Principles Of 2-Stroke Jetting, Prop Pitch, & Reading The Plugs

One of the things that makes Old Timers scratch their heads, and newbies cook engines, is the complex interaction between prop load and 2-stroke carburetor jetting. Here are a few basic principles, and pictures of some sample spark plugs to look at.

If your EGT's are too high or too low, or your CHT's are too high or too low, before you start changing jets, consider your prop load. What RPM is your engine able to attain in flight? This is a major determinant of engine temperatures. For the Rotax engine, recommended factory jetting is usually going to work. If your temperatures are wrong, consider prop load or possible engine problems before making big changes. (Adjusting jet needle height by moving the clip up or down is not a big change)

CHT is a function of jetting and load. CHT can be reduced by larger jets, or a reduced load. It can be increased by smaller jetting, or increased load. An engine that is under propped but jetted correctly will normally have a low CHT and a high EGT. (This assumes an otherwise healthy, normal engine, correctly cooled and operating properly.)

An engine that is over propped (over loaded) and unable to attain normal RPM may frequently exhibit EGT values below normal, while CHT values can vary from normal to high to very high. In such situations, CHT values can be reduced by larger jets, but if the engine can not attain normal RPM, reducing CHT via larger jets is only treating the symptoms. The solution is to adjust the prop load. (Once again, this assumes an otherwise healthy, normal engine, correctly cooled and operating properly.)

EGT is a function of jetting and load. EGT and CHT both will typically be high in an engine with insufficient (lean) jetting and normal prop load. EGT will typically be high and CHT will be low in an engine that is underpropped (insufficient load). EGT cannot usually be reduced to normal in an engine that is under propped and under-loaded. In such an engine, EGT will almost always be too high. The correct action in such a case is to make sure jet size is sufficient, then increase the prop load. Simply increasing the jet size is chasing the symptom and ignoring the cause. In an engine that is correctly jetted, a high EGT will decrease to normal as prop load increases to correct settings. A correctly propped 2 cycle engine should typically be able to reach an rpm slightly below max continuous RPM at full throttle in level flight.

EGT can increase to excessive levels in an engine that is correctly jetted, but is artificially leaned out by overrevving in a fast shallow descent. Typically, the throttle is set to less than 1/2, the airspeed is at or above high speed cruise, RPM is at high, and the EGT gets very high. What has happened is that the fuel supply is less than half normal (the needles are down into the jets), and although the slides are partly closed, air volume is artificially very high due to the high RPM, (the prop turns the engine into an overactive air pump) and the engine mixture becomes greatly leaned out. Now you have a situation of low load, high RPM, high carburetor air flow, and low fuel. This equals very high EGT levels, use caution. A change in throttle position will usually solve the problem.

If EGT levels rise unaccountably, (nothing has changed about the prop or it's pitch) or an engine appears to need unusually large jets to avoid high EGT readings, look for an air leak. If one cylinder's EGT unaccountably rises above the other, look for an air leak specific to that cylinder. If both cylinders EGT's rise with a single carb engine, check the intake manifold. Be suspicious of any oil in the magneto area, it probably indicates a leak around the seal or in the case area behind the magneto. Annual crankcase pressure testing is a good maintainance habit.

The EGT and CHT are useful to get you into the ballpark, and tell you if something changes while in flight, or between one flight and the next. Once you have the gauge temperatures satisfactory, cross check with plug readings. Gauges can be mis-calibrated, but plugs always show what is really happening.

What Do Spark Plugs S'posed to Look Like, Anyway?

   The plugs on the left are typical of an engine with too much oil in the fuel. The plugs on the right show an excessively rich engine.  

   The plugs on the left are rich for a 4-stroke, but not too bad for a 2-stroke. The plugs on the right are close to ideal for a 2-stroke.  

   The plugs on the left might be aceptable for a 4-stroke, but in a 2-stroke they are lean, hot, and dangerous. The plugs on the right would probably indicate imminent failure for a 2-stroke.  

Be aware that the use of 100LL will turn all these readings to a greyer color, but general appearance will still be similar. Use of various brands of synthetic oil will sometimes add extraneous colors to the ceramic portion of the spark plug, which can make things more interesting.

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