And to add to what DaRock said, what he said about "published RPM" makes a good point. That number will change with the prop, as already stated. The mixture richens out when there is a lot of fuel flowing that doesn't get burned.. it spits a lot of it out and sounds like a "four stroke" engine. But, the RPM that happens at will vary with the prop used. It's more of a sense of hearing the "four stroke" point than trying to measure it with a tach.

The same applies for leaning out the mixture. The engine will go to two stroke and increase RPM as the mixture leans out. But, if you are not careful, it will lean out to much and get very hot. Remember, that fuel not only provides the ignition source, but it also cools the engine down. Take to much away and it will get very hot very fast. So, it takes a "tuned ear" to know when it reaches that sweet spot between to lean and to rich... Aaaaahhhh, John, good stuff.. the good old pinch test. Yep, it works great. But, like many specifics about this hobby, it is an art and needs to be learned and perfected. But once you got it, you got it.

All this happens at full throttle, by the way.

If you use a specific prop to break the engine in, you will run it at some rich setting, again, at full throttle (check the manual) and richen the mixture till it blubbers.. spits out fuel. No one knows when that will happen for your given prop and that RPM will be fairly specific for your engine/prop combination. The same for leaning it out. The RPM will increase significantly and somewhat smoothly and the sound sill be very different as it loads up to the prop and settles to an RPM. What that RPM is, well, DaRock said it.. no one knows what that is.... it isn't published. An experience ear will know when all this happens.

You can use the tach once your engine has been fully broken in and is tuned properly, but a lot of things will vary that final RPM.. the temperature, humidity and atmospheric pressure will cause it to change from day today. That's why we all say that it has to be tuned each day before flying that day. Once set, it usually is ok for the whole day, but that too can change as weather conditions change. Quite frankly, I don't use one as often as some at the field do. I depend on tuning by ear using the "pinch method". If I use a tach, it's usually when I change props.

I'll bet you never thought you would get this much information from so many very helpful people.

More than likely, all of us will say that getting some help from some one that is experienced to help you get that first engine properly broken in and tuned properly will go a long way to getting you on your way. But I can also say that you won't get that experienced ear without doing it over and over again.

One important point that I've not read so far ( if I missed it, I apologize).... it is much better to run the engine slightly rich than it is to run it slightly lean. And don't shoot for the most RPM you can get out of your engine. Most engines that are off the shelf are made for the general sport flyer and will perform accordingly. Trying to get 16,000 RPM out of an OS 75 for instance just ain't gonna happen, or it will happen, but not many times. Keep it sane and you'll get many years of flying out of those engines.

CGr.