I bought a 93" unflown Fiesler Storch model (built from robert schweizer plan) and it had a used ST90 in it. Previous owner/builder (who is a nice gent in his eighties and selling up) said he couldnt get it to run properly (it's mounted inverted).

Having some ASP90 two strokes handy, I thought I'd just whip out the ST90 and replace it with an ASP90 - easy peazy lemon squeasy.

But at our club swap meet I came upon a lovely brass (not heavy though) 'in cowl' silencer, not just for an inverted ST90, but specifically for a Storch (with 2 scale exhaust pipes as per full size) and price just £10 (about usd15)

So it seemed worth a try to get the ST90 running and if it seemed reliable actually use it.

To cut a long story short (after some experimentation) it was the ST carb !

Which (to be fair) is old, has been stood inverted in the model for around 10 years, and someone has tried to drill a hole into part of the main needle threaded part (presumably to extend it) but it leaks fuel a tadge in that area.

From what I've read here I thought I may as well try an asp90 carb (have an asp90 engine in bits for spares including a carb) stuck it in and the ST90 fired up straight away. Bit of tuning on the main and idle needle and hey presto.

14x6 apc prop is turning 10500 revs on 5% nitro fuel with asp 90 (2 stroke) carb and incowl silencer (for inverted) . Engine was running right way up on test bed. i.e. not inverted.

Very powerful engine! And it can go back in the storch with the bargain in cowl silencer for maiden flight.

Thanks for everyone posting info here - saved me time & money and a good engine.