I had no problems with tuning my carburator. That was the first rc engine I ever owned on my first plane. I cut the muffler in half on my .40 removed the baffle and then TIG welded it so you couldn't tell I had modified it. That really seemed to wake it up. Sadly the MDS were rejected off-hand around here. Were you running the stock muffler on your .78 ?

Take care, Cliff