Your email address indicates you do not wish to receive any direct assistance from long time users of those engines so I suggest you refer to
scroll down to "Engines,Plugs,Tuning, Petrol, Turbines, Mufflers, Cleaning, Anodising etc" sub section "Carburettors, Tuning & Performance Testing, Glow Plugs" for pdf manual for the ST2500 and follow the instructions, prop sizes etc as recommended by the manufacturer.
I have an article published at my Web Site concerning the delicate adjustment of ST carbs; you might find it useful. BTW, you might receive more answers if you post a valid E-mail within your request; some gents prefer to not reply to the group.
The ST 2300 (1.4 cu) is NOT the same as the ST 2500(1.5). The 2500 is an older engine and likes big props and low oil (12-16%) and low nitro fuels. 18x6, and 18x8 props recommended. The components of the
2500 are more closely related to the St 3000, and 3250.
If this is one of the last of the Italian mks then is it the 23cc or 25cc engine? the 23cc may run on a 16" prop but the 25cc will need an 18 X 8. I tried my 25cc with a 16 and the vibration with rpm was too much for the air frame and returned to the 18" prop which turns around 6700 at full wack. Which is more than adequate to pull a 12lb plane around. The engine will require flooding to start an Enya No3 works fine, run slightly rich.The carb isn't any different to standard two strokes, well mine isn't, in fact it's a replacement ASP from Just Engines(UK). I use 10% nitro mix, I'm told it will fly on straight castor, but that's damned smelly and messy.
The whole purpose of this group is to SHARE information. If a question is posted to the newsgroup, any replies should also be posted to the newsgroup.
What reason could they have for not wanting to reply in the newsgroup? It's more convenient, and the information they provide just might be useful to someone besides the person who originally asked the question. People also have the opportunity to voice different opinions, to correct someone when they give bad advice, BEFORE a disaster happens.
Ego can't take being wrong or someone having a different opinion? Grow up. Learn to accept that you don't know everything. Can't post without being attacked by "enemies?" Grow up. Be the better person. Ignore them. Afraid of spam? Cry me a river. Use an obfuscated email address or get yourself a free email account, let it fill with spam, then the spammer will get the bounces.