Hi!
It is unfortunate that you are experiencing trouble with your ST-3250.
I don't have that engine myself, but I do have extensive experience with ST-3000 engines, which are identical apart from bore and stroke.

The leaning out problems you experience in-flight could be due to fuel starvation, overheating or simply the fact that the engine has not been broken in yet. Do yourself a favor and give the engine a couple of hours of bench time. STs takes a long time to break-in. Typically I use at least a gallon of fuel before installing the engine in an aircraft. As you have noticed it is time (and money) well spent.

BTW, during the break-in process, large SuperTigre engines require fuel with a minimum of 15% oil content by volume. It is only AFTER the break-in that low oil content fuel should be used. In the manual it is recommended to run the engine on the 15% oil fuel for at least two gallons’ worth. I agree wholeheartedly with this. Three to four tanks may be enough for other brands, but NOT for the large STs.

As Bax wrote, make sure that you use large diameter fittings and fuel tubing for your fuel system. If you don't already have it, install an OS F glow plug.

The big STs generate a lot of heat and efficient cooling is a must. Make sure that the cooling air is directed through the cylinder fins.
Block the air inlet opposite to the one where the cylinder is. It doesn't contribute much to cooling, but may restrict airflow due to too little difference between entrance and exit surface area.

The Bisson mufflers provides relatively little back pressure so you could try to block or restrict one of the outlets in order to generate more muffler pressure to the tank. I do that with my ST-3000s to good effect.

I have not found it necessary to change carburettors on any ST of mine. They do take some time to get to grips with, but after that they work perfectly.

If you don't have it here is a link to the [link=http://www.supertigre.com/manuals/supg0268-manual-v1_1.pdf]ST-3250 manual[/link]. Read it and follow it religiously.

/Red B.