Just raise (or lower on a spring lever) the lever when you have no coffee in the portafilter and let water gush from the boiler for a couple of seconds
Just don’t go and tell everyone that lever machines suck and you can’t control the temperature.
To bring the group on a lever machine up to its correct operating temperature if you have just switched it on at the start of the day, or if it has been sitting idle for some time, load the portafilter as you normally would with the stale coffee that has been sitting in the burr chamber of your grinder and pull a shot.
This ensures only a tiny volume of water is drawn from the boiler. Rather than allowing a large volume of water to gush from the boiler at high velocity to heat the group (which results in a group temp of indeterminate value – too hot/too cold/usu too hot in the water channel and too cold further towards the external surface of the group), use a small volume of water and only allow it to travel through the group at approx the ‘normal’ speed for the thermal capacity of the group, i.e. 30mL/60mL in 25-30 seconds. With the coffee restricting the speed at which the water can pass through the group there is sufficient time for the group to warm through evenly.
If you have a full commercial group on your lever machine, like the Bosco, you will need to run 2 double shots to bring the group up to the perfect temp
Try this and be amazed at how much easy lever life is. It works on them all, including a La Pavoni.
ps – do not adjust your grinder in response to the first or second of these warming shots as they will shoot through faster than ideal even if your grinder is at the correct setting when you pulled a shot last time you used the machine (assuming it was recently)
why? because the stale grinds will have dried out as a result of being exposed to the air, which will allow the water to pass them more quickly than the fresh grinds that will arrive on the 2nd/3rd shot. secondly, the water in this 1st (and 2nd shot if a commercial group) will be too cool. Water that is too cool will pass through the ground coffee faster than water that is at the correct temperature)
so pull your 3rd shot before you make grind adjustments would be my suggestion, unless you are very familiar with your coffee, grinder, and lever machine
just remember that while a lever machine is ‘low tech’ the group has had a lot of thought from the design engineers with literally decades of refinement to ensure the group is an effective heat sink in reducing the water temperature down from boiler temperature when it first enters the group to the perfect temperature for your espresso as it exits the shower screen
if you open port flush as so many people who get in trouble with levers seem want to do, you are exhausting the ability of the group to act as a heat sink in the manner described above. you then have no option but to leave it for perhaps 30 minutes to cool.
if you have lever issues please ask; we love them and they are easy to use once you get to know them