In the full-scale world, we always use a solenoid valve on the same switch as the smoke pump. It's a little different situation than the open jet exhaust, but dribbling oil into a hot exhast pipe after engine shutdown can get a stack fire started. The venturi effect does tend to keep a small flow going after pump shut off. On my (full scale) Skybolt, I run a 50 PSI pump and 4 really small injection nozzles which extend 1/2 way into the exhaust pipes. I get really good smoke with very little residue on the plane. A lot of the guys using low pressure systems (basically an open AN fitting welded to the exhaust pipe) end up with the belly dripping oil after a ten minute flight. I'm pumping about 1 gallon/ minute.

What kind of speeds are you getting from the jetluge in 1/4 mile?