You really do not "burn" shots when the water temperature is too high. Although people commonly refer to the espresso making process as "extraction" it is not a simple extraction, rather it is essentially a chromatography in which compounds in the coffee migrate by going into solution, then reabsorbing, and repeating the process as they go through the coffee puck. Ones that migrate fast get to the bottom practically with the first water to come through, others spend more time absorbed and move slowly. Too hot water makes everything move so fast that all the nasty tasting stuff comes out during the shot.
Your machine appears to be a second generation La Pavoni two switch model, the one switch models with pressurestats were phased in over many years through the 1990s. You do NOT need to bleed the models with two switches through the steam valve, but you most certainly do need to do so with the one switch pressurestat models.
You might consider the purchase of an infrared thermometer, they seem to cost about $US12 these days. If you have a plated model you can stick pieces of tape onto the machine on places you want to read temperatures. If you have a polymer coated brass machine you can just aim the thermometer at points and get the temperature there.
Be sure the machine is filled with water. Turn both switches on. When the thing starts to hiss loudly, put a ramekin or other suitably sized bowl under the group and slowly raise the group handle until a bit of water is released. Lower it slowly. Repeat the process. This should clear almost all of the air from the space above the piston in the group so that heat transfer can occur. As soon as this bleeding process has been completed turn off the high wattage element. The group will heat very quickly. If too cool you will get a spongy low volume sour shot. If too hot you will get an intensely bitter shot. If the lower part of the group be near about 90C you will probably do best.
When making the shot you might set the cup and portafilter to the right of the machine. Raise the handle until a few whiffs of steam and a bit of water are released, and lower it just a bit to stop the steam and water. Attach the portafilter, put the cup under it, and raise the handle all the way. You might want to preinfuse by bringing the handle down just enough so some espresso emerges, then recock it and make the full pull.
Remember--too cold-sour, low volume, spongy pull. Too hot-bitter.
Using a deeper basket like the ones from a Micro Casa a Leva(with larger shot mass like 16-17 grams) may reduce bitterness because the puck will be thicker and the compounds will have to travel farther to emerge from the bottom.
It takes some experimenting to develop technique for these machines!