Just an add on to the prior answer.... Many fuel pressure gages are not
really suitable for reading LOW psi and are calibrated for large US size
engines which in the old days required 3.5 to 9.0 psi. Then when F.I. came
into use, gages had to go up into the 30s psi .
To accurately measure an "electric" or mech. pump pressure the most
accurate method is to first get a gage which has a sensative low range
scale (say 0-5 psi). Then insert a suitable Tee fitting into the line as
near the carb as possible. Install the gage and start engine.
Observe psi at idle and also observe it with engine revved up so fuel is
being actually consumed. (Ideal would be to observe psi on the road).
Electric pumps sometime are diaphragm type or vane type, while the original
mech. pump is only diaphragm type. When fuel is NOT being let into the
carb (needle valve is shut) the line pressure can do goofy things depending
on the pump type and potential power of that type pump.
In certain cases the needle valve closes normally, when float chamber is
full, but the pump rams more fuel past the needle valve....causing a
momentary or long term flooding condition.
The use of an adjustable (dial type) fuel pressure regulator lets the reg.
absorb these momentary pressure surges and they don't impact the needle
valve assembly and fuel level remains as desired.
No carb needs HIGH pressure. What carbs need is "enough volume" to
maintain desired "fuel level". The Alpine size engine would have more than
enough volume if the pump would push say 1.5 US pints per minute!!!!
Unfortunately, most pumps have to make high pressure in order to make ample
volume (as required by big thirsty engines) and these powerful pumps can
push one or more quarts per minute.
We really don't care how LOW the pressure is...as long as the vol. is
enough to keep the float chamber level up to desired level.
---------------------------
The Rootes engines have the pump on the "cool" side of the engine so vapor
lock (boiling of the gasoline...it won't pump when it is a vapor) is hardly
heard of with current modern fuels.
However....most of the last crop of carb cars use what is called a fuel
recirculation device which is simply a tiny 1/32" hole in a fitting located
on the fuel pump "outlet" line. A small hose carefully clamped, runs this
tiny amount of continuous fuel flow back into the fuel tank. This allows
the pump to have an increased and continuous flow vol. which is more than
the engine is actually requiring. The affect is that a larger amount of
cool fuel is moving through the pump at all times, thereby preventing the
dreaded vapor lock.
Fuel injection cars sometimes have a similar device which has the affect of
preventing overheated fuel in pipes atop hot engines.
I don't think Alpines need to have this return line installed because there
is no real history of vapor lock with their present OEM setup.
However...I am providing this report so that the LIST will have knowledge
of the principals and reasons why it is used.
-----------------
Dick T.
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