Saturday, April 1, 2017

Bing 64 (CV) Carburetor Part 2 (Starting Carb) Sport Aviation / Experimenter "Technically Speaking" Article February 2017

Bing 64 (CV) Carburetor Part 2  (Starting Carb)

In Part 1, we examined the basic principals of operation of  the CV (Constant Velocity) carburetor. In this article we will take an in depth look into one of the most misunderstood subsystems of the carburetor, the “Starting Carb”.  It is often referred to as the “choke”, however, this  doesn’t properly describe the operation of the Starting Carb.  A choke is, really, a valve on the inlet side of a carburetor used to restrict the flow of air through the carburetor. This results in a low pressure with the intake manifold and carburetor system as a whole. This is different from the carburetor butterfly valve which is located down stream from the fuel nozzle which also restricts the airflow creating a low pressure, but only within the intake manifold. The choke valve which is located before the fuel nozzle presents a low pressure to the entire carb. This low pressure, naturally draws more fuel through the carb and into the intake manifold resulting in an enriched mixture. The starting carburetor, on the other hand, is in fact a separate carburetor within the larger carburetor. This starting carb provides for an enriched mixture by introducing addition fuel as well as air during the starting sequence. The starting carb on the CV carb is different from that incorporated into the slide type carburetors used on the Rotax 2 stroke engines. The (Bing 54) carburetors, used on the 2 stroke engines also use a starting carb, but it operates in an On/Off capacity. On the CV carb the starting carb operation is adjustable allowing for full application during start and cold weather operations, and the ability to reposition the “choke” for less affect as the engine warms.
Figure 1

The starting carburetor mechanical components are located on the side of the carburetor. (Figure: 1) The starting carb is actuated via a Bowden cable similar to the cable used to operate the throttle. The starting carb is  held normally in the off (closed) position by a spring. One of the more common carburetor problems, that we see, is a rough running engine at Idle up through the mid range of operation. If the “choke” cable is sticking in the on or partially on position, the results will be a very rich mixture and rough running engine. We have seen several instances of owners sending their carbs in for overhaul because they were running poorly at lower RPM settings, only to have their freshly overhauled carbs respond just as poorly as before they were overhauled. Frustrated, they brought their aircraft to our shop only to have us identify that the friction within the choke cable was not allowing the very small return spring to pull the choke to the off position. If your engine is running poorly at the low end of the operating range, be sure to look to insure that the actuation arm is hitting the lower stop on the choke housing. Many aircraft manufactures replace the stock choke return spring with a slightly stronger spring.
Exploded View of the Staring Carb
Let’s take a little closer look into the theory behind the function of the starting carburetor. In order to utilize the function of the starting carburetor we utilize the same principles as a choke system. However, recall that we indicated that the starting carburetor is a separate carburetor in itself. As a result, we can use the throttle butterfly as a choke valve to create a low-pressure on the intake manifold side of the carburetor. When the throttle is  fully closed, and only when it is fully closed, the pressure on the intake manifold side of the carburetor is much lower. We use this low-pressure to draw fuel from the very bottom of the float bowl up through a small “starting jet”, then up through a “pickup tube” into passageways within the body of the carburetor and eventually up to the face of the starting carburetor valve. (Figure: 3)
Figure 3

The starting carb “valve” either blocks the fuel from transitioning through the internal  passageway to the outlet to the engine or is positioned to allow fuel to pass. In the partially choked position a small orifice is opened up at the bottom of the starting carb. In the full choke position, a much larger orifice is now opened up. (Figure: 4)

Figure 4

Keep in mind that the basis for being able to draw fuel all the way from the bottom of the float bowl, up through the starting carb, and through the passageways into the intake manifold is based on having a significant pressure differential between the pressure in the intake manifold and the pressure within the float bowl chamber. If we crack the throttle during the starting process, we have essentially negated the effects of the starting carburetor.  The throttle must be closed in order for the choke to function. Trying to employ typical Continental/Lycoming starting procedures generally results in much frustration. The necessity for cracking the throttle has been addressed in the design. The large hole located directly to the left of the starting carb valve is a passageway from the inlet side of the throttle carburetor body into the choke housing assembly. This allows air to enter into the choke housing and be directed around the starting carb valve assembly. At this point the air transitions into the cut-out  slot in the brass plate that makes up one half of the starting carburetor valve assembly. At this point the air combines with the fuel and transitions into the passageway within the carburetor body and exits the outlet on the downstream side of the throttle valve and into the intake manifold. (Figure: 5)
Figure 5
As you can see, the starting carburetor system is really quite simple and brilliant. Properly maintained, starting your engine should never be difficult at all. Now that we understand the system, let’s take a look at some of the problematic areas which we see on a regular basis. Located within the float bowl, but way down inside where the pickup tube is located, is a starting jet. Most people aren’t even aware that it is there. It is not uncommon that contamination will plug this jet because it is located at the very bottom of the float bowl. The size of the starting jet is very small and old fuel can often times form a film over the orifice. This can be very easily inspected by simply removing the starting jet with a small straight slot screwdriver. The pickup tube that is located within the body of the carburetor, which transitions down to the starting jet, can also become contaminated and plugged. Make sure, at a minimum, that you blow through this tube to ensure  there is no restriction.  Speaking of restrictions, the orifice located within the starting carburetor valve assembly, that is used for the partial choke position, is also very small and sometimes becomes blocked. If the engine starts in the full choke position, but quits as you moved to the partial choke position, you may want to pull the choke assembly apart to inspect for contamination on the small orifices. Anytime we see someone installing a primer onto the Rotax engine to help with the starting process, we almost always find that this fuel passageway is plugged up.  There is no need to ever install a primer on the Rotax engine.
The last, but most common problem that we see, is with improperly trained pilots not recognizing the necessity for having the throttle in the closed position during the start sequence. Without that pressure differential we simply can’t get the fuel all the way up from the bottom of the float bowl. On the other end of the spectrum, rather than not being able to get fuel into the engine during the start, if you find your engine running rough, especially at low RPM, you might want to pull the choke assembly apart and look for contamination between the valve assembly and the face of the carburetor body where the two meet. Even a small chunk of contamination will off-seat the valve allowing fuel to leak under the disc and directly over into the passageway to the carburetor outlet. With this scenario it doesn’t matter where the choke is positioned, it simply will leak fuel across the face of the valve. There is a small spring located on the starting carburetor shaft that maintains light pressure against the starting carburetor valve assembly and the body of the carburetor. Ensure that all of these components can float on the starting carburetor shaft. There are both calendar time limits as well as  flight hour limits for the carburetor overhaul. This overhaul should include an inspection of the starting carburetor system as well. We find these carburetors to be nearly bulletproof. The caveat is, you can not expect them to be bulletproof unless you do the required inspections, maintenance, and overhauls. And the key to being able to accomplish  any of these three tasks is a good theoretical understanding of how the carburetor works.