On THIS Optimus 300, manufactured in the mid-1930’s. On the third or fourth firing the light output reduced and the J-tube glowed red. I’d noticed from initial inspection of the lantern that the pricker needle barely projected from the jet when raised, but I could detect a very slight resistance - more a lack of smoothness - on turning the control wheel through the point where the pricker was at its top position. Reading several posts here on the red-hot J-tube phenomenon vapourised fuel getting past the jet threads was mentioned as one possible cause, a richer than optimum mixture burning in the J-tube, making it glow. I wondered if the pricker rod shoulder nudging the jet when cleaning the jet might have jolted it fractionally loose, making jet-to-vapouriser no longer gas tight. Backing off the adjustment of the pricker rod at the eccentric restored the smoothness of the control wheel throughout its rotation and - jet nipped up tightly - the Optimus no longer had a falling-off of light output or a red-hot J-tube. John
I regret not getting a photo/video of the red-hot J-tube, which was eye-catching and almost as bright as the mantle itself (reduced in brightness by then however).
Well spotted! More obvious when removed. Cleaned up and reinstalled. A half-hour into a light-up, the definite yellow gleam of brass still, but no red heat. Cooled down, cap removed to inspect the J-tube. All is well.
