Office of Steam Forum for Model & Toy Steam Gas & Hot Air Engines

Could it be that what you are seeing is actually gas from your torch, being deflected by the hot exhaust rising up your stack. I do agree that it looks like it really is unburned gas from your boiler's flue, but perhaps that is only how it looks?

Perhaps it's just a bad can of gas that you are currently using. Have you tried this experiment using any others?

Meanwhile, I'd mark your air vent collar position, and try adjusting it a little at a time, allowing for a bit more air into the mix and see if your situation changes noticeably. If you get to wide open without any change apparent, then I'd try the next size smaller orifice.