As stated above, a good cleaning is in order before all else.
Blow out the fire tube and burner compartment. Spiders like to do their thing in this area. (That's the 3/4" dia silver tube)
Next remove the short aluminum gas line, being careful not to distort it. Inside the fitting, you will find an orifice. Use a soft bristle to clean the orifice and blow out with compressed air.
Also, if you aren't aware, you should remove the Anode rod and replace if required on an annual basis.
About a year ago, the electric element in mine went south. When I tried to remove it, it turned into a project just due to old age and an accumulation of corrosion over the years. So rather than mess with it further, I replaced it with a 12 gallon unit (original was 10) which slide in slick as duck soup.
I mention this because there comes a time when these things reach the end of their service life and it's time to take em out behind the barn and shoot em rather than keep chasing ones tail. IMHO,,,,,