The Murphy Builders Site

My radiator cooling is a separate duct below the fusealage but
interestingly I also exhaust cowl air out the sides of the cowl and also
take combustion air from the rear top area of the cowl. My intakes to
the cowl are a 3.2" round hole on each side of the prop. My combustion
air inlet temp runs about 25*C above ambient which I would like to
reduce since I don't have a carb. Much of that heat is coming from my
exhaust and muffler. My cowl temp in the vicinity of the vacuum pump is
still much lower than on aircooled engines.

7.5 psi sounds a bit high for a carb. Don't most automotive carbs run
about 5 psi. ?

I am a believer in leaning as much as possible on the ground. Even
Lycoming says that you can't hurt their engines by overleaning below
75% power. That would be even more true for a liquid cooled engine. Of
course you must then train yourself to push the mixture simultaneously
with the throttle.

Ken

snip

FYI - For first flight my cowl had no openings except the scoop at the
bottom directing air to the radiator and outlets on the sides of the cowl in
the upper cowl doors for exit air. I am going to add two 3-inch openings on
each side of the prop mostly to send cooling air to my vacuum pump on the
right side but also to help cool the heads. I don't want too much cold air
in there though because I don't have any carb heat. With the carb on top,
no ram air, and bolted essentially directly to the engine and all that hot
air rising I don't think I need it. Interestingly, the temp and dew point
were 4 and 2 degrees C on the day of my first flight. I did think about
carb ice briefly before I took off. I'm keeping the inlets small because I
want to avoid pressurizing the cowl from the forward inlets because that
could compromise the good pressure differential I currently have across the
radiator based on the coolant temps I am seeing.

I checked my carb floats yesterday. I ran the fuel pumps for a while
(engine not running) and checked pressures with one pump and with both.
It's about 7.5 psi either way. I noted 5 psi in the shop ages ago but I
think that was with the engine running which apparently makes a difference.
After running the pumps I pulled the float bowl level screws and the fuel
was just below the hole for both the primaries and secondaries. The floats
are operating normally.

I pulled all of the spark plugs and they were all black. Mixture is
definitely too rich. I am going to clean them and try using leaner settings
on the mixture control including leaning for taxi. I have been doing a fair
amount of low RPM taxiing and may have had the mixture too rich for that,
even though I did have the control pulled out a bit.

Mike
044SR

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The 7.5 psi I saw was with the engine not running. I remember that it was 5
psi in my shop with the engine running. Either way the float needle was not
overcome even at the 9 psi I saw briefly with both pumps running before it
settled down to 7.5. Since the pressure gauge is mounted right at the inlet
to the carb I need a second person to read it while I am at the controls
with the engine running. I wouldn't be surprised if it reads higher with
the engine not running when the pumps are pushing fuel no where with the
float bowls full and the needle seated.

Mike

P.S. Why didn't I put the pressure gauge in the cockpit? It came with the
fitting to send fuel to the primary and secondary sides of the carb and I
didn't go to the trouble of relocating it to the cockpit. Yet. I'll get to
that eventually.

-----Original Message-----
From: mike.davis@dcsol.com [mailto:mike.davis@dcsol.com] On Behalf Of Ken
Sent: Sunday, October 08, 2006 5:24 PM
To: rebel-builders@dcsol.com
Subject: Re: [rebel-builders] N744MK First Flight Revisited

My radiator cooling is a separate duct below the fusealage but
interestingly I also exhaust cowl air out the sides of the cowl and also
take combustion air from the rear top area of the cowl. My intakes to
the cowl are a 3.2" round hole on each side of the prop. My combustion
air inlet temp runs about 25*C above ambient which I would like to
reduce since I don't have a carb. Much of that heat is coming from my
exhaust and muffler. My cowl temp in the vicinity of the vacuum pump is
still much lower than on aircooled engines.

7.5 psi sounds a bit high for a carb. Don't most automotive carbs run
about 5 psi. ?

I am a believer in leaning as much as possible on the ground. Even
Lycoming says that you can't hurt their engines by overleaning below
75% power. That would be even more true for a liquid cooled engine. Of
course you must then train yourself to push the mixture simultaneously
with the throttle.

Ken

snip

FYI - For first flight my cowl had no openings except the scoop at the
bottom directing air to the radiator and outlets on the sides of the cowl

in

the upper cowl doors for exit air. I am going to add two 3-inch openings

on

each side of the prop mostly to send cooling air to my vacuum pump on the
right side but also to help cool the heads. I don't want too much cold air
in there though because I don't have any carb heat. With the carb on top,
no ram air, and bolted essentially directly to the engine and all that hot
air rising I don't think I need it. Interestingly, the temp and dew point
were 4 and 2 degrees C on the day of my first flight. I did think about
carb ice briefly before I took off. I'm keeping the inlets small because I
want to avoid pressurizing the cowl from the forward inlets because that
could compromise the good pressure differential I currently have across the
radiator based on the coolant temps I am seeing.

I checked my carb floats yesterday. I ran the fuel pumps for a while
(engine not running) and checked pressures with one pump and with both.
It's about 7.5 psi either way. I noted 5 psi in the shop ages ago but I
think that was with the engine running which apparently makes a difference.
After running the pumps I pulled the float bowl level screws and the fuel
was just below the hole for both the primaries and secondaries. The floats
are operating normally.

I pulled all of the spark plugs and they were all black. Mixture is
definitely too rich. I am going to clean them and try using leaner

settings

on the mixture control including leaning for taxi. I have been doing a

fair

amount of low RPM taxiing and may have had the mixture too rich for that,
even though I did have the control pulled out a bit.

Mike
044SR

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