my 61 FX used to quit all the time while it was brand new. The 61 FX is similar to the 46 FX only larger disp.

I broke it in per booklet, 2 tanks or so. Used Bisson pitts muffler.

What happened was during break-in and shortly after, it needed a lot of backpressure which the pitts did not supply.

I also had an in-line fuel filter.

The engine would start and run but would not hold idle. Then it would quit in the air.

I plugged one of the two pitts exhaust tubes and removed the inline filter (dubro).

The engine ran fine then, didnt quit and held idle.

After 10-20 flights the pitts restrictor i put in (1/4-20 set screw) fell out in flight.

Then engine liked the lesser restriction and was fully broken in, at least enough so that it ran like a champ ever since. This engine is the best I've ever owned.

Not sure if the problem would have even occurred had i started with the stock muffler, but the engine was broken in as i was planning to run - - with pitts.