Hello your not doing anything wrong. Your problem just lies elsewhere. You may or may not have the tools needed but this is what we need to check and or adjust. First, the gas pressure is not factory set and has to be adjusted to 3.5 IWC for natural or 10.0 IWC for propane. Next would be the combustion air and or draft inducer. A pressure guage need to be T ed into the draft hose going to the vent switch and verify we have enough negative vent pressure ( there will be a rating on the switch and it need to be more negative and the rating) Burner alignment and also check to see if any type of debris is in the burner. Sometimes this is very difficult to see. If all this checks I would replace the burner or burner manifold. One more thing and I obviously can't see the job but if it has been doing this from the beginning it maybe the way the venting is run or terminated. Since it is a 90 it may have two pipes going outside one exhaust and one combustion air. If the combustion gas or flue gas gets back into the combustion air or fresh air pipe this will happen as well. Let me know if I can help you further. Thank you