Often a high idleing RB26 is due to a damaged Inlet manifold gasket,
Most RB26's idle at around 1,000RPM (well mine and a few others i've noticed).
Your screw may already be wound right in, so next may be your inlet manifold gasket, but that is quite a big job.
Spray, Water from a spray bottle I think it is around the seal of the plenum and if the idle differs at any spot you spray at you know the plenum gasket isn't sealing thus the idle.
Hope this helps mate