Hot Rods Mech Fuel Pump, how much vacuum?

I just spun a motor over on the starter and a vacuum gauge connected to the suction side of the fuel pump, it read 4-5" vacuum, seems quite low to me, anybody know what it should be?
Carter pump, 60's era Ford FE motor. It runs out of fuel after 20min of driving, excellant fuel tank, new lines, new pickup assembly, new fuel pump, 50's not vapor lock.
Thanks, Oj

 

Sounds like a weak diaphragm maybe. I know its new but that is what I would guess.

 

Is the fuel tank vented somehow?

 

I've never read a vacuum spec in the shop manual, just volume, and static pressure. Typically something along the lines of 1 pint or better in 30 seconds, and 3.5 to 5.5 psi.

 

My old Motors manual say 7-12 inches of vacuum at 1000 rpm. That is testing the pump alone, not connected to intake vacuum.

 

Interesting, I hadn't seen that, my copy is 1950

 

Yes, I had the gas cap in my pocket after it quit on me.

 

I wonder how you'd get to 1000rpm? Rig up another line to the carb?
I did more testing after I posted, I ran the idle screw down to get the engine running at 3000rpm and connected a fuel pressure gauge at the carb. The initial pressure was 6psi and stayed solid for about 20minutes, then it started to fluctuate and dip down to 5psi. Another 10 minutes and the pressure wasn't stable at all, I saw it dip to as low as 4psi and thats when I returned the rpm to normal and quit the test. It has done the same with 2 brand new fuel pumps, a 110gph Holley and a 120gph Carter. They both quit at exactly the same distance from the shop. Either the Virginia highways are at fault or the drive mechanism for the pump is failing.
I just don't know how the eccentric can do what it is doing, any thoughts? Other tests?
Thanks, Oj

 

I think that applies to the fuel pumps that have vacuum pumps for the wipers.

 

I think 4"-5"hg is probably about as good as it gets. Mechanical fuel pumps, by design, are made primarily to compress and pump liquid fuel to the carburetor. They are far less efficient at pulling air and/or fuel from the fuel tank. They depend almost completely on having a gravity-fed supply of liquid fuel available at the pump inlet port. This is why it's critical to have your fuel pump mounted lower than than the pick-up tube in your fuel tank. Actually, this rule of thumb applies to electric fuel pumps as well.

 

4-5 inches of mercury is equivalent to 56-68 inches of water column, so your pump could suck water up from that distance below it, and gasoline even higher since it is less dense. Seems like the suction is sufficient.

4 psi should be plenty to supply a carburetor, but low pressure gauges are often quite inaccurate.