Choking your two stroke

Choking Your Two Stroke
By  James Goss


     Another item of interest to me that I would like to write about deals with choking a two-stroke engine. While having the instrumentation equipment set up to perform pressure checks that I used in the article “Fuel Tank Pressures”, I decided now would be a good time to run some experiments on how choking a two-stroke  engine also affects pressure in our fuel tanks. Using the u-tube manometer is inconvenient, being 4 feet long and it must be secured in place, so during this test period I wanted to answer all the questions I had accumulated over the years and not have to set it up again. Plus it required having some of my good engines on the test stand instead of on airplanes.  

     Now I am sure everyone knows how to choke an engine, but I wanted to know how the fuel tank pressure was altered while the engine is being choked.  Like most people I place my finger over the carburetor's venturi, and then flip the prop a couple of times, this draws fuel into the carburetor.  This seems to get the job done, but have you ever seen someone place their finger over the muffler's exhaust port to choke their engine? I don't know if we can call this method choking because it does not choke off the air to the carburetor, which is where the expression choking comes from.  I do know this procedure works on some engines better than others.  To begin with I did not see how this method was allowing the carburetor to draw more fuel until I performed the following experiment and then it was very apparent how this method works. Before you read any farther see if you can figure out the answer.  

     While you are cranking the engine by hand the carburetor does not develop any sucking action at all because the venturi of the carburetor is open to the atmosphere. Even with the barrel closed to idle there is almost no suction of fuel from the fuel tank.  Now when the piston goes up it does create a vacuum at the induction port through the shaft, but it pulls air through the venturi instead of fuel.

     With an electric starter, which by the way turns at over 5000 rpm's with no load and about 3000 rpm's when loaded with a .40 to .60 size engine, it will only create a vacuum of about 4 inches of water. This would be about the same as having .1 psi on the tank. This is based on the fact that a pure vacuum, 29 inches of mercury, will lift a column or water 34 feet. So this is why your engine starts better with an electric starter, it is pulling a little more fuel from the tank plus at this speed some pressure is returning from the muffler. If you choke the carburetor while hand cranking the engine, the sucking power will increase.  It will now be equal to a tank pressure of .4 psi. I think for this article it will have more meaning if we think in terms of pressure and not in terms of vacuum.

     If you choke the carburetor while using an electric starter the sucking action will really go up. It may go up to about 3 to 6 psi. This is estimated because my manometer only reads about 1.5-psi max.  It almost blew the top off.  I could place two or three manometers in series and add the results or start using a pressure gauge at this pressure of 3 psi or greater. Trying to use a bourdon type pressure gauge when maximum pressure is less than 1 psi is not very accurate, the manometer is the best instrument for this low pressure.  This higher pressure shows us why it is not a good idea to choke the engine for any length of time while using an electric starter. It can fill the crank case full of fuel in short time.  This can cause the engine to lock up in a hydro lock. A hydro lock is when the piston can't move; it is trying to compress a liquid instead of a gas and as you know you cannot compress a liquid. On smaller size engines this could bend the crankshaft if you are using a strong electric starter.  I would say never choke your engine while using a starter if the glow battery is not connected.  Always have the battery on so the engine will fire when fuel does start arriving.  Without the battery connected to the glow plug you could fill up the crankcase with fuel and not know it until it is too late. By that time you have already subjected the crankshaft to a lot of stress. That stress is also transferred to the ball bearings and may shorten their life. I have never liked using an electric starter on an engine with bushings; they are made of brass and are easy to wear during high stress.  

     If your engine does get flooded and will not start you can get your engine running faster if you remove the glow plug, place a paper towel over the engine's head, shut off the needle valve, and use the electric starter to turn over the engine. This will blow all the excess fuel out of the engine. Without using a paper towel you will get a face full of fuel, which doesn't taste very good.

     Again if you are hand cranking your engine you are not getting any pressure from the muffler to pressurize the fuel tank. If you place your finger over the muffler's output and prop the engine by hand, you will now have pressure going to the fuel tank. On this .46 os it was .15 psi and this was without the check valve in the pressure line. With the check valve the pressure will double. If an electric starter is used while your finger is restricting the muffler's output you can expect to have about 3 or 4 psi on the fuel tank. This can also flood the crankcase and lock up your engine.

     So even though this method of stopping up the muffler's output does not choke off the air to the carb, the increased pressure on the fuel tank will push more fuel to the engine. By and large, the results are the same in most cases the engine starts.  If you think about what is happening, it's so simple. The glow engine is an air compressor just like the big ones, except it has no valves as such. It uses porting and timing to control the intake and exhaust. So in this case the fuel tank becomes the air storage tank. When you restrict the exhaust port of the muffler the fuel tank receives more pressure thus the engine receives more fuel.  But the engine still does not receive as much fuel as it would by choking the carburetor  because it still draws air through the venturi. Why was this so hard for me to comprehend???


     If you really would like to prime your engine for starting, try this: Leave the carburetor cover in place and use your finger to place over the muffler. Prop the engine two or three times and it's ready to go.   In my case I use a carb cover for the carburetor and on the muffler's exhaust. This makes it really easy to prime my engine plus when storing your engine between flights it is sealed from the surrounding atmosphere. This prevents or reduces oxygen and moisture from entering the engine.  Without oxygen and moisture steel can't rust. Remember the crankshaft and ball bearings, if your engine has ball bearings and not bushings, are made of steel.

     So when you see someone with his finger over the muffler's output, they are not choking the engine they are pressurizing the fuel tank, but lets call it choking anyway. You may also see this done on a four-stroke; it will give the same results.