MSTS Conversions Driving Help

[AdmiralLeviathan]

I've been playing around with the excellent MSTS Conversions offered from Noel and I'm having trouble driving some of the engines

In particular I've been trying to drive the Golsdorf 310 and Golsdorf 380 engines (Listed as OEBB 16/BBO 16 and FS479/Golsdorf 380 in the menu), and I can't seem to get above 23 KPH

I'm manually firing and keeping the steam pressure up. No combination of cutoff/throttle seems to get me going fast. I'm driving on level track with only 4-5 carriages in tow. Brakes are all off in running, and the handbrake is off.
The AI seems to be able to drive the engine quickly, so what am I missing? What is the best combination of cutoff and throttle to drive these engines? Thanks in advance