I know some of you have tried the Fora 21 for slow combat, and there have been problems.
My buddy Oleg got a couple the season before last. They broke in pretty easily, maybe 6-10 runs, starting a bit rich and working the needle in a bit after each flight.
But they never settled down and ran well. By comparison, the standard Fora 15 would break in in 4-5 flights. By then it steadied down and started running very reliably and steady while gradually getting faster with each flight.
The 21 never settled down. We tried different fuels, different size venturis, different props, new bladders, and different needle settings. He gave up for awhile and switched to a couple LA 25's which broke in and ran fine(Japanese built silver ones).
This summer he brought the Foras out again and still had the same problems. Out of frustration he swapped the long plugs he'd been using in the LA 25 for an OS A3(a short plug). Next flight it ran perfectly with the usual slightly rich setting on launch. It also ran fine on the APS 8/4, the 9/3N, and a few other props. As a bonus it ran over 6 minutes on barely 3oz. of fuel.
This may not be a cure-all, but it's worth a try.