Hello,
I have an early .45 Sportster with the o-ring type head. It was new when I purchased it (and I started it briefly, then stored it), but has been sitting for some time. I ran it again recently and it seemed to operate fine with muffler pressure. The fuel tank is a little low, so I added pressure and it cured the problem.
To try and avoid any future issues, I ordered a newer type head that has the fins machined down and a head shim. I installed these items in place of the o-ring type head and have had nothing but problems trying to get a consistent engine run or needle setting. Any suggestions?
CG