Thought I would pass along some info on the smoke system in my Extra 300-S. The 300S started out with a ST 90. I plumbed up one of the Dubro smoke systems. This system has a valve body that has two fuel tubings running through it. The valve is driven by a servo - OFF pinches the two lines closed - ON opens them up. The system runs on 2C crankcase pressure. One line runs between the crank case pressure tap and the tank. The other line runs between the clunk in the oil tank and the muffler. A one way check valve in the pressure line keeps the pressure up in the tank. This system worked well and I got lots of good smoke. I then converted the 300S to 4C power. I installed a Sato 120. A 4C does not have crankcase pressure so I tried using muffler pressure. This almost worked but I didn't have the volume of smoke I wanted and at time the smoke was intermittent. A search of smoke equipment turned up the Sullivan smoke pump. Probably would work but is pricey, ($100). The other item that popped up in my search was the Perry Oscillating Smoke Pump, ($40). Price was much better and it looked easy to install. I bolted the Perry to the Sato 120 per the instructions that came with the pump. I plumbed it up between the oil tank and the muffler. So now the muffler pressure is pushing on the oil supply and the Perry pump is pulling on the oil supply. A couple of check valves and the system was ready for a test run. I haven't flown it yet but on the ground it makes tons of billowing white smoke.
Anyway if you are wanting smoke on a 4C the Perry Pump along with muffler pressure seem to work well.