Hi ??
I flew a YA A4 on the Australian circuit for some 8 years, it is a great model. Springair retracts are the way to go (never had a problem). use the OS 91 VRDF and Ramtec combo, it suits this model exactly. You do not need the cheater hole in fuse bottom like some A4's, but do cut out the 4" hole where the bifurcated inlets meet, the Ramtec breathes just fine with this.
Do not reef the model of the ground, ease the model off, then push slightly down until it builds up speed, and it will fly great.
i can put you in touch with some jet guys in Qld if you want, they could steer you in the right direction as well.
Dave Burns
IJMC Rep. Australia