Strange to hear about problems with the FX91. I have 3 of them, one with a tuned pipe on a patternship, and have nothing but good things to say about this engine. I have modified mine (the tuned pipe one) by installing a Perry pump inside the backplate with incredible results. The engine never runs lean and the "bubble problem" disappears entirely.

If anyone is interested in making this modification, please see below:

1) Remove the backplate and drill a hole in the dead centre the exact diameter of the pump pulse input tube. The best is to ask a jeweler shop to do this, it cost 5 Dollars and a perfect job was done.
2) Insert the pump into the backplate with the pulse input tube protruding slightly through the hole.
3) Fill up the backplate around the pump with silicon rubber to both secure the pump and make a good seal.
4) When the silicon is dry, cut off the slightly protruding tube with a razor blade.
5) Reinstall backplate and connect fuel tubing as per the Perry Pump instructions.

NOTE: The blue hard plastic tube used for remote pump installation is not used as the pump now receives it's pulses directly from the crank case. MUCH more efficient!

When you start your engine, you will notice that it needs a lot of leaning out due to the higher fuel pressure, but once you have set this, the engine is MUCH more friendly with a far greater distance on the needle from too lean to too rich.

Since doing the modification, the engine has not even approached getting hot, & has not had one deadstick in over 400 flights. Many of my friends that fly pattern with tuned pipes, and many more flying with the standard exhaust have tried this and I have not heard one complaint. I cannot understand why more people haven't done this as all I did was take a look at the Heli version of the FX91 which comes standard with the Perry Pump in the backplate and copied the idea for a fraction of the price!

As soon as they come back into stock I will be ordering & building a DV. All of your posts in this thread are most inspirational & have given me many ideas as to personalising my aircraft. Example: Mine will have a side mounted FX91 with a Macs tuned muffler going beneath the wing, a cowl, and a canopy.