I suggest, to facilitate your adventure and to learn better how your machine behaves, to use some sort of rather accurate temperature measuring device while playing with these techniques. Some options are:
- a scace thermofilter
- a "schomer" style portafilter thermocouple
- a temp strip on the group
- black masking tape on the group and an infrared thermometer
- aluminum tape and a Taylor Digital Thermometer (my personal favorite for its sheer convenience)
On The Fly Temperature Management
Without modifications to the operating parameters of the machine temperature management on these machines becomes an interesting game of modifying the group temp right before pulling a shot to get the desired results:
You can cool the group a few degrees by locking in a cold portafilter for a minute or so before the shot. If you have an extra couple double spout portafilters you normally don't use for anything locking a cool one in during your prep between shots is a very convenient and effective way to keep a group that likes to overheat from going out of control.
A disadvantage though is that if you forget to lock in the portafilter before prepping the shot you either have to pull it hot, wait around for a minute or two for the portafilter to pull enough heat out of the group, or use another technique to cool the group quicker.
You can cool the group with a cool wet towel a or small fan: Oops, I forgot to cool the group during my prep and now I have a prepared basket and a group that's too hot. I can use one of these techniques to cool the group in a more rapid fashion.
You can cool the group by putting a bowl of water under it and pulling down on the lever a bit to suck up water into the group. This is a very effective way to cool the group almost immediately. It also has an extra bonus of helping clean the dispersion screen of caked on coffee grounds. It can be too effective however and you can drop the temp too far in a hurry. It can also get kind of messy if you aren't careful.
You can flush right before the shot: On most levers this will be a "cooling" flush to cool the water to the right temp; however on some machines such as open thermosiphon styles, you raise the temp of the group a few degrees by flushing right before the shot.
You can use a pinhole blind disk to get the group up to temp quicker: Levers tend to get hotter when pulling shot after shot without letting them recover. If you have a group that is too cold and you want to speed up the wait, you can pull some "blank" shots through a scace or some other device which regulates the flow to simulate puck resistance. You waste a little bit of water but it can shave a ton of time off of the "warm up" time in the morning. Popular DIY versions involve poking small holes in blind baskets or blind disks.
Advantages to these methods are that they are flexible, meaning you can change the desired temp easily shot per shot; and you don't need to make any modifications to the machine (although you may need extra portafilters laying around);
Modifying the "resting" Group Temperature
You may want to set the "resting temperature" higher or lower depending on personal taste however.
One common way to do this is to adjust the boiler pressure higher or lower. This works but depending on your machine this may also affect preinfusion and steaming, which could be undesirable side effects; know your machine.
Another less popular but also effective way is to change the ambient temperature of the room the machine is in. Those big lever heads are simply heat sinks; a colder room will cause the groups to run cooler; a hotter room will cause them to run hotter. If you don't mind changing the temperature of your room this may be a great option as it doesn't have so many side effects except for possibly the need to wear a hat and scarf every time you make espresso. I have had the opposite problem where due to the location of my machine sometimes the room it was housed in was in the low '50s (farenheit) I had to put a space heater 3 feet in front of the machine and turn it on when the machine turned on to get the ambient temperature around the groups to the proper temp to pull shots at a high enough temp.
NB: f you have a two group machine like me, the above two "resting temperature changers" affect both groups. It may be desirable to change the behavior of just one.
To control individual group "resting" temps, if your machine has a thermosiphon, you could attempt to modify the operating temperature of these levers is to control the flow of the thermosiphon on each group, thereby controlling the heat transfer to the group, and then to the coffee: the slower the thermosiphon flow, the cooler the group, the cooler the shot. Here is an example thread of doing just that.
For machine specific threads related to the concepts above, here are some other good HB articles around the same topic:
The temperature profile of a commercial lever group
Apparent Overheating of Lever Espresso Machines Measured with Scace Thermofilter
Temperature management of commercial dippers
Faema Lambro Brew Temperature
Commercial lever: pre-infusion effect on brew temperature