Carburetor Install, Connection and Calibration

Carburetor Install, Connection and Calibration
By: George Hurt

The carburetor serves an important function to much more than just your engine running. It must meter the air and fuel quantities going into the engine, mix them together properly, and do it in precise ratios over a wide range of engine loads, speeds, altitude, and atmospheric conditions. The purpose of this article is to assist you in the initial set up, installation and connection of the carb(s) to your engine. A correct initial setup and adjustment, as well as precise installation and connection of components at this time will have a great effect as to how well and efficiently your engine performs for you. Care and precision here are crucial.

There are three basic fuel circuits in the carburetor. The first is the idle circuit which is in effect up to about 2000 rpm, more or less. This circuit is controlled by the idle jet size and the idle air screw adjustment. The second is the mid range circuit which is then in effect up to about 5800 rpm and is controlled by the needle jet size, the jet needle size and taper and the jet needle clip position. The third is the full power circuit which operates from about 5900 up to wide open throttle and is controlled by the main jet size.

All three of these circuits have some amount of overlap which enables a smooth transition from one to the other so all circuits are somewhat interdependent. These circuits all have changeable or adjustable components which makes the carb customizable to satisfy the needs of differing engines and operating conditions.

In our case, we are setting up these carbs for operation on an aircraft engine. What we need our carbs to deliver for our engines are long life, reliability, smooth transition between circuits, a wide power band (usable torque throughout the rpm range), engine cooling and fuel efficiency.

**CALIBRATING THE CARBS (Step 1)**
Fig. 1 *Click on images to enlarge.* Fig. 3 *Click on images to enlarge.* Fig. 5 *Click on images to enlarge.*		The first step in calibrating the carbs is to disassemble them and verify that the jetting is correct for your altitude. All the jet and needle numbers as well as clip positions can be compared to charts found in most of the catalogs provided by all the Rotax Distributors such as Leading Edge Air Foils, California Power Systems and Lockwood. The correct jetting and needle numbers can be found in the Jetting Chart and Altitude Main Jet Conversion Chart shown in (Fig. 1 & 2). NOTE: After you bring up Fig. 2, place your cursor over the image and it will turn into a magnifying glass with a PLUS symbol in it. Click on the image to fully enlarge for readability. After the jetting has been verified and/or corrected, and everything on the bottom (in the float bowl) has been reassembled, the last step before installing the carbs is to set the Idle Air screw. This is the small recessed screw on the left (as viewed from the carb throat) of the carb just above the float bowl level (Fig. 3). It should be screwed in lightly and backed out 1/2 turn as an initial setting. Caution, screwing it in too harshly can ruin the needle seat. After everything is verified correct, you can reassemble the carbs. It should be noted that during reassembly of the carb slide, the small O-ring should be placed on the jet needle with the O-ring on top of the e-clip (Fig. 4). Failure to do so will promote premature wear on the jet needle as the O-ring is used for vibration resistance. Also, if the O-ring is placed under the e-clip the carb jetting would result in an enriched state of operation. This photo shows the needle assembly reinserted into the bottom of the carb slide (Fig. 5). Photos (Fig. 6 & 7) show the order to reassemble the carb top, return spring, white spring cup, and how to hold that assembly together while inserting the cable back into the carb slide. Insert the cable tip into the top of the slide and into the larger hole in the bottom of the slide then slip the cable over to the smaller hole. Now let the cable tip slide up into the recess in the smaller hole and release the spring and cup. You will have to reorient the white plastic cup so that it goes correctly down into the carb slide. Once that is reassembled, you can reinsert the carb slide assembly back into the carb and finish the carb assembly. Note: The carb throttle cable does not exit the carb top in the center of the plate, it is offset. The offset should be towards the rear of the carb.		Fig. 2 *Click on images to enlarge.* Fig. 4 *Click on images to enlarge.* Fig. 6 *Click on images to enlarge.*

Fig. 7
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**INSTALLING THE CARBURETORS**
The first step in installing the carbs is to be sure that the intake manifolds are mounted and oriented correctly. At this time, you should inspect the oil injection lines between the manifolds and the injection pump to be sure the crimp connectors are installed correctly. Also check that the injection lines are not pinched or otherwise restricted. It is very rare for there to be a problem but it does happen. Photos Fig. 8 and 8a show the carb sockets. They slip over the manifolds and fit into a groove that helps keep the carbs securely mounted to the engine and oriented correctly. Note the arrows on the sockets in those photos (Fig. 8 & 8a). They indicate the fuel/air flow direction with the arrow going towards the engine. After the carb sockets are installed onto the manifolds, loosely install the socket clamps onto the socket, do not tighten at this time. The clamps should be arranged so that they are accessable once the aircraft is assembled. Personally, I like the screws on the bottom with the rear carb clamp screws facing aft and the forward carb clamp screws facing forward. Next install the carbs into the sockets. This will take a bit of force to get the carb deep enough into the socket to fit into the groove. Once the carbs are in position, place a straight edge such as a ruler on top of the carb top plates and rotate the carbs until the straight edge sits squarely on top of both carb top plates as seen in photo (Fig. 9). The carbs are now aligned correctly. Once the carbs are installed and aligned the socket clamps can be tightened. Note that the clamps fit into a slight groove so be sure that they are in the correct position and tighten them down. At this point, the fuel lines and control cables can be attached. Go back now and recheck the alignment.

Fig. 8
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Fig. 8a
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Fig. 9
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**CALIBRATING THE CARBS (Step 2)**
Fig. 10 *Click on images to enlarge.*		After the carbs have been installed and connected, the next step is to get the carb slides sitting on the bottom of the carb bores. The idle stop screws should be just barely positioned to begin raising the slides for idle adjustment with a small amount of slack in the inner throttle cables. This position can be found using two identical pencils or drinking straws inserted into the carb inlet with the tip just under the edge of the carb slide (Fig. 10). The pencils will detect any slight raising or lowering of the slide. The object here is to get the slide on the bottom of the bore, not resting on the idle stop screw. This is the starting point for all further carb adjustments. This point must be set precisely and extra care here will result in a smoother running engine with a bit better performance. The next step is to adjust the throttle cables so that they just begin to open precisely together. This can be detected with the pencils (Fig. 11). This photo shows one carb beginning to open slightly ahead of the other. Again, adjust the cables so that they begin precisely together. This is a point where care to get it correct will result in smoother and better engine operation. Many of these adjustments may have to be repeated several times to get them precise.		Fig. 11 *Click on images to enlarge.*

Fig. 12
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Once you have the carb slides setting precisely on the bottom of the bore and the throttle stop screws precisley set to begin opening the the carb slides exactly together, you need to verify that the slides are arriving at the top of the bore together. This can be seen in this photo (Fig.12) where the slide is just barely at the top of the bore. This is verified visually or with your finger. If the carb slides don't arrive at the top of the bore together, then you probably have some type of throttle cable problem that will have to be resolved.

I want to make one point about idle speed adjustments to Bing carbs. Bings are very subject to atmospheric changes from day to day. I like mine to idle about 2000--2200 RPM where the engine idle smooths out but if it is set at one idle speed one day, it is not likely to idle anywhere close to that idle speed the next day. I always advise to not idle by the idle stop screws but by the tachometer. This is especially important on final approach in your aircraft where if you pull the throttle all the way back it may very well just stop running.

The next step is to adjust the idle screws in two full turns each. These need to be done exactly the same and all further adjustments to them must be exactly the same. This can be done now or at the earlier point where the throttle slide was sitting on the bottom of the carb bore with the idle speed screws just barely beginning to raise them. The last step is to lock down the cable ferrule on the carb top plate (Fig. 13). Now go back and verify that the inner throttle cables have a small amount of slack in them and that the slides are beginning to rise at precisely the same time and that they are arriving at the top of the bore together. You will need to recheck these adjustments several times in the first few hours of operation and occasionally thereafter. Install the air filter and you’re done.

Fig. 13
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