On my old 850, which I rode fast on the street and at the 1/4 mile drags, it was very noticeable how it ran out of power when the tach got close to 6000 rpm. This bike was in standard trim except for being run with various mufflers and megaphones after the originals rusted in two. None of the replacement exhaust parts seemed to make much difference.
My bike had a 19T gearbox sprocket on it when I got it, and I later changed to a 20 or 21 and can not remember it making any real difference in performance. The bike was so torquey down low it would pull off the line easily, and with the power dropping off at high revs it did not fight wind resistance well above 5k. When I raced it I never revved it above 6k when shifting, it probably would have gone slower if I did.
I rode it a lot in high gear with the tach bouncing close to 6K, it did not ever want to pull much higher than that unless going downhill with the right wind and temperature. I blew a lot of oil out onto the highway too.
At the drags it would hit about 99 mph, probably limited by the skinny street tire I had on the rear, it would spin quite a ways through first.
I don't think these bikes were meant to be anything be anything but mildly tuned practical transportation as they were supplied.
When ridden hard , which was all the time it got pretty bad fuel mileage, which did not work well with the small roadster tank, probably in the low thirties during high-speed touring above 70mph.
With the 850 Norton made the engine larger, kept the same camshaft and dropped the compression, a good recipe for less hp per cubic inch and moving the power to a lower rpm range.
The 650 Norton which I also rode for years and also took to the drags, I think had the same camshaft profile as all later 750/850 twins. It had higher compression than the 850, a crank that weighed about the same, much lighter pistons (68mm bore), smaller valves and smaller carbs at 1 1/16 vs. 1 1/4 for the 850s. It was several mph slower through the quarter mile with a granny launch, no dropping the clutch or spinning the rear wheel.
You could feel the 650 pulling hard at higher revs, it would though with the same cam in a smaller engine, it would pull right up over 6k in lower gears easily and I think it would go just as fast as the 850 on the top end. Not because it had all the power the 850 had, but what it had it made at higher rpm. In Cycle World's 1962 test of the 650 Norton Manxman, they say that the factory recommended rpm limit was 6800 rpm, but in the test they used 7500 as a shift point in tests and said there was never any sign that it caused the engine distress, they also said the valves would float at a bit under 8000.
So knowing the 850 had heavier valves and pistons than the 650 would make me think that in standard form and in good shape 6000 would be a good limit for the bike. I eventually bent the valves in my 850 missing a shift made at full throttle where I was just slapping the clutch lever to snick it into the next higher gear. It was fun while it lasted. How accurate is the tach on those bikes as supplied anyway? When I missed my shift, by the time I looked at the tach it was still not down to 6k. It got me home but never had enough compression to re-start after that.
Thanks for your time......
