The probe location is; I would have to look, it has been so long since I installed it. It is in the ash pit about 4 inches, either at the level of the grate or above it. With out revisiting the location exactly, I would say it is just measuring the temp of the gas in the ash pit.
The vacuum can run about 5 to 7 inches H2O. Can go over 12 or so when the cooling rails go up to 30 inches. I use to have 10 on the rail and 5 in the pit when all was well. The temperature in the ash pit use to be around 900 degrees and that is what it was running today when everything was going well. Then the truck lost power and I looked at the temp and it had gone to 1300 F. At that temp the gas gets weak.
I got new rope gasket material and put it on and drove 2 miles to the airport and all was good ( aspic was 5 in. and temp was 900 F ). Turned around and headed home and the gas got week and the temp jumped to about 1300 F. Of course when the gas gets weak I give it more throttle and all the vacuums go up.
Jim. That really sounds great, but doesn't that mean you use an entire tube for one gasket. At least you have opened it on both ends. Can you save what is in the tube. That sounds like it makes a perfect gasket. I have already installed new rope. but tomorrow is another day.
My conclusion today is--- something is broken in there that I could not see when I removed the char / ashes. Something like a fatigue crack in the air chamber that when it heats up, it opens up and allows air in to burn the gas OR SOMETHING. I missed my dead lines for this week so now I have time to take it apart and really inspect it. I'm thinking, something will pop up during a close inspection.TomC