Principles Of 2-Stroke Jetting, & 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.

The prop load, and only the prop load, determines maximum RPM. If engine RPM changes, then the engine has lost power, or something has happened to cause some change in the prop pitch. If a given prop works on sombody else's engine that is apparently identical to yours, but your engine won't pull it, your engine is weaker than theirs, or else the prop load (diameter/pitch) is not the same.

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.)

EGT is a function of jetting and load. EGT and CHT will typically be high in an engine with insufficient (lean) jetting and normal prop load. EGT will increase and CHT will decrease in an engine that is underpropped (insufficient load). EGT cannot usually be reduced to normal in an engine that is under propped and overrevs. 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 engine will typically be able to just nudge RPM red line at full throttle in level flight.

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 can increase to damaging 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 cruise, 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.

If EGT levels rise unaccountably, 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 would be OK 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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