You won't get a lot of performance with a 12x8 prop in vertical. Put a 12x6 or 12x4 on it and you will see a large increase.

I have always liked the ST engines, but if you try to do the fun stuff with the wrong prop, you will be dissapointed with any engine you run. Your not going to get max RPM out of anything with an 8 pitch prop. There is no way!

With all of that being said. I will try a 13/5 or 6 and see if my performance gets any better. By the way, my muffler did'nt vibrate loose. A washer inside of the muffler came loose. These mufflers do not come apart so a new muffler is my only option. Thanks again for all the input.

I had that happen with a Saito 180 muffler. It didn't come loose, it blew the muffler right off the stand pipe. Ate the threads right out of it. Hate paying for a new muffler for an engine that has been run twice. That bit.

Hope you get it working well!

Also consider some for of Loctite on all of the muffler connections and screws. Those mufflers have a history of coming loose and falling off.

Hi,,,there a couple of things that are NOT in the instruction book,,,and that i had to learn myself...the fuel nipple on the carb. has to be pointing right at the rear mount bolt,,,next...NEVER trust the factory settings....close off the idle needle and put a piece of fuel tubbing on it and blow...with the throttle in the almost closed position..open the needle till you just hear air comming through. when you have this set...open the high speed needle about 1 1/4 turns and this engine will run fine...i have a 80 inch plane with a ST3000 and itll pull till it gets to the moon..LOL...and,,,,,,,,,,it weighs 24 pds with all the crap i have in it....spinning a 16-8 3 blade...i will say this....this engine is an Italy made engine,,,and the new ones,,,,im not impressed....if you look at the heads of the 4500 Italy,,,and the head from 4500 China...there is a big differance...things do change,, and sometimes not for the better....It really shocks me that i didnt see anyone ask what elevation you were at....remember..prop the engine for the place you live....not what the book sais....Better performance...as far as 2 strokes....the old ST's are one hell of an engine...cant beat em'...good luck..i hope this helped...Rod-AKA Cublover

My ST 75 muffler's baffle came loose and was making a metallic sound and you could see aluminum specks in the fuel residue. I shipped the muffler back under warranty and got a new one.

I'm running a 12 x 7 on a 6.5 pound Kaos with unlimited verticle. Ground level = aprox 400'

One added note to all ST owners.


The barrel screw that holds the barrel in place NEEDS to be lock tight. I have seen two beautiful planes get damaged (one was my p-51) from locked barrels. Let me explain, if the barrel screw vibrates loose it will slip out of the barrel grove and Lock the throttle in what every position it is in when it does.