A few facts to consider

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1" of fuel head is equal to .022 psi
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46.17" inches of vertical fuel column is equal to 1 psi

Fuel boils at 170 degrees F

An average engine will consume between .435 and .450 lb. of fuel per
hp per hour

Cowling air typically runs 60 F warmer than outside ambient air
temperature, with isolated hotter spots.
Fuel boils at 150 F. Being a witches brew of hydrocarbons, it will start
to boil at a "low" temperature but not completely boil or vaporize until 450F.

A fuel system must have an adequate fuel vent.

 Ellison, Rotec are types of diaphragm carburetors. These
carbs usually require some fuel pressure to operate correctly.

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These carbs do not operate well on more than .5
lbs of fuel pressure; yes that is less than pound of fuel pressure per square inch.
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These carbs do not vent any fuel vapor that may form in the body to
the venturi as there is no float bowl well or diaphragm vent.


If a vapor lock forms in the body of the carb or the fuel line
preceding the carb, the engine will start to surge and eventually
quit if the vapor lock cannot be moved through the carb. It requires
the absence of heat to remove the vapor lock from diaphragm style carbs
as fuel pumps are usually not used.

The extreme demand on the Rotec diaphragm regulator is to keep the pressure (in terms
of inches of fuel) at the jets the same level as a float in a regular carb would do.
In other words plus or minus ten or twenty thousands of an inch or better. +/- .010
to .020 or better below the jets.


You should have a Manifold pressure gauge

You should have a air/fuel ratio meter. That implies a wide band O2 sensor.

I would also get a compound pressure gage and connect it between the diaphragm
Rotec pressure regulator and  the pressure regulator after the pump for testing and
trouble shooting only. Mount it at the same height as the diaphragm pressure
regulator. Just like measuring blood pressure. Exact fuel pressure is critical to the
operation of a diaphram carb.

The air/fuel ratio meter should read as follows:

Cold start to warm idle 8:1 0 ­ 800 prop rpm

Idle 12:1 800 ­ 1000 prop prop rpm

Mid range 16:1 F/A 1000 ­ 2500 prop rpm

Cruise @23 inches of map.

14:1 air/fuel ratio
2700 to prop 3300 rpm
Take off 12:1 to 13:1 A/F Wide open throttle

Here is compound pressure gage from Mcmaster carr


Paul Lamar


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