see gauge back down and thus you return to idle to play it safe ?

Is the fuel psi gauge in dash or engine mounted ? If engine mounted is it a liquid filled gauge or dry ?


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