ORIGINAL: MOTORMAN37
They are within 1-2 oz's of each other. I would have no problem balancing it, but it does not seam like it is more powerfull of enough to warrant it, plus you need a very stable low rpm for landing approaches. My TH .75 idles at 1,800 reliably with excellent transition.
Well if you try to idle your ST90 that low, your transition will be shot to hell because you'll have to richen the low-speed need a *lot* due to the poor carby design.
ST themselves claim
"Any idle below 2,500 RPM is considered good".
I think that probably rules out the ST90 for your application, unless you're willing to put a new carby on it.