Let me pick your brain

I see at least 2 problems. Even if introduced after the carb, the propane needs oxygen to burn. I the carb is set to deliver a proper air/fuel mixture (which I hope is the case), then you pump propane into that mixture, it will be too rich (needs more air). Also, since motors suck the mixture in from the carb, pumping propane in after the carb will result in less air/gas being delivered.

Now, I'd like to see you (anybody but me) try fuel and oxygen. I bought a small setup for brazing. It has 2 small tanks, the size of small propane tanks, one with MAPP gas and one with oxygen. Mapp seems pretty close to acetylene. Pump a mixture of that in your motor. That should give you a boost!