Getting the Most From Your 2-Stroke

Article Type: How-Tos
Author: Jim Booker
Posted: 11/22/2006

2-stroke engines have been around at least since the 1950’s, perhaps even earlier. It’s probably the most common type of model aircraft engine and, debatably, the most popular at the club level. For many, the sound of Saturday morning would never be the same without a .40 whizzing around at your local club. But have you ever wondered why some 2-strokes run better than others? The truth is, almost all modern model aircraft engines are well-made and run well with high levels of reliability. There have been times in my life I would have challenged this statement, but I can honestly say that 98% of the trouble I have had over the years with 2-stroke glow engines (and I’ve been flying since I was six) was totally due to my poor tuning. To make any engine run properly, it needs two things: ignition (in this case, a glow plug) and fuel. Where we get into trouble is when either of these items varies from the optimum. Both key elements must be operating consistently.

The Secret:
Good Glow Plugs +
The Corret Mixture +
Clean Fuel

Glow plugs are quite often misunderstood. They are not kept working by ambient heat in the cylinder. Once you remove the glow igniter, they are kept hot by methane produced from the burning methanol chemically reacting with the “platinum” in the glow plug. Sure the engine temperature and compression do have some relevance to the correct ignition, but the primary reason glow plugs do what they do is because of this chemical phenomenon. So guess what happens if the platinum in your glow plug starts to deteriorate? You have poor ignition.

Lean runs are the biggest cause of glow plug break down. The platinum will start to deteriorate to such a point that only the perfect fuel air ratio will allow continuous running. A well set up 2-stroke glow motor should be capable of at least fifty flights on the same plug. For sport flying, that’s a lot of Saturdays. So why do we have so many glow plug problems? The reason is the incorrect fuel mixture. Most of us can achieve the correct mixture on the ground, but what happens when the aircraft is in the air? Most will notice an increase in rpm’s. More rpm means the engine is more than likely making more power in the air than it was on the ground. More power means more fuel.

I have a term: “Set to Kill.” This means I lean the high-speed needle to a point the engine can run no leaner before it stops. For an unpumped 2-stroke, this is way too lean. Instead, for sport flyers, I recommend finding the maximum rpm on the ground by running up the engine and setting the high-speed needle as lean as possible, then richen up the mixture 1/8 to 1/4 of a turn. Your engine will make more power in flight than it ever has if you apply this tip.

Setting the highspeed needle 1/8 to 1/4 turn rich of the peak lean point will provide an ideal fuel/air mixture for most sport engines.

Another big reason for lean runs is poor fuel systems. At some point of our model’s life, the fuel flow to the engine worked well, but, after time, things move around. Fuel tanks can be pinching fuel lines. Muffler pressure holes will carbon up. Holes can appear in fuel lines, and my favorite, junk in the carb. All these things are avoidable. I am a fuel filter guy. I have a filter in my fuel can, plus one in the aircraft. I check my fuel tanks every 100 or so flights. I replace fuels lines each winter and so on. Preventative maintenance is the way. Some of you have precious little time for flying. Spending some time preparing your model will always pay off when you get to the field. The model field is for flying, not aircraft maintenance.

Evolution’s needle limiters make tuning your engine simple.

The final little element in the 2-stroke “fun meter” equation is the low needle settings. The low speed needle is not just for the slowest idle. It is responsible for the fuel flow to at least ¼ throttle, and, in some designs, even more. A poorly set idle needle can be just as damaging as a poorly set main needle. This is one of the reasons we have needle limiters on all our Evolution glow 2-strokes. They allow people to be in the ballpark, where only final tweaking is needed for optimum settings.

Setting the idle needle is pretty simple. Start your engine, warm it up, and then pinch the fuel line going to the carb. If the engine dies straight away, you may be a little too lean on your low speed needle. If you pinch the fuel line and the engine speeds up, then dies, you are rich. The optimum setting is where the engine momentarily speeds up, perhaps ½ a second, then dies. Once you have your low speed needle set here, it really should never need to be adjusted again.

Evolution’s needle limiters make tuning your engine simple.

One final piece of advice—make sure you have “consistent” fuel. Methanol is very hydroscopic, meaning it attracts water. Make sure your fuel is fresh and once you are happy with a certain brand, stick with it. Each fuel manufacturer uses slightly different ingredients, which will cause variation in your needle settings. Also try to stick to the same nitro content, whether it be 5%, 10% or 15%.

I hope reading this article gives you more success at the field. Next time we’ll talk about tuning 4-strokes. Until then, happy flying.