I was going to write that I'm a bit of a novice with nitro but actually I'm a complete novice so bear with me!!
I've got the engine idling without the wheels spinning but when I push the throttle on the receiver the engine revs, but won't sometime might take a few seconds to reach full throttle. It sounds like it's thinking about bogging down but then decides to actually react to the throttle input.
Once it reaches full RPM, I can then cut the throttle so it drops to idle and then if I put full throttle on again (within a few seconds of it idling) the throttle will pick up straight away. Hope that all makes sense.
I'm using 20% nitro fuel and the engine is a tamiya FS-12SW. It's pretty old but has had 2 or 3 tanks through it recently.
Thanks