Well, I dare say that I have broken in two OS 70s, four OS 91s, two Saito 82s, a Saito 100 and a Saito 125 all in the last two years (And just last night I found my tach which has been missing for three years
)
The only reason I even OWN a tach is because a few years ago I was reviewing an engine and wanted to show a picture of the digital RPM readout.
Here's the deal...
You start the engine and bring it to full-bore. Assuming the mixture is rich, you start to lean it out. You will hear the RPM increase.
At a certain point, you will find about 5 or 6 clicks where there is little or no increase and then you will hear the RPM begin to fade - this will be followed very closely by the engine running very poorly and if you don't bring the needle back, it will quit.
Ok, so when you get to the point where it starts to fade (let's call it click 7), bring it back to click 1 or 2.
Now, nose the airplane up. The RPM should stay the same or SLIGHTY increase. If you hear a drop in RPM, you need to richen the mixture up another click or two. This is not rocket science.
Now, here is the problem with a beginner using a tach:
ORIGINAL: Campy
USE A TACH !!! In order to set up a 4 stroke you will need a tach.
Your MAX rpms are going to be ABOUT 10,500 (depending on the prop and nitro content).
Ok, so this guy reads that he MUST use a tach, and that he is going to get 10,500 RPM (which sounds a little high to me). Now, in all fairness, Campy DID say, "depending on the prop and nitro content", but will the newbie remember that? Probably not. He'll remember that he MUST use a tach and to shoot for 10,500rpm.
I have personally had someone ask me to help him get his engine running right. The engine was toast because he was told he should get 10,000rpm and he was only getting 9800 on his tach, so he kept trying to lean the engine out to get that last 200rpm.
Tachs do not take altitude, humidity, prop size, prop length, fuel brand, nitro content, or the condition of the fuel into consideration. A tach tells you numbers. Numbers don't mean anything. You might get 9800rpm today, and have a warm front move in tomorrow and you only get 9700rpm. No matter what you do, your engine will not put out 9800rpm today.
I can see the tach junkies flipping out!
Now, someone please explain why you can tune a 2-stroke by ear, but you MUST use a tach for a 4-stroke when the procedure is identical?