By John Larson
"You ought to try the adjustable carb needles, they really
The advice might not have been taken too seriously, except that it came
from Rod Snider, a man who has spent much of his life in the air as an
ATP, hang-glider or Avid pilot. When Rod speaks, I listen! At this point
in time, I had a vague remembrance of hearing something about adjustable
needles for the Bing 54 carburetor (used on most Rotax 2-stroke engines),
but couldn't visualize in my mind how they could work reliably and thought
no more about until Rod offered to let me look over his installation.
We were at a breakfast fly-in at Glenns Ferry, Idaho when Rod took me
over to his Landphibian (Avid Amphibian without floats). He showed me two
small knobs protruding from the panel, one for each carburetor. "Go
ahead and turn one." he offered. I did so and found they had a 'click
feel' every time they were turned a revolution. He explained that each
revolution was equal to changing the needle one notch. By watching the
EGTs and turning the knobs, you can dial in the desired mixture for the
altitude. When you open the throttle, the two knobs come out to wave at
you and they move according to the position of the carb slide.
called Mike Jacober, the needle maker at Arctic Sparrow in Alaska, and
wanted to know more. Mike explained that he developed the needles because
he flies out of Anchorage, which is at sea level, and has to climb
thousands of feet to get anywhere on the mainland. This means the
carburetor setting, which works perfectly at sea level, will be so rich at
8000 feet, that the aircraft will be at its ceiling.
Mike wanted to fly to, or possibly over, Mount McKinley and the
carburetor situation made this next to impossible. Jacober developed his
system so the needles are welded to a threaded portion which is welded to
the cable. By turning the cable, the threaded portion is screwed back and
forth in the slide, raising or lowering the needle. With these needles
installed in his aircraft, Mike became the first man ever to fly over Mt.
McKinley in an ultralight. He assured me that they were perfectly
reliable, I reluctantly shelled Out the money and started watching the
mail box. Soon, the slides I sent to Alaska arrived back with the needles
ready to be screwed in place.
The procedure is quite simple. With the needles installed (he sends
instructions) and screwed all of the way out, run the engine up to 4000
rpm and note that the engine will be four-cycling, an indication that it
is running very rich. Begin screwing each needle in clockwise until the
engine clears up. With one eye on the tach and the other on the EGTs, you
can see the engine clean up as the EGTs and RPM rise. You are safe for
takeoff when the engine clears out. The part of the carburetor used at
full power, that is the main jetting, has not been changed. The carburetor
operates on the main jet at full throttle. The Jacober needles do not
change this-you climb out as you have in the past. The needle position
will take effect when the throttle is rolled off of the main jetting, as
in cruise. Now the pilot can adjust the mixture to his desired heat range
by manipulating the needles found that I could dial in near 1200 F and
cruise at 90 mph in my Heavy Hauler and burn about 4.6 GPH rather than the
5.5 I burned previously. The near-one gallon per hour saving would be
reason enough to use the needles but I also found that I could be close to
gross weight and still be climbing at 400 fpm on a hot summer day at
12500' altitude. With oxygen for the pilot, the plane undoubtedly would
crack 20,000 feet.There is another feature I like. Should the carbs become
Out of sync, due to one cable wearing longer than the other, you can
synchronize them in the air with the needle settings until the cable
adjustment problem can be cured on the ground.
At first I wanted to take some offense as I was constantly being
`observed' by the adjustment knobs on the dash, which seemed to be two bug
eyes on stems moving to and fro with the throttle. I soon came to
appreciate them because watching their position, and the position of the
throttle and the tach, I could learn a little bit more about how
everything was working together inside my humming engine.
Want to know which carburetor to work on when things don't look right?
Because the cable will pull up on the carb slide, just reach over and pull
on the first one knob and then the other while watching the EGTs. If you
tuned your engine for perfect cruise—say 1175º F—you have encountered
the annoying situation of watching the EGTs go for 1300 when you reduce
power and nose over for descent. Rotax has convinced me that 1300 is an
unlucky number, so without the adjustable needles you have to set the
needles on the ground to a too-rich setting to have it in the safely zone
for descent (unless you want to descend by constantly playing with the
throttle). With the adjustable needles, you can dial in more fuel and this
will leave you in a nice safe range when you are ready to take off again.
If you fly over a large range of altitudes, the needles are a must!
Jacober said they would pay for themselves with fuel savings and this was
the first time in my life where such an extravagant claim about fuel
consumption turned out to be TRUE.
Also read the follow up article
For more information contact:
- Arctic Sparrow Aircraft Inc.
7321 Rovenna St.,
- Anchorage, AK 99518-2177