Overhauling diaphragm carbs and theory of operation
I try as much as possible to cover new topics not previously found in any other model helicopter literature and I believe this is one of those precious times. Rather than only explain which parts to replace and in what order I will also cover basically how the pressure carburetor functions internally. By knowing how the system works should offer the operator a sensible way to rectify a faulty unit. In any event I hope you learn something new and I shall endeavour make this as “Murphy proof ” as possible. At first it may seem a rather complicated device next to the glow engine counterpart which is true to a point, but still overall it is a simple enough device. Please view the schematic of the fuel system first noting the fuel flow and the internal components then refer back if and when you need.
The glow carb relies on venturi pressure to draw fuel from the tank and is usually assisted by muffler pressure. A venturi is nothing more than a compartmentalized airfoil which creates a low pressure when air flows through it. By tapping into this venturi at specific points we can expect different pressures. This can be used to an advantage because the velocity and mass airflow will vary under different throttle openings and airflow. By clever designing an optimum fuel/air mixture can be assured through all throttle settings. Unlike the glow engine carb fuel is not drawn from the fuel tank by the low venturi pressure. The gasser carb has an integrated fuel pump which uses engine crankcase pressure as the operational force. When the piston rises in the cylinder a negative crankcase pressure (suction) is created and when it falls a positive pressure develops here. Most engines use a passage through the cylinder to the carb insulator adapter for this purpose. Other engines have a tap off in the crankcase which is piped to a nipple on the carb. These carbs can be adapted to the G-23 by blocking the nipple and plugging the crankcase tap off. A small hole may need to be drilled into the carb to align its internal chamber with the insulator passage. The reason I say “may” is because some carbs have both a nipple and drilled passage. This is a very easy job to do.
The pump is located under the front cover of the carb. This cover is retained by one large screw. The cover serves two purposes. One is to offer various tiny compartments and directed crankcase pressure to the pumping diaphragm which is located directly under it and the gasket. The gasket also helps to form these compartments. The other purpose is to act as the low and high throttle stop. The low stop is adjustable and this is where idle speed or engine cutout is to be set after the completion of correct mixture settings.
The pumping diaphragm moves in and out with crankcase pressure directed on one side. On the other diaphragm side is fuel from the tank. The pump has two check valves which are in the form of a flapper type or reed like in nature. The valves are schematically positioned on the fuel side of the diaphragm only. The pump inlet opens while the outlet closes so that the pump cavity can fill with fuel (negative crankcase pressure). When the crankcase pressure changes on the diaphragm towards a positive value the inlet valve closes and the outlet opens directing fuel flow into a passage leading to the carburetor’s internal filter. Once through this filter fuel is directed to the back side of the carb’s main body. Here it will wait behind the needle valve until commanded to flow by the metering diaphragm. The metering diaphragm is located under the back cover and is retained by four small screws. Depending on the specific unit it may or may not have a primer bulb attached at this end. The metering diaphragm through a lever opens and closes the needle valve on the opposite end of this lever. In actual operation the amount of opening and fuel flow will vary through the diaphragm operated needle valve. The point I want to make is that in operation it is not like a switch on or off. When the engine is not running it will however be full seated blocking the fuel due to a spring force supplied to the lever. All weedeater pressure carburetors function similarly up to this point.
The thousand dollar question is how do we move the metering diaphragm to vary fuel flow? To expand further where does the fuel go from here? Well the diaphragm is moved by the venturi negative pressure (suction). The greater the negative pressure on the diaphragm the more the needle valve opens. Since pump displacement is a function of engine rpm we now have another variable that is implemented effectively since fuel demands are higher at higher rpm. Four fuel jets or ports are strategically located in the venturi so that at various throttle openings and mass air flow (rpm) a proper fuel/air ratio will be maintained. These jets are controllable by the high and low speed mixture screws. The low speed jets are progressively located near the throttle butterfly valve in the venturi at more closed throttle positions since air velocity is higher here in this operational configuration. As the throttle moves to a more open position the efficiency of these jets is reduced as the local pressure over them rises. So now the larger high speed jet comes into effect which is fine tuned or adjusted by its own mixture adjustment screw. The high speed jet becomes effective because mass airflow over it has increased due to the bigger throttle opening. In both cases these screws control separate openings between the diaphragm fuel chamber and their respective jets. The low pressure from the jets does two things. It moves the diaphragm through low pressure changes over the jets and allows the varying but higher pressure fuel to flow into the venturi and mix with incoming air. The theory is intended as very basic, but you must understand that each individual jet is more effective under different throttle and rpm conditions. I shall leave it at that for now.
The currently supplied carb for the very popular Zenoah G-23 helicopter engine is part or model number WA167(A). It is made by Walbro. There are certain design features that distinguish it from its brethren. One is a primer bulb which allows the diaphragm chamber to be quickly filled manually rather than pulling your guts out on the starter and slowly filling a fuel depleted system. The second is the manner in which the metering diaphragm fuel chamber and the passage ways to the mixture screws are connected. The WA167 has a rubber sprung check valve which has to be open for normal fuel to flow to occur. The higher fuel pressure will open it due to the lower pressure on the other side from the jets and this my friends is commonly referred to as differential pressure. The main purpose of this valve is to allow the primer circuit to function so it will not pull air through the jets instead of fuel through the nornal flow path. As a minor side benefit this valve also prevents fuel dripping out the jets from the diaphragm chamber while statically at rest. Don t be poking around here as this part is not included in carb overhaul kits. It is a chrome piece with a rubber center pressed into the carb and located under the diaphragm. Also pressed into the carb are two welsh plugs which respectively each cover the high and low speed fuel jet chambers. Underneath the high speed welsh plug is another filter screen most people do not know about. It is retained to the chamber floor with a small ring and the welsh plug has to be removed to gain access. Dirt and crud can get past the first screen located at the front section before the metering diaphragm needle. If everything else looks copasetic and you are having erratic running with a drop out in a hover look here. The proper carb repair kit comes with new welsh plugs, two screens, snap ring, front diaphragm/valving, metering diaphragm/needle/lever/fulcrum/spring and all gaskets.Next time I hope to cover problem areas associated with this engine. I hope you have learned something useful and it would be nice if you could sign our guest book. This way we know if this type of article is of value to the masses. Thank you.