50 Years of Norton Design: Interview with Brian Slark and Mick Ofield

Thomasa said:

I ditched the plastic airbox to fit in a bigger battery for the e-start. I reverted it to the earlier MKII parts,'74, it think. It involved battery mounts etc. Lately I find it seems modern batteries have so much greater cranking power that I needn't have bothered but I'm not going back. It was too much work.

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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.

I hope I'm not writing too much personal info but Mick has been talking about it openly. He lives about 3 miles outside town in his retirement home in Tennessee. About 6 -8 weeks ago, he pulled out of his driveway to go into town and before he got about a quarter mile down the road, another pickup truck veered into his lane and hit him head on. The impact was so strong that it pushed the engine in his pickup into the transmission tunnel area. He was pretty badly hurt, multiple contusions and cuts, broken collarbone and ribs with lung bruising (I'm not so sure about the details on this part but this is about right). He was hospitalized for about a week but once he was stable he's continued to heal and last time we communicated about 3 weeks ago, he told me that he's continued to feel better and is able to be out and do usual activities.
Brian, of course, has continued to be an Energizer bunny in his mid-80s with seemingly unending energy. I look forward to seeing him again at Barber in October.

Blewdy Yaink said:

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.

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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!



Glen

Just to make sure that the myths are identified as myths- here is what Jim Comstock had to say in response to a poster who wondered if his MK3 airbox would be adequate for a hotrodded 850-

"No problem with the MK3 airbox. They flow great. About the same as the ham can.
Either are considerably better than the K&N . Jim"

As most of us know, Jim actually does flow tests and dyno tests to get real information.
This agrees with what I've found on dyno hill. When I remove the hoses to the airbox the bike goes 2kmh slower over the top than with the hoses connected. Some restriction!

So there is the myth about air flow sorted.


The bloated weight myth was already sorted by the hanging scale.

One other myth was that Black Caps don't negatively affect power.
That was sorted by Dyno Hill.

Nonetheless, I'm sure that some folks will continue with all of these fictional stories.
They make good yarns to have a beer over!

Glen

I left out 2 other myths

-750s are much lighter and more nimble than 850s.
The scale showed that the kickstart Interstate 750 was just 4 lbs lighter than the kickstart Interstate 850. This would not create a noticeable difference in performance, especially with an extra 80cc of displacement for the 850.


-750s are faster than 850s

I'm still working on this one but it's not looking good for the 750s so far.
And that is using the slug of all slug 850s as tester, the bloated , constricted, some even say detuned, MK3!

Glen

worntorn said:

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.
(snip)

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.
(snip)
Glen

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Nice data, but it conflicts with months of dyno tests, road tests, and experimental work at the factory. I was there (as were Mick and Brian). I'm glad you have a Mk3 that works well for you.

Well the scales certainly don't lie and I have complete confidence in Jim's airflow findings, moreso than any anything anecdotally from the factory.

I would very much like to see the dyno tests from back then rather than just go by a memory of something that happened almost 50 years ago, especially when that memory conflicts with the real world results obtained now.
I can imagine that dyno tests were done as the bikes were built, - with black cap silencers fitted. I don't doubt that those results were poor. The acceleration performance of every Black cap equipped Commando was very poor in all road tests.
Because of that and my own testing of the Black Caps, I don't use those.

If you put a set of those on Nigel's fire breathing Maney 920, the fire would go out!
There are straight thru Black caps available now. I'm sure those work well.


As far as the on road tests- all I can do is video the results. If the result isn't what is supposed to happen and therefore isn't accepted by some, that's OK, your mind was already made up.



Glen