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

To do right by the MK3, if you reduce the back pressure on the exhaust, you also need to ditch the plastic airbox and replace the 230 main jets with 260s, and adjust the needles.
The result is undeniable, and sweet.
But I agree about the noise.
A free flowing exhaust is great around town for short periods, but for long trips on the super slab, I'll take a quiet pipe every time.

I did exactly what you suggested, even replacing the airbox with a ham can I managed to cram in despite the starter. Sound sweet for as long as I ride it these days. Now for a quiet muffler for my hard-working pickup.

BritTwit said:

Well ya know Harley production motorcycles had to meet the same exhaust regs as Norton, or any other manufacturer. But dealers were more than happy to install louder (better) exhausts before the bikes rolled out the door.
No self respecting Harley owner would ever be seen, or heard on a Harley with a stock exhaust.

My thinking is that you should just accept that the 750s were quicker than the MK3, and that the MK3 was a simply superior motorcycle to the 750s in every other way.

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Did you see the vid of the stock MK3 vs the hotted up 750 from 10 mph to 110 mph?
Glen

BritTwit said:

To do right by the MK3, if you reduce the back pressure on the exhaust, you also need to ditch the plastic airbox and replace the 230 main jets with 260s, and adjust the needles.
The result is undeniable, and sweet.
But I agree about the noise.
A free flowing exhaust is great around town for short periods, but for long trips on the super slab, I'll take a quiet pipe every time.

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The plastic airbox is fine. It is huge and gives great flow. Some of the smaller K&N's might hinder performance, but the plastic airbox does not.
Yes to bigger jets once the potatos are removed.
The same 30 minute treatment is needed for Mk1a and MK2a.

Glen

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.

worntorn said:

Did you see the vid of the stock MK3 vs the hotted up 750 from 10 mph to 110 mph?
Glen

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I did not.
Do you have a link?

I was planning to renovate mine to do a switch to either ham can or K&N when comnoz posted some very useful info on airflow with various air cleaner/ no air cleaner configurations.
It seemed that the right airbox could be better than open carbs and that just about anything was better than open bellmouths with screens fitted.
I removed the rubbers connecting the carbs to the plastic airbox and ran the bike up dyno hill. The speed at top was the same as with the airbox connected. It liked the same jets as with the airbox connected.
Conclusion- the plastic airbox and air cleaner don't hinder or help performance.
Glen

Was that with the black cap silencers or peashooters?

BritTwit said:

I did not.
Do you have a link?

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The MK3 is stock and still on original Pistons/rings/valves from 1975.
The 750 has a performance cam, bumped compression and Jim Comstock modified head with Fullauto style D exhaust ports.
The lead time is about 1 second? for the 750.
We both would like to try again as Jim hit the rev limiter on his 750 and I stayed in 3rd way too long.
We will also reverse the order and let the MK3 roll on first.

BritTwit said:

Was that with the black cap silencers or peashooters?

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That was with pea shooters.
I have only had borrowed black caps on the bike for a few runs up Dyno hill.

Impressive!
Doesn't look like much difference between the two bikes.
Can't wait for the rematch, maybe you can have a scantily clad young lady start it with a flag.
Winner buys the beers!

Maybe when doing rematch, have a run switching riders also.

"The MK3 is stock and still on original Pistons/rings/valves from 1975."
Is it not a little misleading to call it "stock" if it does not have Black Caps?

NickZ said:

Maybe when doing rematch, have a run switching riders also.

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Yes that should be done, Glen has a very good technique for gaining max acceleration, dunno if his buddy does…?

worntorn said:

Mick appears to hold the same view that Alan mentioned awhile ago- that the MK3 was a changed bike with great loss of performance.
The big loss of performance came with Mk1a and Mk2a models that were fitted with black caps.
I have an original road test of one of those bikes. It is slower through the quarter mile than a BSA A10 , and that is pretty slow!
It is quite possible that these fellows are not aware of how the black cap silencers destroyed the performance of any Commando they were clamped on to.
The official word from Norton was that they were Scientifically designed to offer quiet running without power loss. Nothing could be further from the truth.
It's a strange situation, the Norton twin engine was developed from 25 HP to 60 over about 20 years. Doug Hele played a big part in that, getting the head design so right.
I've read endless inter Company memos where tiny performance enhancing details are worked out. All interesting stuff and quite well executed in general.

After all of that they stuck a potato up each exhaust and dropped it back down to 25 HP all over again!!


Glen

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I have a MK3 with the original bean cans, not aftermarket, and I would put it against any close stock Commando, it runs fine and quiet. Note there are differences in original ones and aftermarkets are not even close internally ,John Favill went on to work with Harley. I like mine and if they were made to a higher standard they would have been even be better, I took one apart and even though it was badly corroded I could see that it was not built to the patent . CNC machined components in SS would have been better , I thought maybe someday I would have some made.

NickZ said:

Maybe when doing rematch, have a run switching riders also.

"The MK3 is stock and still on original Pistons/rings/valves from 1975."
Is it not a little misleading to call it "stock" if it does not have Black Caps?

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Fair enough.
My MK3 engine is internally stock, the other bike has internal engine modifications for added power. The modifications are -aftermarket performance cam, raised compression and Jim Comstock modified head with D exhaust ports.
Hopefully that description makes the grade.

Glen

Plus John would have been a good person to have in the interview

Funny Slark was dishing fellow Norton owners/restorers for asking questions about certain parts on his "new" MK3s. However he never mentioned the particular offending parts , just a general wave, stting in front of a JPN with nonblack chromed mufflers. I am sure he has an explanation. Two frowned upon MK3s not a S type that was so prized by him in the day. I don't remember seeing one in the museum maybe they don't have one( and that would hard to believe) .

It is easy to get more torque out of a Commando engine, as long as you can live with the noise. However, a friend of mine once fitted a megaphone exhaust to a Thruxton Velocette and did not change anything else. He immediately burned a piston and got towed home.
A two into one exhaust is much better on a Commando, if you intend to ride fast around corners. However, for it to work properly, there must be no reduction in its total cross-sectional area beyond the collector. That area must be at least the total of that of the two header pipes And the exhaust valve opening point must be earlier, to pump the pipe harder. A 2 into 1 exhaust with some form of Exup would be good.
The other thing is, when you get more torque out of your Commando engine, you usually do not know you have it, until you raise the overall gearing and the bike becomes faster..
I suggest Exup is primarily intended to defeat noise, It is claimed Exup increases torque, But with superbikes, if you run the hottest cams the noise levels increase and something is needed to cut the noise levels. With Exup, the overall effect af the hotter cam and more restriction is more torque ?
If you ever watch a Superbike race, listen carefully to the noise they make compared with the revs they are doing. Sound meters only work under certain conditions.

jimbo said:

Funny Slark was dishing fellow Norton owners/restorers for asking questions about certain parts on his "new" MK3s. However he never mentioned the particular offending parts , just a general wave, stting in front of a JPN with nonblack chromed mufflers. I am sure he has an explanation. Two frowned upon MK3s not a S type that was so prized by him in the day. I don't remember seeing one in the museum maybe they don't have one( and that would hard to believe) .

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Hopefully the slug slow MK3/blazing fast 750 nonsense is pretty much sorted.
The weight bs was easy, the hanging scale sorted that.
Put an Alton Estart on your willowy 750 and it now weighs the same as a bloated MK3.
The MK3 power problem- you just need to chuck out silencers that were designed 50 years ago and replace them with the 54 year old design instead.
Then you are good to go against any most any model of Commando including some ( not all) hotted up ones.
The rest of it is just talk!

Glen

My mk3 has the cone end silencers, with balancer pipe still on. Single amal carb and 23 tooth sprocket. Contrary to all expectations, it flies along. I generally keep below 5,000 to 5,500rpm, but it still pulls very well above 6,000rpm, which is 70mph plus in 2nd. It absolutely purrs at 3600, 4th gear, 70mph. Love it.

Not properly tried a 750, so I don't know what I might be missing. But, couldn't be happier with the mk3!