First of all I can't get 18K with a 6x3 prop. I have been using a 6x2 APC prop for those rpm. I'm not sure how much I can get from a 6x3, but I am thinking maybe 15,000. In a few more weeks I will be able to get back to testing if weather permits. The biggest thing that I do is remove the back plate, remove the spray bar, and reed assembly. I chuck up a 3/16 drill bit into the drill press, and drill very slowly from the reed side to the opening where the spray bar goes through. It is very thin and narrows where the spray bar goes through. If you go to fast it will crack the opening. Apply pressure from the needle side of the spray bar when removing the spray bar. If you try from the other direction it will also crack the tube, (I learned this the hard way). I use pliers and caution, I tried pressing the spray bar out. If you apply pressure to the bar without support for the tube it can snap off. After I clean up all the openings from small plastic shavings the drill leaves I reinstall the spray bar. I then carefully open the chock tub in both directions to 3/16”.
I use about 30% fuel for high rpm, I also reset the piston. That’s the first thing I do. Drilling out these openings give me between 2,000 to 3,000 more rpm., and the 6x2 prop gives at least 1,000 more rpm. I find weather, and humidity also make a difference in my rpm. I have had the engines tach over 19,000 rpm, but so far I have not been able to maintain those rpm. It will run in the 17,000 to 17,500 constantly.
I have a video on utube of a sure start with a chock throttle. It was done inside a garage on a very cold day after dark with a flashlight about ¾ of the way through you can see the tach at over 17,000 rpm. This is not my best running engine. When the weather gets better I plan on making another video outside in good lighting.
I know 17,000 to 18,000 sounds like a lot, but It truly will turn those rpm. With a stock unmodified sure start running a 6x3 prop I get an average rpm of 12,500 = 13,500 rpm. I get a lot more rpm out of APC props than Master Airscrew props. But as I said the biggest thing is drilling the tub and venturi tub out. You don’t have to have a drill press but it helps. I tried it with a hand held drill and broke the venturi tube.
The engine really screams after making this modification. After I do this the engine starts better if you drop 4 = 5 drops of fuel down the chock tube before trying to start it.
In the video the needle turns with the throttle so it doesn't get the best rpm. I am still working on the chock throttle to get it to work better, again this also will be a warm weather job.
http://www.youtube.com/watch?v=KKXThGEl_FE