Carburetors do not appear to be well understood. The carburetors used in hit and miss engines are simple one jet units with only a single mixture adjustment. There is no choke or throttle that can be adjusted. The power cycle in a hit and miss is always at the same speed and the stroke power is always the same, excess energy goes into the flywheels. When the flywheel speed exceeds the regulator limit, miss cycles allow it to slow back down. This simple operation is ideal for a simple carburetor.
Some real hit and miss engines came with 2 or 3 different throttle plates to be used to restrict air flow to the carburetor to match the engines speed and power to the load. Restricting the carburetor and engine power reduces the number of miss cycles and improves average speed of the engine. For model engines operating at high cycle power and having lots of miss cycles makes the engine sound more interesting. If the engine was actually doing work, we would be more interested in constant speed than in interesting sound.
Carburetors have a Venturi, which is a smooth narrow area that increases the velocity of the air passing through. The high velocity air creates a lower pressure. If a jet or nozzle is put in this area fuel can be pulled into the air stream be the low pressure created by the Venturi. Add a valve to control how much fuel can be sucked into the air flow and it is a carburetor.
From the book “Gas Engine Troubles and Installation” by J.B.Rathbun, originally published in 1911 but reprinted in 1995, the Venturi size of the carburetor is normally ¼ of the piston diameter.
This guide line was developed for larger engines. As the engine size reduces the area of the fluid spray bar increases in proportion to the Venturi size. The area of the spray bar blocks a percentage of the Venturi area making it smaller than the drilled size. It is necessary to slightly increase the size of the venture to compensate for this. In my engine, the bore was ¾ so the Venturi should have been 3/16. However, with a 1/8 diameter spray bar passing through the Venturi nearly half the Venturi area was restricted by the spray bar. I chose to increase the Venturi size to 7/32 to keep a reasonable air passage.
Should the carburetor be in a horizontal or vertical air passage? Horizontal are logical and simple, but Vertical appears to be a better choice, as most carburetors will leak at some time. In a horizontal deign the raw fuel is likely to move directly into the cylinder and dilute any oil causing premature failures. In a vertical up draft design, leaking fuel will flow into the air cleaner and then leak out causing no problem. In many older auto engines with down draft carburetors a hole was added to the intake manifold directly under the carburetor to allow fuel leaks to escape the manifold before the fuel can enter the cylinders and cause trouble.
In actual tests with several carburetors and elbows, I observed no difference based on carburetor direction. It was possible to flood the engine with either orientation.
Some builders believe a check valve is needed so once fuel is pulled into the carburetor it will not drain back the fuel tank. Draining back is a logical idea but does not happen with vertical lifts of about 1 1/2 inches. The surface tension of the fuel prevents 1 1/2 inch of fuel from draining back. If the fuel lift is more than about an inch then a check valve is required to prevent draining back to the fuel tank.
The check valve does require more vacuum to pull the fuel into the jet but not so much as to be a problem.
If the engine design has the carburetor thermally coupled to the head, it can over heat and boil the fuel. The boiled fuel can force the fuel back out of the carburetor into the tank. The check valve will prevent the fuel from being forced back.
For some good notes on check valves go to Bob Shores Engine Building Tips #5.
As the engine vacuum pulls fuel into the moving air, the droplets evaporate. Liquids need heat to evaporate. Fuels evaporate at low temperatures and the evaporating process cools the carburetor.
Enough heat needs to be transferred back to the carburetor to keep it warm enough to continue evaporating fuel. The carburetor is mounted directly to the head or using a short pipe. Either method transfers heat to the carburetor. If too much heat is transferred to the carburetor the fuel can boil in it, pushing the liquid fuel back into the tank and causing a vapor lock.
This happened on my hit and miss initially. I was using a thick Teflon washer for a head gasket to reduce compression. It reduced heat transfer from the head to the engine body and the head got very hot causing the fuel to boil in the carburetor every time it stopped. I replaced the head gasket with a metal one and this cooled the head and solved the problem.
On my first vertical engine, the carburetor ran so cold in the beginning that dew formed on it then the dew froze. Shortening the tube to the head allowed more heat to get to the carburetor and solve the problem.
Air cleaners are important. A small speck of dust or grit could prevent a valve from sealing or scratch the cylinder. Wire cloth that is at least 200 mesh can be used as a reasonable filter element. Two layers are best and keeping them oiled so particles will stick seems to work well.
On my hit and miss I used a cylindrical air filter. It looks good and was easy to make, but there is no way to restrict the air flow to speed priming the fuel lines. A simple disk filter allows a thumb to be used to cover the inlet and quickly prime the fuel system. I made a small filter from the screens used in a faucet aerator.
The adjustments are frustratingly critical on these small carburetors. The range for operation is about half of a turn on my carburetors with a normal setting of about 1 turn out...
When I first start a new engine I begin with the screw about ½ turn out. I open the screw about 1/12 turn and try again. Fist the engine just seems to spin an extra turn, then a pop, and then it will catch and begin to run.
Once the engine has warmed up, I adjust the mixture for maximum speed with the speed regulator disabled. Then the speed regulator is enabled and the settings are left alone.
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