Chessie.
Your description is one of a blockage to the gas jet, leading to fuel starvation. I had exactly the same symptoms with a brand new model recently. After lighting the gas, it was barely warm ten minutes later. A quick clean of the jet and it was blowing off within 3 minutes.
It is a simple matter to loosen the complete jet holder from the drag beam of the loco, disconnect the rubber pipe, then with suitable spanners, unscrew the jet from its housing. Take your can of gas, invert it holding the nozzle over the hole in the jet, at the front. Squirt a stream of gas backwards through the jet to remove any blockage. If successful, you should see a stream of liquid gas emerging from the rear of the jet. This stream should be axial and parallel to the jet. If it seems to be one-sided, then soak the jet in citric acid solution or vinegar for a couple of hours to loosen any material, then repeat the squirt of gas. Do not be tempted to poke a piece of wire down the hole as you may result in enlarging the hole with unwanted effects.
It is not unusual for a new gas fired loco to suffer from blockage to the jet. As a result of the silver-soldering process, scale will develop inside the gas tank. The movement and shaking of the loco in transit can easily cause some of this scale to loosen, enter the gas flow and block the jet. I usually put a piece of cigarette filter inside the jet to intercept any subsequent rubbish that gets into the system.
When you replace the jet, it is important that you do not have any gas leaks around the thread. Two ways to deal with this: one, wrap a short length of PTFE tape around the thread and screw it in place, two apply some sealant to the thread - silicone, gasket cement, etc before screwing in place.
Keith
