The next few posts are Triumphcentric. They summarize what I have learned about this particular bike. Folks with new Bonnevilles may benefit. Most of the rest of you can hit the ignore button.
The inlet tract has a hole at each end. One is a nice round opening and it is always open. The other end is closed most of the time and when it is open it is cluttered up with all sorts of obstructions like valves, valve stems, and valve guide bosses. It is obvious which end is the most restrictive - it is the end with the valves. Experience has taught me this is the first location to consider for tuning.
There are various options for the Triumph. Larger inlet valves are available from Black Diamond, Triumph performance, and others. Inlet valves in 1 mm and 2 mm larger diameters were available years ago when I had the head work done. Now larger sizes are available. The 2 mm oversize worked great with the standard Triumph pistons. There were no clearance problems.
The inlet valve seats need to be reworked to fit the larger intake valves. When this was done I had all of the seats redone with a five angle valve job. Flow around the inlet valve is enhanced by this. Most performance shops can do this. The head was ported when the valve work was done. Triumph Performance did it and others offer similar services.
These bikes are sluggish in standard form. This porting and valve work makes them come to life. An excellent street engine was all standard Triumph 790 cc parts except for this head work, a black box equivalent to the Triumph Performance Stage II, Triumph off road mufflers, and the standard air box with the baffle plate and snorkel removed. The engine with louder Norman Hyde mufflers and velocity stacks ran in the 120's at Bonneville.