Manifold Pressure Question

PCHunt · #1 04-07-2009, 11:07 PM

I have a MP gage that is acting "funny" (aviation technical term). It is on an RV-6A, O-320E2D. 150hp, carburetor, fixed pitch prop. Yea, I know you don't need MP on a FP prop, but it came with the airplane when I bought it.

On the ground, the MP gage seems to be correct, showing low (12") at idle or near idle, and 28" or so at full throttle, which would be about right for the DA of around 2500' or so.

BUT, when airborne, at say 7500 feet, WOT, 2650 RPM and indicating around 170 kts or so, the MP reads about 28.5". I can't imagine that the ram effect through the FAB would be that much.

Seems to be reading way too high.

Any thoughts, ideas, or experiences?

PS: The gage is hooked up to the #2 cylinder primer port.

TX7A · #2 04-08-2009, 06:47 AM

Do you have a restrictor installed in the manifold hose?
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OOPS!
Pete, I just realized that you bought the plane and may not know about the restrictor. The MP gage most likely worked at one time anyway. Like Vic says below, it just may be dirty. Sorry about that.

vic syracuse · #3 04-08-2009, 06:55 AM

Manifold pressure gauge lines can become clogged with crud from combustion, fuel, and oil. Disconnect it from the engine and the firewall (hopefully there is a disconnect there) and clean it out with some brake cleaner, carb choke cleaner, etc., then try it. If that doesn't work, it is most likely the instrument.

Vic

PCHunt · #4 04-08-2009, 02:14 PM

I dunno about a restrictor. The MP line is small copper tubing, just like the primer line. I will clean the line out and see what happens.

I don't see how it could be the instrument, because it works fine on the ground??????? Maybe a leak in the case of the instrument?

I just might take everything out and test it..................

Anyone have other ideas?

Thanks,