for a beginner learning to use a 4st engine get a tach its a helpful tool for tuning a 4stroke.

when I run up my engines before flying I usually get them to max RPM and richen them up about 300rpm so in flight when they unload the prop there not getting too leaned out.

over the years the same guys have posted the same thing about not needing a tach, so last week when I was dialing a 120 surpass I got off ebay that I was rehabbing, did a valve adjustment, and was tuning it on the test stand so I figured I would give the ear test a try,what I found was I was consistently off on the high end by at least 3-500 rpm sometimes more, even when I was sure I had peaked the engine, and when I did peak the engine with a tach I was 5-700rpm off my richening when I rolled back the needle,the 1-300 rpm changes are nearly imperceptible to the human ear really subtle sound differences .

I have pretty good hearing, but when your dealing with prop noise and not running engines every single day, you might not be as sensitive to the subtle changes to rpm, also running engines your not totally familiar with its not a bad idea using a little technology to help you along,I use a tach every time I run up my 4strokes not so much with the 2stroke engines.

with a 4 stroke a couple hundred rpm does make a difference in performance,when your swinging a larger prop it helps alot