Ken....your comments sound to be mechanical [contaminate] in nature rather than the physical behavior of gas....so just a few thoughts [and assuming the actual placement height of the jet is as was 2 years ago]
1. disconnect the gas tank, take it outside & exhaust the contents to atmosphere.......leave the discharge fully open and invert the tank a few times
2. unscrew the discharge gas valve gland nut, so unscrewing the valve needle......inspect the needle for evidence of contaminate
3. unscrew the Ronson filling valve with a fork shaped tool......[shown below & available from tank suppliers]
4. if there is any contaminate, gently pour warm water with some kitchen washing up liquid to say 1/4 fill the tank
5. swill the contents around for a few minutes, drain into a glass jar and inspect for any contaminate fall out or settling
6. rinse the gas tank with warm tap water a few times & drain to waste......keeping the tank with valves trappings open to allow the tank to dry out
7. carefully reinstall both the Ronson valve & the discharge valve
8. fill the tank & check for flame characteristics as from that day or 2 years ago
9 if the flame is still retarded, remove all copper/brass tubing spools between the tank & the jet
10. with compressed air, blow through all lines with a white handkerchief on the discharge ends
11. place the tubes in a saucepan of water + a squirt of kitchen washing up liquid.....boil rapidly for 10 minutes...moving the tubes around with a fork
12 . take the tubes & rapidly cool in tap water in a glass jar
and inspect for any contaminate fall out or settling13. rinse the tubes and let dry in the sun for an hour or two14. the burner jet should sit squarely in the jet carrier & once the tightening screw is gently tightened, eliminate any movement
15. reinstall the tubes & check for flame characteristics as from that day or 2 years ago[an alternate to 11. & 12. would be to place the tubes in a 20kHz ultrasonic bath for 10 minutes]........
Let us know what differences occur & when
... Derek