Anyone know how to tune a gas heater?

I have a Stewart-Warner Southwind gas heater in my '67. The last couple of years, it hasn't put out as much heat as it used to. I've cleaned the glow plug and checked the gap as well as checking the power to the coil. I've also clean and adjusted the thermostat. I've noticed that the heat exchanger doesn't turn orange like it used to and it puts out quite a bit of smoke too. The only thing different is that I've been running 100 octane in it for the last couple years. It seems to me that the fuel mixture needs to be adjusted, but I can't figure out how to do it. Any ideas?