I've noticed with the ST carbies that there *is* a problem with the linearity of the low-speed fuel metering system.

Bax frequently points out that many people have problems tuning ST engines because they set the idle too low.

After much experimentation and analyzing the setup, it would appear that raising the ST's idle *will* improve the mid-range transition -- because it allows you to lean out the low-end needle a little more.

The problem is that many folks are using these engines to over-power models and a fast(er) idle just won't work -- unless you've got a *really* long field that allows the "hot" landings that a fast idle produces.

The STs really do idle beautifully at lower RPMs -- but because the carby is not designed for these low idles, it requires that the low-speed needle be opened up or the slow-idle is too lean. Unfortunately, when people open up the low-speed needle to support a slow idle, the mid-rnage goes to hell in a handbasket because it gets way too rich. The low-speed needle affects not just the idle but the whole range up to about 2/3-3/4 throttle.

Turning the spraybar has a small (but insufficient) effect on this -- a *low* idle will always produce a fat mid-range -- it's just a characteristic of this carb design.

So Bax is quite correct when he says that raising the idle will solve the transition problem -- but it's only a symptomatic solution, it doesn't address the fact that the carby over-restricts the fuel-flow at any *reasonable* low idle setting.

Of course this doesn't change the fact that the ST engines are incredible value for money and will run forever if you don't abuse them. I've got several and intend to buy more -- but I am now building my own carbs for them and it makes a *huge* difference. Throw a properly designed carby on them and they'll idle *really* slow and have flawless transition with no deadsticks.