You would probably gain a little more power if you pull thr inner snorkles and increase the main jets to around 132 - 136, your sparkplugs will tell you for sure. Also, you can make a quick test by running up beyond mid range and easing the choke out *slightly* with the engine fully warmrd up. If it runs a bit stronger, the main jet is lean.
That won't do anything for the poppping though, with the throttle closed you are running only on the pilot jet. It isn't so easy to check for, but you may have an exhaust leak sucking air when you back off. If you take out the air filter, spray a bit of WD40 or some such in the carbs to make the exhaust smokey and block the end of the pipes with rags, you might see a stream of smoke if there is a leak.
I suspect that part of the cause of exhaust popping is the ignition system. A small or zero throttle opening and above idle engine speed is a sure indicator of a light load condition. In the interest of lower emissions and better economy, the ignition controll will most likely use the max advance curve. To get an idea what this does, tey to watch a vintage hotrod run in a hilly area. The vacuum brake distributer on pre - '49 Fords will go to full advance coasting down hill and the pipes pop and bang much like the Triumph twins.
