This is a very debated subject and i tend to agree with PRTom.
My own oily frame t'bolt has a 21t g'box sprocket fitted, this makes 1st very tall
for traffic etc use but at 4500rpm the old crate is doing around 75mph so it's
With a pair of +1.5mm i/l valves and some judicious work on the inlet ports
together with work on the piston crowns, retaining the 28mm carb and running
the low end pazon or Boyer
at around 33 degs fully advanced allows the old heap to
be ridden at 30mph in top and will pull to 100mph around 6k rpm. It will also
run on 95RON fuel quite happily which is a consideration when living here.
Being a lazy sod , i don't like changing gear more than i have to so flexibility
was the thought behind the head work and retaining the smaller carb. As standard
the old thing would ping if trying to pull from low down and was not happy
at much below 40mph in top.
The silencers/mufflers are the emgo dunsatll replicas lagged a little. The centre hole
is also plugged as they tend to be too free flowing, the a65 likes a little back pressure
at the lower end. Carb jetting as i remember requires the needle to be lifted to centre
and a slightly larger main, 240 or 250.
I also own a 69 t120 and in honesty the difference in performance 30-85mph is negligible
when given some stick. At 75mph the beezer is about the same vibes wise as the trumpet.
What Tom says about the 750 kit is true, but expensive and i'm a mean old git.