Air is drawn in thru the carb, which is mixed with the fuel as it enters the intake port. As the piston is on the down stroke, the mixture is forced up the bypass ports into the combustion chamber. As the piston travels upwards, it compresses the mixture to the point where the glow plug ignites the mixture  and forces the piston back down and at that point  the exhaust port opens and the exhaust is pushed out by the piston forcing the new fuel/ air mixture into the combustion chamber.  Not all of the expanding gases can fit thru the muffler at once  so some of it will stay in the muffler as the next  cycle begins and that backpressure will keep the unburned mixture from entering the combustion chamber and blowing over the top of the piston and out thru the exhaust. Hence  better fuel economy  Timing is achieved by a few factors: Glow plug heat, port locations, and length of stroke (length the piston travels up and down).  Any one of these out of tolerance will cause "detonation". Hope this helps anyone with any questions on how a 2-cycle engine works. 

The Pinch Test

If you pinch the fuel line and the engine speeds up, it is on the rich side of the adjustment.  HOW MUCH is speeds up shows how close you are.  If it speeds up a lot, you are rich.  If it speeds up just a little, you are just right.   If it doesn't speed up, you are just going lean.  If it slows down, you are LEAN.    This test temporarily starves the engine for fuel ,and is reliable to test for a too-lean condition.  At full throttle, quickly pinch the fuel supply line.   The engine should momentarily increase RPM's before starting to die.  It starts to die immediately, then it's already too lean and should be adjusted.

Initial Setup and break In (ABC Engines)

The ABC engine break in process is similar the ring engines with one major difference. The ABC engines are not run as rich as the ringed engines in the early process . 

1.Choke and start your ABC engine in the same manner as detailed above in the Ringed Engine section. 

 2.Let it warm up for 30 seconds and advance the throttle to full open. 

 3.Now slowly lean out the mixture while you listen to the engine. As you lean the engine, you will hear it increase in RPM. At some point, you will hear the engine sound like it is jumping up and down in RPM. The exhaust note will be jumping up and down in pitch. You want to continue to lean the engine until it is running at mostly the higher pitch sound, with just an occasional break to the lower pitch sound. Let the engine run at this setting for 5 minutes.

Idle Mixture Setting

The idle mixture is adjusted with the brass screw that's located in the center of the throttle arm. It operates in the same manner as the high speed needle.    Clockwise leans it out and counterclockwise richens it up. It needs to be open at least 1/2 turn. The basic adjustment of the idle needle has been set at the factory and should require little adjustment, if any. Use the same pinch and release method to determine if the idle mixture can be leaned out. You may have to hold the pinch a little longer because the fuel flow is less at lower RPM ranges.

 1.   The best way to adjust the idle mixture is to have the engine running at full throttle, and slowly decrease the throttle to less than 1/2 open.

2.Once the engine has been throttled back to less than 1/2 throttle, try the pinch and release method. The engine should increase in RPM slightly. This means you can lean the idle mixture a bit, about 1/8 of a turn.        

3.Reduce the throttle a little more and repeat.    

4.You should be able to work down to a nice slow  idle.  The Carburetor should be about 1/32" to 1/16" open when the engine is idling properly.                                              

How To Adjust Top End

  1. Close needle valve, then open 2 full turns.
  2. Make sure you have a good glow plug.

  3. Start engine, and run at full throttle.
  4. Slowly close needle valve to lean the engine.
  5. Pinch the fuel line briefly between every couple of clicks to see if the RPM will rise any further.
  6. If the engine picks up RPM, continue leaning.
  7. A properly adjusted engine will pick up just a little when you briefly pinch the fuel line.
  8. If the engine immediately dies when you pinch it, it is to lean.
  9. As a final check, hold airplane vertical to ensure the engine will not lean out.

How To Adjust Low End

  1. The procedure resembles that of adjusting the top end.
  2. Close low end needle or screw, and then open 2-3 turns.
  3. Start engine and run at full throttle for at least 10 seconds.
  4. Bring engine to idle (about 2500 RPM) and let idle for 10 seconds.
  5. Pinch fuel line and hold.
  6. If the engine speeds up and then dies, the low end is to rich.
  7. If the engine slows down and then dies, the low end is to lean.
  8. Turn needle or screw 1/4 turn in the direction required and repeat process.
  9. Ideally when you pinch and hold the fuel line while the engine is idling, the engine will not gain or loose any RPM before it dies.
  10. When you get close to this, make smaller adjustments to really dial it in close.
  11. In some cases, you will have to readjust the top end while adjusting the low end. Always make sure the top end is properly adjusted before adjusting the low end.

Air Bleed Screws    

When adjusting air-bleed carburetors (the ones with the little hole in the front), a good rule to remember is the word “richen”. Split this word in half (rich-en), and when you want the carburetor rich, turn the screw in. Of course leaning the carburetor would be turning the screw out.

 Engine Overhauls

When you take the engine apart and you want to get those nasty varnished in place cylinders and bearings out to clean or replace just put the crankcase (with the stuck parts) in the oven on the bottom with the oven set to a high "baking" setting. Must be baking so the lower heat element is hot. Do not put parts that have rubber or silicone parts; i.e. carb, O-rings, gaskets in the oven. Bake for 3 to 6 minutes. Pull the crankcase out with vise grips or burn your hands, your choice. Watch that the bearings or cylinder do not fall out when removing the engine. If they do not come out easily bang the back of the case on a block of wood and they should fall right out. If not try heating longer.  Nasty, dirty and varnished engines tend to run hot. High temperatures will reduce engine power and life. To clean the crankcase and cylinder head of vanish first remove all parts, i.e. crank, piston, liner, bearings and anything else that is loose. Take the crankcase and cylinder head and wash oil and lose dirt off with hot water and soap, "This will not hurt the cast aluminum parts". Then dry with a rag and soak in a jar of good old paint stripper from a hardware or paint store. Use a fine stiff acid brush or an old tooth brush to remove stubborn varnish.

CAUTION! DO NOT GET THE STRIPER ON SKIN, IT'S PRETTY CAUSTIC! USE RUBBER KITCHEN GLOVES AND EYE PROTECTION. (This is a serious warning I used some of the commercial cleaner sold for cleaning engines. It did not seem all that bad, did not burn my hands or anything, so I did not use gloves two days later my hands looked like they had been sun burned and most of the first layer of skin pealed off. ) Also in time the tooth brush will melt if you don't wash the stripper out frequently. After you have got all the varnish off the engine rinse all the stripper away with water and dry the part with a rag then coat with a good after run oil. I use air tool oil and it works fine. ( NOTE: the reason you want to use air tool oil is that it will not attack the o rings and other silicone parts in the engine like normal oil will. Don't forget most of our engines are designed to burn alcohol not oil based gasoline.)  To install a new rear main bearing, drill a hole in a hard wood block large enough to put the threaded end of crank in. While inserting the crank through the case (with rear bearing in place on the crankshaft)  then use a long dowel through the back of the crank and tap with a small hammer until the bearing is seated.

What is meant by Schnuerle Porting?

Some engines are advertised as being schnuerle ported.  This means that there are multiple (usually three) bypass ports in the cylinder which allow fuel up into the combustion chamber.  On a non schnuerle ported or "loop scavenged" engine there is only one bypass port which is opposite the exhaust port.  This may result in some of the fuel passing over the piston and right on out the exhaust port.  With schnuerle porting the fuel arrives from different directions and converges at the center of the cylinder which allows more fuel to be burned, hence more power.  Most newer engines use schnuerle porting.

Plug Reading

Here is a rule of thumb to go by when reading your glow plug.

1)Wire and surrounding bottom of plug wet, with like new shiny wire = rich side of optimum power 85%
2) Wire and surrounding bottom of plug starting to dry and wire starting to gray = Very close to optimum power 95%
3) Wire and surrounding bottom of plug dry, wire totally gray but not distorted optimum power 100%
4) Wire and surrounding bottom of plug dry, wire distorted = slightly lean
5) Wire and surrounding bottom of plug dry, wire broken and distorted or burnt up = extremely lean possible engine damage!   
Note: You can only "Read" your plug in a nearly new state [Wire like new and shiny] A gray plug can still operate well. But after it has totally gone gray performance can start to fall off. To test just put in a new plug and if there is no difference in performance save the gray one or put it back in. If your engine does not feel or run right try a new plug before making major tune change's

Engine Preventive Maintenance.

The best thing you can do to prevent engine problems is to keep your engine clean. Dirt causes a lot of problems that are not always obvious as to cause. Most dirt enters through the carburetor, but can also enter through contaminated fuel or loose muffler connections. One of the most risky times is after a crash or landing in high weeds, dirt and plant material may enter the engine at this time. Never turn over an engine after a crash until you make sure all dirt has been removed from the carburetor. If dirt may have entered the carburetor you should remove the carburetor from the engine and wash in clean fuel. Check the input hole under the carburetor for dirt before replacing the carburetor. When fuel burns it chemically changes, the exhaust gases and waste oil become acidic and can quickly corrode ball bearings and other parts in your engine. The best way to prevent this is to use after run oil. You can buy bottles of after run oil at most hobby outlets, but a lower cost alternative is to buy air tool oil. Most large hardware stores have several brands. The reason you have to use air tool oil instead of motor oil is that it is not petroleum based. Petroleum based oils can attack the silicone o-rings in a glow engine. If you have a gasoline engine use any good 2 cycle motor oil, do not use air tool oil, as it will attack your neoprene rubber fittings. Make sure you check the manufacturer’s instructions before using any after run oil, some glow engines with pumps can be damaged with after run oil. With 2cycles engines simply drip some after run oil into the carburetor while turning over the engine by hand until you have put about one half ounce into the engine.  With four cycle engines you will have to connect a fuel hose to the crankcase breather port and inject a half ounce of oil with a syringe while turning over the engine by hand. This is best done with the airplane held tail up so the oil is more likely to run into the bearings. Next inject another half ounce into the fuel input port with the throttle open this time make sure the barrel of the carburetor is pointing up or the oil will just run out.. This is messy and oil will be dripping out of your engine until you go flying again, but it is better then having your engine freeze up. Next time you go flying make sure that you turn the engine over a few times by hand before starting to make sure the engine is not full of oil and liquid locked.  Four cycles require that you check the valve timing and clearance every so many hours. Your instructions should tell you how to do this. Basically you turn the prop until it is at TDC on the compression stroke at that time the valves should be closed and there should be a small amount of clearance between the valve arm and the valve stem. The hard part is determining when the engine is in the correct position. It is critically important that the arm not touch the valve stem. Remember that when the engine is hot the clearance may be smaller do to expansion. If the valves are left open even the smallest amount they can be burned, you also lose compression and power.

Engine Size Conversion Chart

Cubic Inches to Cubic Centimeters


.049 .8
.09 1.5
.15 2.5
9 3.1
19 3.5
.25 4.1
.29 4.8
.35 5.7
.40 6.5
.46 7.5
.50 8.2
.61 10.0
.80 13.0
.91 14.9
1.20 20.0
1.50 25.0
1.60 26.2
1.80 30.0
2.00 32.8
2.40 39.3
2.70 44.3
3.00 49.2