DP
There are two operating points (only) that can be easily set without instrumentation:
1) Full-rich
Fairly obvious how you do that one
2) Peak EGT
You lean until the engine RPM starts to drop off noticeably, and then go back (rich) a little bit. Verify that the carb heat lever still works as normal: a small RPM drop. Every Lyco-type aero engine I know of is authorised to run at peak EGT when
below a certain power setting (65 or 75%).
So, in climb, use 1).
In cruise, set the power to or below 65% and use method 2).
How to get 65% or below? Look at the engine manual (which is likely to be a separate publicaton to the POH, or the info could be in the POH somewhere) and look up what power the engine is supposed to be developing at a given altitude and RPM (with a specified prop attached).
You will find that at a certain altitude, say 7000ft, it can't make 65% anyway, so if climbing to/above that altitude, the issue goes away. If below that altitude, you have to set the specified RPM when top of climb is reached.
I flew a PA28-161 recently for a couple of weeks and vaguely recall a page from the POH which would show max RPM of some 2600, and a 65% power RPM of something like 2300 at sea level.
That's the only way to do it. Top of climb, set the correct RPM and lean for peak EGT.
The case where you are stuffed is where the engine cannot make peak EGT without excessive vibration. I have no suggestions for that one.