Simple answer, yes buy a tach!!!! This is one of the best tools EVERY RC pilot should have!!!

I am not to familiar with the YS engines, sorry. But if it is a normal 2 stroke, judging by ear can be done to some extent, but there is ALWAYS the risk of setting it to lean. 4 stroke engines on the other hand can be almost impossible to judge by ear.

2 stroke setting by ear is to run the engine to full throttle. SLOWLY lean out the high speed needle until you can no longer get the engine to run faster. Then RICHEN the engine until it starts to run with a quite rough setting. Then lean it back out just to the point where the engine regains a little higher pitch and the running gets smooth. Then pitch the airplane to a 45 degree nose high attitude and if the engine gains a lot of rpm by sound, richen the needle one click at a time until the rpm stays close to the level attitude. you may only need about 1-3 clicks of richening, but remember that you could be gettintg to close to running it to rich again with the more you go. Also, the engine should gain a slight amount of rpm when pitched up due to engine leaning out slightly; you are just merely trying to keep it from running to lean.

The problem with this is that you have NO idea how close to peak this setting is without a tach!

THE SAFEST WAY TO SET AN ENGINES HIGH SPEED NEEDLE IS TO USE A TACHOMETER!!! With a tach, you can get precise measurments on what your engine is doing! Without a tach, it is just a guessing game.

I always have a tach with me when instructing, and advise all of my students that purchasing one should be done as well. When I am at the field, my tach is avialable to anyone wanting to use it! With today's airplane costs, I would rather let someone borrow and use my tach to set their engine properly, than to have them do something that may ruin their engine and take them out of the hobby!

DBChery has given you some good advise as well!! I just like my engines to run more rich than to risk a too lean run.

Just my thoughts,

Reg