I have the exact same unit that I've been using on my briggs 4 stroke mower for some time now. I pretty much use it for keeping a record of hours for my oil changes. That being said, I have used it a number of times to check/set governers and carbs on old mowers that have come my way for fixing. I read the ENGRISH manual that came with it (I bought from china on ebay) and it made very little sense to what was acutally happening. When pressing and holding the S1 button for more than 3 sec, you cycle through the list of modes from 01 to 05. In my experience, I've found that you use 02 setting for both single cylinder 4 strokes AND single cylinder 2 strokes. 02 Measures every time the plug fires (or rather every time the current shoots down the lead) 03 doubles that figure and would be suitable for 2 cylinder engines bearing in mind that you are measuring only 1 cylinder that is doing half the work despite having 2 cylinders firing for overall revs. I have no idea what 01 would be suitable for as this halves the revs of 02.
Example: My briggs single cyl 4 stroke mower measures around 2940 at run (full) speed (should be governed to 3000) on setting 02 and my 2 stroke echo trimmer measures around 3400 idle (should be 3000) and 7700 flat chat (should be 7000 max). I know my trimmer idles too high as the head spins at idle and it probably runs a little lean as the carb has never been serviced since new. As a side note, I run 98 RON BP Fuel and use with STABIL additive l so I'm probably getting a little more HP from it also.
I'm assuming you bought it cheap from China like me - about $8 so there may be some inconsistancy with the workings between yours and mine BUT there should be a dramatic difference between 01-02 and 03. Maybe you have old fuel or worn rings or an oil mix ratio that is too high or a spark plug that isn't firing well or even a high ohms reading on your ignition coil. All this asside, 12,000 RPM seems a little high to me. Maybe this is the RYOBI Redline of the engine itself with nothing attached to drive shaft. You might need a RYOBI expert to weigh in on this. If it runs, accelerates and cuts well, I wouldn't get too hung up on the numbers.
I normally have about 3 wraps around the plug on mine and keep the lead away from heat sources. Hope this helps mate.
