Bing carb mixture control - by Arctic Sparrow.

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Bing carburetor mixture control for Rotax aircraft engines.

By John Larson

"You ought to try the adjustable carb needles, they really work!"

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.

[Image]I 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 MORE NEEDLE NEWS

For more information contact:

Arctic Sparrow Aircraft Inc.

7321 Rovenna St.,

Anchorage, AK 99518-2177

Phone (907)272-7001

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