How much slower is a MKIII?

I happened to meet a fellow who has a very nice 850 MKIII with 15000 original miles. Untouched, still sporting black cap silencers and points ignition.

So my riding buddy / fellow Norton owner starts telling him that he needs to lose the mufflers and install peashooters, swap the intake for a K&N, send his carbs out for sleeving, and of course get a Boyer or whatever. His reasoning is that the stock system is so restrictive that he'll never get over 85 mph, and that the power of the MKIII is simply lame compared to earlier versions.

For those of you who own and ride a MKIII, is the overall rideability so bad that it demands being modified? Or is my buddy correct in his assessment?

DRC

STOCK Mark3 versus STOCK generic say 73 850 all figures are averages taken from actual period road tests Cycle World, Cycle, Motorcycle New, etc tec

Top Speed

Mark3 around 102

73 850 107


quarter mile:

Mark3 15.2 sec

73 850 13.4 sec

Performance improvement possibilities aside, 40 years after manufacture, I'd suggest keeping an original spec bike with the factory parts intact.

I bought a new 1975 silver Interstate in 1976 and put a lot of miles on it. As things like mufflers wore out , I replaced with peashooters, then pulled the box air filter for KN.

The bike will never be able to return to standard again.

When I bought my Mark III the black cap mufflers were rotten out. I bought new ones and installed a single Mikuni carburetor as well as
a Tri-Spark electronic ignition. I wasn't too happy with the performance. It was just plain a dog, unlike my Mark II which is a hot rod.
I had thought that the Mikuni jetting was the problem but was advised that, while I might gain improvements, the black cap mufflers
were the culprit causing the poor performance. So I bought new pass-through pea shooters and mounted them. Voila!!! I had a new
bike that actually performs like a Norton should. A night and day difference! My Mark II is still a peppier at all speeds, but then it's 40 lbs lighter. While I am sympathetic with the notion that a Norton
should be kept original, I have no regrets about having a Mark III that actually performs well. What's the point of having a Norton if
it's a doggy garage ornament, an investment?

While I agree that re-installing a Mark III air box is a major PITA, it can be done. All other changes are very minor and all removed parts
saved.

I would suggest that the answer to that question is ....that it depends on the rider some ride slow bikes fast, while others ride fast bikes slow :roll:

cjandme said:

I would suggest that the answer to that question is ....that it depends on the rider some ride slow bikes fast, while others ride fast bikes slow :roll:

Click to expand...

I kinda do both...LOL

They run pretty good when you strip off all that electric starter/leftside shifter garbage and drop 'em in a 750 frame...... :roll:



...and lose the base gasket, lighten the rockers, match up the ports, manifolds and carbs, lace up some Borranis, trash the passenger pegs, drill the rotors to within an inch of their lives....... :mrgreen:

If you want a fast bike by an R1. Our maximum speed limit is 110kph, yes I'll run mine up to 130 occasionally, horses for courses. A well tuned MK3 will give you a nice easy ride.

The MkIII is faster than some bikes and slower than many others. 1975 vs 2016.
I believe a person needs to consider the MKIII for what it is, a reliable classic comfortable ride for anyone who expects reasonable performance.
Capable of extended periods of highway motoring if wind buffeting is not a major concern. 86mph is easily attained.
Readily provides sharp crisp handling at reasonable speeds and is superior to many of the same period.
Sounds good, looks great.
It's simply not a fast motorbike by today's standards and the brakes are only adequate. The same may be said about previous years Commando's as well.
If you tweak the front brakes and then really need to use them you end up sliding up the fuel tank somewhat.
Yeah, ok, it is not a 72 combat, your granny could drive one and my right hip thinks that is just fine.
Stall it in traffic and push the button, no muss, no fuss.
I gave up trying to turn various vehicles into something they were never intended to be, an extra 10 or so mph is irrelevant to me.
Performance wise mine is bog standard save for the Dunstall silencers it came with and the EI I added to rectify the run away AAU.
I have never felt under powered, but then I don't feel the need to ever be the most rapidly moving object on the road.
To the OP, see for yourself. Trade rides for the day with the fellow you met.
I have no regrets owning my MKIII, it is always a thrill to ride. The older I get the more thrilling it becomes.
All the Best.

Mine is stock other than Boyer, open pipes and a skinny head gasket. It pulls really well.
On the seven mile long Salmo Creston grade, which I believe is at 8%, it showed a tick over 90 MPH on the speedo, in top gear.
I'm about 250 lbs with riding gear plus I had luggage on board.
Would a Combat or any other model of Commando do any better than that?
More importantly, would it survive the pull?

I've done dozens of those pulls on that MK3. It doesn't seem to care.

Glen

What are the key performance related differences between a mk111 and earlier bikes?

I can think of:
Lower CR (claimed 8.5:1 vs 9:1 of earlier bikes. But my own '74 mk11a was under 8:1, I suspect they were lowering on purpose after the Combat saga)
Bigger inlet ports (too big according to Sir Comnoz)
More restrictive airbox (some say)
More restrictive silencers (Black caps)
Extra weight (electric start and associated components)

Is the list correct?

Fast Eddie said:

What are the key performance related differences between a mk111 and earlier bikes?

I can think of:
Lower CR (claimed 8.5:1 vs 9:1 of earlier bikes. But my own '74 mk11a was under 8:1, I suspect they were lowering on purpose after the Combat saga)
Bigger inlet ports (too big according to Sir Comnoz)
More restrictive airbox (some say)
More restrictive silencers (Black caps)
Extra weight (electric start and associated components)

Is the list correct?

Click to expand...

Another point to consider is the taller gearing - a 22T gearbox sprocket if I remember correctly? The A-N website is no help here as it lists from 19T to 24T :shock:

It seems the 'easy' fix is to ditch the black caps. I'm running a pair of original ones on my MkIIA Roadster and it's a real pleasure to ride; quiet and smooth. It does feel quite un-British and with 'lower' (still high) gearing (I dropped to a 21) I rather like it for local riding. My otherwise identical Interstate with peashooters is a slightly more sporty performer, and better suited to high speed cruising. If I only had the one bike I'd be running peashooters.

While the weight of the electric start can't be helping outright performance much, the emissions control 'enhancements' strangle the bike far more.

Fast Eddie said:

What are the key performance related differences between a mk111 and earlier bikes?

Lower CR (claimed 8.5:1 vs 9:1 of earlier bikes. But my own '74 mk11a was under 8:1, I suspect they were lowering on purpose after the Combat saga)

Click to expand...

Depends if you are comparing the Mk3 with the 750 models or just earlier 850s? The claimed compression ratio for all 850 models was 8.5:1.

Although the Mk3 was certainly heavier, theoretically there shouldn't have been much if any difference in engine power output between the Mk3 and the Mk1A and Mk2A models.

Fast Eddie said:

What are the key performance related differences between a mk111 and earlier bikes?

I can think of:
Lower CR (claimed 8.5:1 vs 9:1 of earlier bikes. But my own '74 mk11a was under 8:1, I suspect they were lowering on purpose after the Combat saga)
Bigger inlet ports (too big according to Sir Comnoz)
More restrictive airbox (some say)
More restrictive silencers (Black caps)
Extra weight (electric start and associated components)

Is the list correct?

Click to expand...

I thought all the 850s had a quoted 8.5 to 1 compression ratio.

I was comparing to the 9:1 of 750's.

I've never measured a 750, but I know when I measured my own 850 it came out at under 8:1, despite the claimed 8.5:1.

AntrimMan said:

The MkIII is faster than some bikes and slower than many others. 1975 vs 2016.
I believe a person needs to consider the MKIII for what it is, a reliable classic comfortable ride for anyone who expects reasonable performance.
Capable of extended periods of highway motoring if wind buffeting is not a major concern. 86mph is easily attained.
Readily provides sharp crisp handling at reasonable speeds and is superior to many of the same period.
Sounds good, looks great.

Click to expand...

Which was my input to the fellow. He's a weekend rider, never had a Brit machine before, and doesn't have anything to compare it to. I suggested he get a workshop manual and go through the "routine maintenance" chapter. Keep it stock and just ride it as is. OK, maybe take the silencers off and swap some peashooters on it, but otherwise leave it alone. If he keeps it long enough to wear out the AAU then install a Boyer or equivalent.

Only issue he has with it right now is the idle is high and a bit erratic. I tried dialing it down for him but it became unstable and wanted to stall. When I started going on about cleaning the pilot jets I kinda got a blank stare in return. I was hoping not to scare him into selling it, but Norton owners need to be mechanics and I'm not sure about this guy.

I guess if you are comparing the MK3 to the 750s, then you have an extra 8 ft. Lbs torque with the MK3, 56vs 48 or 49 for Combat.
That more than makes up for the starter weight, which I doubt makes a noticeable difference in felt performance.

Glen

As far as I know, my MkIII is stock as far as engine internals and final drive gearing. It does have peashooters and K&N pods with proper jetting, as well as a Boyer analog ignition.

I have had it to somewhere between 105 and 110 indicated; the needle was bouncing a bit, but I'm quite certain it was over the ton.

Road was flat and smooth, and I weighed 130# at the time. That was the day me and 2 friends did 620+ miles from 8AM till about 10PM. We cruised between 70 and 80 on the Interstate (about 200+ miles of the trip).

The bike DOES feel a bit zippier than it did with the airbox & beancans, but I had it over the ton with that stuff on it at least a time or two.

maylar said:

AntrimMan said:

The MkIII is faster than some bikes and slower than many others. 1975 vs 2016.


Only issue he has with it right now is the idle is high and a bit erratic. I tried dialing it down for him but it became unstable and wanted to stall. When I started going on about cleaning the pilot jets I kinda got a blank stare in return. I was hoping not to scare him into selling it, but Norton owners need to be mechanics and I'm not sure about this guy.

Click to expand...

Click to expand...

I did clean the idle jets, synced the carbs with a carbtune and installed a tri-spark. Made a different motorbike out of it. No loping idle, just smoothness.
At the end of the season I just made sure I had closed the petcocks. That's it. It is in a garage.
This season I charged the battery, drained the sump, checked the air pressure, opened the petcocks, tickled the Amals, put my leg to it as I pushed the button and that is it. Away we goes.
If you talk him out of it make sure you buy it.
All the Best.

I pretty much agree with your first post on this, the bikes are not rocket ships but they are no slouches either. Fwiw my one with standard airbox (rubber venturi removed), peashooters and original amals is much quicker than my other which has a 34mm Mik and peashooters, however they both do their own job admirably. End of season I drain the carbs, and disconnect the fuel lines. The Roadster tank goes upstairs in the spare bedroom, normally the Interstate with the Mik is used all year round, I don't have a modern bike .
sam