You have finally soloed and convinced your instructor that you are capable of landing your model wheels down most of the time. It's a beautiful Saturday morning, your former instructor isn't around but you're ready for a great day of flying on your own.
After a bit of fiddling, you get your engine started. You then taxi onto the runway and start your take-off roll but before you rotate, the engine sputters and dies. You bring the model back to the flight line, re-start the engine, run it up to full throttle and everything seems to be fine. So, you taxi back out onto the runway, begin your take-off roll only to have the engine die again! Being the intrepid flyer that you are, you are not about to let this bit of adversity ruin what promises to be a good day of flying.
Once again you bring the plane back to the flight line, re-start it and because you think that the engine must be too rich, you lean out the high speed needle and give it another go. Your third take-off attempt ends the same way as the first two. By this time, you are beginning to wish that your instructor were around so that he could perform the secret rituals and incantations that will get your engine running properly.
If you will follow along with the rest of this article, I will share with you some of the techniques that I've learned over the years and you will see that the steps to properly tune an engine are really rather straight forward. For the purpose of this discussion, we are going to assume that the engine has been properly broken-in. We won't get into the proper break-in procedure in this article but if you are interested, there is one on our web site about breaking in a ringed engine.
The first step is to determine the type of carburetor on your engine and while you are doing that make sure that it is mounted tightly so that there aren't any air leaks. The two most common types of carburetors are twin needle and air-bleed. There are other types but they are not as common and not likely to be used by sport flyers. The exception to this is the carburetor used on K & B engines but we won't complicate this discussion by including them.
The air-bleed carburetor can be identified by a small hole on the front of the carb. body. A small needle valve is used to partially block off this hole. The twin needle carburetor has a high-speed needle valve and a second needle valve directly opposite and on the same line as the high-speed needle (except for those engines with a remote needle valve). This second needle valve could have either a stem on it or a recessed screw type head. Now that we have correctly identified the type of carburetor we are going to be adjusting, we can get started.
The first step is common to both types of carburetors: adjusting the highspeed needle. Any attempts to adjust the low speed with the high-speed needle set incorrectly will be an exercise in futility. There are two tests that I apply to determine if the high-speed needle is correctly set. The first is to momentarily pinch off the fuel line while the engine is at full throttle. When the needle is set correctly, the engine rpm will increase briefly. If the rpm drops immediately, the setting is too lean. To insure that the needle setting is not too rich, I will continue to lean out the setting until I've gone too far and then back off. The second test is to have a helper rotate the model from a level position to a vertical nose up position. The rpm should remain constant or slightly increase. If the rpm sags, the high-speed needle is too lean. Keep in mind that you will have far fewer dead-stick landings and your engine will last longer if the setting is slightly rich. Now that we have the high-speed needle set correctly we can now work on the low-speed adjustment. Because of significant differences in the function of the low speed needle for the two types of carburetors, we will discuss each individually.
he low speed needle on the air-bleed carburetor controls the amount of air that is allowed to enter the carburetor through the air bleed hole. The correct air / fuel ratio is adjusted by changing the amount of air in the mixture. The setting of the air-bleed or low speed needle will affect not only the idle speed mixture but will have an impact on the mid-range mixture as well. The degree varies from one brand of engine to another. Before making any adjustments, make sure that the end of the low-speed needle is visible in the air-bleed hole. If the needle has been set so that it has completely uncovered the hole or screwed in so far that the hole is completely covered, one or two turns on this screw will not have any effect as the hole will remain either fully open or closed. I suggest adjusting this needle so that it covers one half of the air-bleed hole as a starting point.
Now we are ready to re-start the engine. Before removing the glow driver, run the engine up to full throttle to make sure that it is clear and up to normal operating temperature. Adjust the idle speed to about 3,000-rpm and let the engine continue to idle if it will. After idling for about 5 seconds, advance the throttle quickly to full and note if the engine accelerates smoothly, dies, or stumbles and sputters before reaching full rpm.
If the engine accelerates cleanly, we are close to being correct.
If the engine immediately dies, the setting is too lean. Richen the mixture by turning in (clockwise) the low-speed needle to close off more of the airbleed hole.
If the engine stumbles and sputters, the mixture is too rich. Turn the lowspeed needle out (counter-clockwise).
Another test to apply is very similar to that used to check the high-speed setting. While at a fast idle, pinch off the fuel line. If the engine speeds up briefly before slowing, the mixture is about right, if the rpm drops immediately, the mixture is too lean and if the rpm increases for more than two or three seconds, the mixture is too rich.
After you have reached the point at which the engine will idle for 5 seconds or so and accelerate to full rpm cleanly, increase the idle time to 15 seconds or so and note the results when the throttle is opened fully. If it doesn't accelerate cleanly, apply the steps above. Once you are satisfied with the idle, check the throttle response after the engine has run at 1/3 to ½ throttle for 15 or 20 seconds. It should accelerate cleanly now as well. If it doesn't, some final tweaking of the low speed needle is called for. It's not uncommon to need to compromise on the setting of the low speed needle for acceptable transition from both idle and mid range throttle settings.
The low speed needle on a two or twin-needle carburetor controls the amount of fuel that enters the carburetor at idle and mid-range to achieve the correct fuel / air ratio. On the air-bleed carburetor, turning the screw out leans the mixture. On the twin-needle carburetor, the opposite is true; turning the needle out richens the mixture.
Although the direction the low-speed needle is turned is the opposite on the twin-needle carburetor versus the air-bleed carb., the test procedures as described above are the same and I won't waste space and repeat them all here. You just need to keep in mind that the low-speed needle is turned in the opposite direction. Two other differences that you should be aware of are that the low-speed needle on the twin-needle carburetor is significantly more sensitive than that on the air-bleed versions and it also has a greater impact on the mid-range fuel / air ratio.
Hopefully, this article has given you some useful tools to help you get a balky engine running correctly. Once you know them, you'll understand what I mean when I say that a two-stroke glow engine will talk to you and if you properly interpret its signals, you to will be a member of that special group that knows the secret rituals and incantations.
One last word: there is a proper time and place for everything and on the flight line next to a pilot is neither for anything other than a quick tweak of the needle valve. If you need to perform these tests, please do so on the picnic table in the engine test area.