Fuel pump. Prime or gravity feed or ?
 

When you install a fresh mechanical fuel pump (60 389) does it suck gas from the tank or do you have to prime it somehow? Thx for any replies.

Fill the carb bowls through the vent and it will light long enough to start sucking.

Yes to the above provided that there are no small leaks between the tank and the pump.

Mechanical pumps suck air far easier then heavy fuel.

There is NO suction going on in a mech fuel pump.
The pump creates a vacuum on the tank side of the membran and s pressure on the carb side.
Technically speaking it's atmospheric pressure that pushes the fuel from the tank to the pump.

FWIW

I often leave the line off to pump if the tank or lines were dry
put a little air pressure in the tank with a rag around the air fitting to make a seal
and pump some air in the tank not alot ,,, just purge it ,,,
gas will come out at the pump
hook up the hose and fire it up

Good thread and was timely for me. Thank you PY people.

I was skeptical but it really did take only a second or two to start pumping after getting fuel in the line from tank to pump. Shaker455 recommended I leave the line off the carb and this was good advice. I put about 1.5 oz. of gas in the vent of the dry carb and the engine fired after a couple pedal pumps. The line was routed into a gallon milk jug with a couple brass fittings and I could see gas flowing way before the carb ran dry.

The first gas out of the pump carried quite a bit of debris, even though I had a new tank, a new sender, and a new pump. Shaker's advice kept that stuff out of my carb.

This was a good idea but I did not want to risk blowing gas all over so I pulled the line at the pump and used a MityVac to prime the line to the pump. It took only a few pumps and was done without a mess.

TRUTH which is why tank vents that let air into the tank are so important.

In the old days you also had a vented gas cab that would dump gas on the race track, when the car launched,
and really piss off the track crew.

Much better to have proper tank venting lines and then you can run a a non-vented gas cap.
A lot of cars in the early days had a small restrictor in a looped line that went into the trunk
and back out to air under the floorboard. That started in the 1965/1966 time frame.

Tom V.

Tom, Kenth...

Help me with your use of the term "suction". To me, suction is a reduced pressure acting on the gas in the tank. What is the distinction you are trying to make?

Yes, it's the atmospheric pressure "pushing" the gas, but only because the pump is creating a vacuum relative to the atmospheric pressure inside the tank. If this is not suction, I'm confused..... just like sucking water through a straw from an open glass, right?

But for sure, if there was no vent, the gas would stop flowing when the pressure was reduced inside the tank to the max capability of the pump.... or until the tank fully collapsed!

The tank in my car has two vents, which I don't understand. One at the front and one in the neck at the rear. The vent in front is larger and feeds a vertical rubber hose "standpipe" with a filter as a baffle, but not otherwise protected against fuel flowing through it. Probably a few safety incidents with that deal... like those movie car wrecks that ALWAYS end in a fire or explosion.

Sounds like the 2nd vent was a "Add-On" at some point in time:

The vent in front is larger and feeds a vertical rubber hose "standpipe" with a filter as a baffle, but not otherwise protected against fuel flowing through it.

When Pontiac Engineering went from the single vent next to the filler tube on the tank they then went to fuel tanks that had multiple nipples on the tank and went to a "vapor liquid separator collection box". Evap gases from there went to the carbon canister.

Tom V.

not sure what the vented caps were like in the mid 60's, but the vented caps used for 1970 cars were one way only, IN. gas or air wont escape out on launch or any other time unless the caps membrane went bad.

i use a 1970 non vented tank in my 72 since i didnt want or care about the EEC set up and am not using a factory return type mech pump. i did add a tanks inc vent with roll over protection but didnt notice any difference in driving before i added it. with the robbmc pickup, i have always kept the tank full at the track, running low 11s it has never dumped a drop of gas on the track.

This creates a pressure differential and just like when the piston goes down, atmospheric pressure pushes the fuel in the line or air into the cylinder.

Stan

YOU DO NOT NEED SUCTION TO HAVE FUEL FLOW STAN/ETC.


Vented for "some" enthusiasts means air in or air out.
As fuel will not flow without atmospheric pressure on the upper-side of the fuel supply
in the tank, the atmospheric pressure is what moves the fuel to a new location:
(On the ground if a hole in the bottom of the fuel storage container, be it a gas can, fuel cell, etc.)

Fuel will also flow to a carb even if there is no fuel pump in the fuel system.
I posted about this years ago when a fellow employee at my work had a fuel line fail before the mechanical fuel pump. Here it is again for newer members or old guys
who can't remember stuff.

My friend needed to get home, It was midnight, and he could get the fuel system repaired in daylight the next day at a local shop.

So we filled a 5 gallon fuel "Jerry Can" with fuel after welding a fuel nipple at the bottom of the 5 gallon "Jerry Can".

We taped the can to the roof of his car and ran a hose to the carb bowl fuel inlet nipple.
Now we had a Model A/Model T Ford fuel system. GRAVITY FEED TO THE CARB.
It worked great because he was driving at midnight and on country blacktop roads with the emergency flashers going. So that proved you did not need a fuel pump at all if the fuel requirement to the engine (per minute) was low. It WORKED GREAT!!

Tom V.

Atmospheric pressure is almost a constant, you need suction ( a delta ) or gravity for flow

Suction produces the delta in pressures to start fuel flow in a semi open system, high pressure seeks to fill low pressure void then after the pump fuel is pushed by positive displacement

Gravity feed is a bit more simple for the human mind to understand in terms of flow look at a water fall to understand that putting a fuel tank on a roof is an example of this

You need lower than atmoshereic pressure to work with the atmosphere in a semi open fuel system or gravity to drop it in

YOU ARE CORRECT! We actually used Gravity Feed to supply fuel to the carb but without air
from the outside of the fuel can the flow still would not have occurred in a "sealed system".

Take a plastic straw, put your thumb on the bottom of the straw and fill the straw with water. Then quickly flip the straw over and the water will remain in the straw. NO FLOW.

Gravity still will not cause flow in a straw/hose without a "vent" open to atmosphere.

Even Water flowing in a Stream has Atmospheric Air AND Gravity working to cause flow
down a hill. In a cup or a pond you have gravity but still no flow.

Tom V.

Well...

This thread was about priming a mechanical fuel pump so I apologize for opening a scary box.

I wasn't trying to start a discussion about fluid flow, just curious why Kenth stated "there is NO suction going on in a mech fuel pump."

I'm confident he understands what makes fluids flow, same as you all have shared, and I'm also confident he knows how mechanical pumps work. I figured he was making a point, but wasn't sure what.... and given the topic about priming, I know he has experience to add.

I did try to suck fuel at the carb by pumping the fuel line with the MityVac THROUGH the pump. It did NOT work. Most likely, the vapor return line inside the pump doesn't allow a vacuum to build up unless there's a lot of airflow. That return line has a restriction but I think acts as an open vent back to the tank. I speculate a higher-flow vacuum pump would be able to overpower that vent restriction and pull fuel through the pump. That would truly prime the whole system up to the carb but might get pretty exciting when the fuel showed up. And in the end, the pump primes itself fast.

No worries, Shiny.

The only suction in the pump is when the pump diaphragm moves upward (caused by the fuel pump cam on the end of the camshaft and the the pump lever) and opens the check valve(s) to allow fuel into the pump cavity and then to the carb fuel line.
When the fuel pump cam moved the opposite way, the process starts again. Normal pumps have one inlet valve and one outlet valve.

Tom V.

Thanks Tom

Yes, he probably was saying the same thing.. the suction inside the pump wasn't generated by air at a reduced pressure, but rather by displacement of the membrane in the pump. Either way, I personally consider it suction but it's all good either way!

Mike

Fluid seeks level. Without being acted on by a vacuum/suction (pressure lower than atmospheric), or a positive pressure (higher than atmospheric), it will never go above level. The fuel line comes out of the tank and (almost always) goes above the level of the fuel in the tank. If the car is parked facing uphill, the pump is MUCH higher than the fuel level! Therefore, if there is no vacuum/suction on the pump inlet line (tank outlet), not a single drop of fuel would ever leave the tank. The tank vent allows air in to replace the volume of fuel that is drawn out. Otherwise it would create a vacuum in the tank and suction line. The air isn't rushing in and pushing the fuel out!!! Diaphragm pumps are capable of generating tremendous pressure both on the inlet and outlet. A diaphragm pump positively creates vacuum/suction on the inlet. A spring pushes the fuel out and limits maximum pressure.

Mike, if a lower pressure than atmospheric pressure of about 14.7 pounds per square inch is created by the fuel pump (or the moving action of the pistons in the engine under the closed throttle blades) opening the valve(s) to the lower pressure area, in the fuel pump or intake manifold, will cause the atmospheric pressure to to fill the void (push fuel or/and air) in that area.

From Doug Roe´s book "Rochester Carburetors":

If you are unfamiliar thinking about the density of the combustion charge during the intake phase, concepts such as mass flow, pressure differences and volumetric effect are likely to become confusing.
Here is a simpler description of the principle, worth pondering, which should make things a little clearer.

The engine is an air pump - it takes in air and compresses it.
A common assumption about how the air gets into the cylinder is that the piston sucks in the air - the piston creates a vacuum that pulls the air in.
However, it is effectively atmospheric pressure - about 14.7 psi or 1kg/cm2 - that pushes the air in, assuming its path is unobstructed.
At WOT, the atmospheric pressure helps to fill the void above the piston during the intake phase.
If you close the throttle valves to a small opening, a smaller amount of atmospheric pressure will pass the throttle, the result is a smaller amount of air to the cylinder and thus less power.
Consequently, a small venturi or throttle opening throttles the amount of air that atmospheric pressure can force into the cylinder.
Thus, it is the pumping of the pistons that creates a negative pressure in the inlet under the closed, or almost closed throttle valves, which the atmospheric pressure equalizes more the more the throttle valves are opened.