Thank you for your reply. I am not certain of the heater age (I recently purchased this home) but I would guess it is about 6-8 years old. It last worked approximately 3 months ago (last time we used spa). I will post pictures as requested I did check input gas pressure (before changing the NG valve). It was approximately 7"" WC, but I returned the manometer to my friend when I decided to replace the gas valve (thinking the new one would be calibrated). The old gas valve was defective (I could not hear the valve open like I can now). As far as smelling gas at the vent - I do not - I assume it is the high air mix from the draft induction fan because I did test the gas valve before installing in the burner (just vented above heater) and I could smell/hear the gas then. Note that I did not replace with a Hayward part. I used to work in commercial HVAC thus I had a friend of a friend get me a wholesale equivalent Honeywell valve. The ~7 seconds seems correct then for the initial gas release. I have watched the voltage during the ignition sequence and do not see a significant drop during the 7 second period (it was always approx 27 VAC). As far as burner cleaning I removed the burner orifices and blew out with compressed air, then removed all SS burner tubes, removed minor corrosion with steel wool and cleaned with compressed air. There was only minor corrosion - mainly at the burners most distant from the gas valve. If you think the issue may be outlet pressure related I can get the manometer back and check next weekend. One question for you - you mention the Fenwel ignition control. Does this refer to the control on the gas valve or the Hayward control board?
