XJet

Sorry Ed, but that means you're calling me inexperienced and incompetent - but I don't consider myself either of those two things :-)

I've flow-modeled the ST90 carby and my simulation *proves* that it is a flawed design if you want both a good low idle and a reliable mid-range/transition.

Unless the three ST90 carbs I've got here (which are identical) are all anomolies that happend by sheer coincidence to make it into my posession, then the reason people are having mid-range problems is down to bad design.

*Very* careful tuning and the use of crutches such as idle-bar plugs, the "ideal" prop, etc can partially mitigate the effects of th design flaws -- but ultimately these are a bad carb.

If OS, Thunder Tiger, ASP/Magnum, Saito, and the vast majority of other engine makers can design carbs which are able to be tuned in just a few short minutes by relative novices -- doesn't that give you just a little clue that there's something woefully wrong with the ST design when so very many people are having so much trouble?

And how come, simply throwing an OS or other brand carb on a fussy ST engine completely solves the problem?

Because the ST carbs are a legacy of a time that has long-passed when people didn't need low idles and good mid-range.

The ST twin-needle carb was highly praised when it was released because it was the successor to air-bleed carbs so it *was* good for its day. Today however, it's a dinosaur that hasn't been updated to meet the needs of a totally different breed of model.

We overpower our planes these days, putting 90-sized motors in airframes that would have been made for 40-50 sized motors when the ST carb was first designed. A 2750 RPM idle was considered *very* low back then and because of the poor power to weight ratio, most models would have no problem landing even with an idle up as far as 3,500.

Today, we really need low idle speeds to allow our over-powered planes to land and taxi properly -- so when people put an ST90 in a plane that would previously have flown on half the power, they need an idle as low as 2.000-2,250 -- or the damned things won't come down or sit still on the runway.

So, what happens is that people tune the bottom-end of their ST carbs for a nice smooth, reliable idle (and these engines will idle well under 2,000 RPMs when broken so that's easy to do). Unfortunately, to get them idling this low, the low-speed needle has to be opened quite a bit (because of the design flaw) -- which means that when the throttle is advanced past idle, they run *very* rich. This produces hesitancy and sometimes is so bad that the engine will actually load-up and quit altogether.

The US ST agent's solution was simple -- raise the idle speed.

Of course this works very well. Bring the idle back up to 2,750 and the rich mid-range goes away because you can lean out the low-speed needle without the engine quitting. But then you have the problem that most lightly-loaded, overpowered modern models can't be flown with such a high idle.

In summing up -- it's a design fault, the carby is way out of date, it could be fixed with a *very* small change to the design so that you didn't have to be highly"experienced" and "competent" to make it work.

So why are ST sitting on their thumbs and selling such a poor carb, which reflects badly on their engine performance -- when, for just a few hours of NC programming on their CNC gear, they could turn these engines into ones that were just as easy to tune as OS and just as flexible in their performance?

Kind of makes you wonder, doesn't it?