let you engine come warn at the ground with a little rich set (high end) put your plane nose up and open full throttle to see if your engine come lean nose up, if it come lean ,make your high end more rich
if that way does nt work you have to close one exhaust port.
using a check valve is a good idea to keep pressure when the engine is slow and this way the transition wil be much better

to post from Jim and lafus maximus they are right , I just comfirm

let me know if your problem is not fix after you try this

Gilles