I worked at the factory as the Mk1A (which basically carried over the plastic air box monstrosity all the way through the Mk3) was being developed. Nobody liked the black caps but -- except for the boat anchor effect -- it was clear that the issue with performance with the "noise reduction system" was the airbox. Every test, dyno and road test, showed that the horsepower loss and killing of top end power was because of the airbox. Some test riders would swap assignments with others to avoid riding a day with a plastic air box. I remember Bob Manns* describing the first test bike with the plastic air box as "awful - the most constipated motorcycle I've ever ridden".
I don't care if someone has taken modern examples of the old motorcycles out and run them through whatever test that they can think up -- the Mk3 was an overweight sled with a constricted engine, mostly from the air box.
PS My office door was 8 feet from Mick's in Bob Trigg's design office. Mick was there and he was part of it. He knows what he's talking about.
We used a hanging scale to weigh 4 Commandos, a 71 750 with alton , a 73 750, a 73 850 and my 75 Mk3. All are Interstates.
The Kickstart 750 and 850 weighed virtually the same at about 445 with fuel (3" down in tank, all bikes). The 750 with alton and the Mk 3 were also virtually the same at about 475,
There's no extra weight on a Mk3 other than the estart, about 30 lbs.
These stories get started, then perpetuated and eventually turn into long running myths that many accept as fact.
The scales say otherwise.
I've raced two 750s now, beat the stocker and was almost even with the hotted up bike, as shown in the video. Another friend has a stock 750, fresh rebuild, we are going to give that a try. Should be interesting.
There's nothing modern about my Mk3 powerplant. It is still running all the same internals it left the factory with 48 years ago. And pulling air thru the stock MK3 airbox.
There's no holdup in the airbox. That was all done by the Black caps. Might as well stuff a potato up each silencer as run those things. I have tested the bike's ability to pull on a hill with regular open peashooters and with factory Black caps, as well as with 7 other exhaust combinations. Separate skinny pipes with open peashooters were the best combo. This is what most Commandos came with before the Black caps.
With the open peashooters it pulls hard and will accelerate from 100kmh to 113 kmh in top gear on a 14% grade . With the Black caps and appropriate jetting, it can't even hold a steady speed, the speedo needle goes backwards as you go up!