I have a ST-90 that is not fully broken in yet and made in China. I have it installed into an old Super Kaos 60. I am using a APC 13/6 Prop 2-blade. The engine runs very good, and has unlimited vertical from the time the wheels lift off you can yank it vertical and climb out of sight. The muffler is stock, but I will tell everyone, the original mufflers for the China made ST-61, ST-75, and ST-90 run well but they are junk. After a few flights the internal baffle plate will dislodge and start pinging around inside the muffler and you will definitely hear it from the ground. If you want superior performance and are willing to forget about price, buy a tuned muffler from Jett Engineering. If you have the cash, buy his replacement carburator also and then you will have a real screamer of an engine. Here is why most people say ST Carbs are junk. They do not know about putting a squeeze clamp with rubber jaws like you will find in any hardware store to hold wood on top of the carburator and on the bottom of the engine and squeezing it slightly before you tighten the carburator clamp screw down. If you do this, you will avoid all the varying carburator adjusting and lean burn scenario. This will ensure you will not develope an air leak! Do not use a metal clamp to do this. The low speed adjustment is the hardest to set and get right, but a little minor tweaking of not more than a 1/8 of a turn at a time will get you a good idle and will affect the high speed adjustment also. Lift the model vertical and see if it dies out or slows down, if it does, you have the high speed too lean. If it will not idle and take the throttle opening smoothly, then the low speed adjustment is not right. There is nothing worse than the engine quitting when you advance the throttle for a go-around. Spend a little time on the ground getting the low speed right, and you will save yourself a lot of grief. You will not generally have to touch the low-speed until you get a gallon of fuel through your ST engine. After that you can lean it down or richen it up, whichever will result in a smooth throttle transition and a good idle. What you want to avoid is over propping the engine and getting it excessively hot. I use my fingers to tell me this. I go once around the pattern, quickly land and feel the head. If I can not stick my fingers on the head for a couple of seconds then it is too hot and too much prop, generally too much pitch. The engine should not slow down and labor as your pulling a loop. It should remain fairly consistent throughout the loop. Too much pitch and the engine labors and will get hot very quickly. I have seen more people smoke their engines trying to use too much of a pitch prop and then they wonder why they never run right after that. I am fussy on the fuel I use. I use OMEGA 10% Fuel that has some castor and the rest synthetic and my engines last a long time. The Castor Oil will save you in the event you experience a short lean burn say from a pin hole in a fuel line as long as you land and check it out before trying to continue to fly the plane. If you want a screaming engine, then use a good tuned muffler that is made for RPM increase. I have seen the difference between a MAC tuned pipe and a tuned muffler from Jett Engineering. A Jett Muffler will make your engine really scream in comparison. They may cost you up to $85, but worth every dollar. Well worth it when you keep a US Manufacturer in business that stands behind his product. Super Tigre Engines do not like a lot of back pressure. Back pressure holds in heat. I have pulled out with a lot of patience the baffle plate that came loose in side one of my defective ST Mufflers. Such a tiny hole in the center of it. The engine ran far better without the baffle than the muffler with the baffle. We just do not like the increase in noise level, so a good Tuned Muffler is in order. Now I have tried the MAC Tuned Black Muffler, but that is not where the performance is either. There is too much back pressure even with those mufflers, as MAC recommends reaming out the two holes inside of the muffler for ST Engine use. If they would buy ST Engines and test them before releasing their product to us, we would see far greater gains than the stock Tuned MAC Muffler perhaps. But with JETT Engineering, they buy engines and test them before releasing their final product to the public and we do not have to play Mr. Test Engineer after we buy it. It is ready to use out of the box. I have been running ST Engines for years and I enjoy their performance once broken in and properly tuned.