>I recently fitted a manifold pressure gauge to our Rotax 912, and have
>encountered a problem with fuel being drawn up into the gauge to the point
>where the dial is half submerged.
>Getting fuel into the instrument panel area sounds rather dangerous. If the
>fuel is making it all the way to the gauge it makes me suspicious that
>there might be a small pressure leak within the gauge sucking the fuel.
>Have you tried doing a pressure check on the gauge? Also the connection at
>the back of the gauge could be leaking slightly.
As the pressure in the manifold and therfore in the gauge is generally
bellow atmospheric pressure, a leak in the gauge or fittings would result in
air being drawn into the manifold, not fuel being drawn out. This would
result in a weak mixture and affect the engine running, but not as I see it,
cause the problem I have.
My theory :-
The gauge consists of a sealed case which is connected to the manifold, and
a capsuel linked to the needle, the pressure inside the case acts on the
capsule to give a reading on the scale.
The manifold pressure when the engine is off equals atmospheric, so the
system is all at equal pressure. When the engine is running the manifold
pressure is less than atmospheric, varying according to throttle setting.
I think the gauge is acting like a vacuume reserve chambre, so that when
the manifold pressure is low there is a corresponding volume of low pressure
in the gauge. when the pressure increases in the manifold, fuel vapour is
drawn into the gauge to equlise the pressures, this vapour then condenses
and over a period of time builds up in the gauge where it is trapped.
Some form of vapour trap close to the manifold tapping is required, I'm
hoping the restrictor I have fitted will do the job.
Any other theories ?
Jim Naylor
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