You won't get as much smoke fro the .91 FX as from a .91 FS. The 2-stroke exhaust just isn't as hot.

You can get decent smoke from it though... and it will give you some experience with smoke systems.

To help overcome the lack of exhaust heat... wrap the cylinder with a turn or two of brass tubing as a smoke fluid pre-heater.

Make sure the injection point is close to the engine exhaust port. (in the flat portion of the muffler, near the connection to the engine) This is a low pressure area compared to the main expansion chamber. (lets you pump the fluid in easier)

You CAN pump in too much fluid. I've found it better to tune the smoke fluid flow for maximum smoke at 1/2 to 3/4 throttle. This helps prevent excess fluid expansion creating too much backpressure, robbing power from the engine. Some people preach to tune for max smoke at full throttle. They don't run thier smoke systems at low throttle settings... (where you can get fluid splashing into the cyl and quenching the glowplug in some flight attitudes...)