Love my Mark III - but how about a little more power?

Had my Mark III out for some exercise yesterday, put it through the paces on twisty back country roads for about 100 miles, and it performed flawlessly, as usual.

Couple of pictures here -





However, there is one thing lacking. I would like to have a bit more oomph when I ask for it. (Probably spoiled by my other bike - a Ducati 999, which has a very different reaction when I open up the throttle).

For example, yesterday, I wanted to pass a truck and trailer in front of me, as they were blocking the view of the oncoming roadway, and i was getting impatient too. On one of my other bikes, or even in my Toyota, I have enough acceleration to pass quickly and be done with it. The Norton, despite all its other remarkable virtues, is lacking in that dept.

Don't get me wrong. I love the bike. As a matter of fact, of the 5 bikes in my garage it is my favorite for recreational riding - so smooth, so visceral, so..... you know what I mean.

I would like to hear from others some suggestions for increasing the HP, specifically to improve acceleration ability - not necessarily to increase the top end (as I rarely have taken it over 80 MPH).

Current state:
- 23,000 miles. Bought it 4 1/2 yrs ago with 14,500 miles on the clock.
- Original owner had a cam failure at 12,000 miles and replaced the cam and a damaged left piston, and re-sleeved that cylinder to standard spec
- I had a valve job done at 15,000 miles because a valve seal had deteriorated and the bike smoked badly. (new valves , the works, at Rabers)
- Have:
- Boyer (was on the bike when I bought it)
- Sparx 3-phase alternator and Podtronics reg/rectifier
- re-built original starter motor which works 1st time, every time (after replacing sprag & associated gears)
- single 34 M Mikuni carb
- peashooter mufflers, and am minus the stock Mark III header crossover pipe
- Corbin seat; short & low handlebars; Avon tires; rear sets
- upgraded front brake (re-sleeved master cylinder)
- stock-size sprockets.

I ride the bike regularly, and appreciate it's amazing reliability - considering it's 36 years old. If only it had a bit more juice....... my silly grin when riding it would be even bigger.

Here's what I'm looking for -
1. Increased power for acceleration (not doing rocket launches off red lights - just looking for more oomph on the throttle in the mid-range)
2. No compromise to overall reliability
3. Suggestions for a set of mods in the Under $2,000 range
4. In case I find some cash under the Christmas tree - what if budget was higher - say up to $4,000

Oops - should have included this in the orig post.

A friend who owned a Mark III years ago said he had one simple mod: he had put in the same cam as a Combat, and it made for a significant power gain.

Anybody have personal experience with just a cam change? which cam? Compromises? Should something else be done at the same time?

Note - I have the typical garage tools, but definitely nothing fancy like machine -shop stuff, nor do I have technical/mechanical knowledge beyond following instructions in a manual. (have removed / replaced heads, pistons, etc.)

Appreciate the suggesitons,

Keith

You have voluntarily castrated the poor thing by putting a single carb on. Going back to twin carbs won't make it a race bike, but will give you the useable power back between 5000 and 7000 rpm. Which is where the power peak should be.
More serious modifications cost serious money, and mean the engine has to come apart.

Ditch the boyer and Mikuni, both not helping. Buy a set of Jim S. Flatslides. And a better ignition, the boyer seems to be stuck in the 70's. Old britts sells a nice new setup that comes with a specific commando tune, and can be changed with other ignition curves. Your Mikuni is killing the top end. Just a start of things that can be done.

You really should do a compression test and a leak down test to figure out if you are losing some power through wear. I am not familiar with the response from a Mikuni but there are those that suggest that dual carbs are gonna be a lot better. If you don't want to go Amal, you could go to the flat slides as available from JS Motorsports. You comment that it has stock sized sprockets. Frankly I don't know what was stock in '75 but a drop in size on the front sprocket by a tooth or two will give you a lot of additional response so long as you don't mind cruising at higher RPM.

If it turns out that you need some new rings or valves then it pretty much opens up a whole world of possibilities. You cold also ask for a Fullauto head for Christmas.

Russ

Hi there,both exhaust pipes look really bad:how is your Mikuni settings?.

[video]http://www.youtube.com/watch?v=lX0rn_QosTg[/video]

I love the opening of this video...

how much do you weigh? :shock:

doubtful if a single carb or a boyer is the REAL problem (unless the car/trailer your trying to pass is going 90+ mph) - maybe tune better, check engine condition, and as suggested change front sprocket down a bit (how many teeth is it now 21?)

investigation further before throwing money at the problem - how un-forum like eh?

Mike, Nitro kit :lol:

I guess I should be clear - I am not trying to solve a problem, or fix an issue with my Mark III.

It runs perfectly ! There is nothing "wrong" with the bike or its performance.

It is not missing any compression or power . It runs the way it should - for a Brit bike built in the mid-70's.

What I am after is to make it better - i.e. more acceleration/ power. And since i frequently have it on the freeway at 75-80 MPH, I am not too crazy about a smaller sprocket which will have it revving even higher at that speed. (Apparently the stock gearbox sprocket for the Mark III for the USA and Canada was 20 T, versus 22 T for other markets).

Going back to twin carbs seems like an obvious step. Does anyone know what that does to HP? Increase of x?

Thanks

Keith

One stop shopping for Norton DIY spunk
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Click to expand...

I would agree that the single most limiting power item is the single mikuni or any other single carb conversion.

If you should decide on a cam change whether a 2s, a JS stage 1 or a 560nr, you will never get the true Commando performance with a single carb.

As limiting as a single carb is, so is a duel carbs advantage. I ran a 36mm Mikunt VM, I then ran a 34mm Mikuni TM flatside and experienced a major increase in upper end power.

That being said and knowing that all other aspect of the stock 850 is in good running condition then the Twin 32mm Flatside carbs is the biggest bang for the buck.

Let me add, any head/cam/piston/rod work you might decide on, you will be then for all practical purposes required to lose the single carb, for as limiting as it is now, the single carb will be even more limiting with performance upgrades.

So, no matter what you do, duel flatsides will bring you up to and past that car quicker that anything else. This is within your budget.

Also, don't be scared to drop into third and run it up to 6500. Anything over 5000rpm is a gas but........as you must now realize, almost impossible with a single carb.

john robert bould said:

Mike, Nitro kit :lol:

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nice! hahaha - now thats funny John!

i think that switching from a single carb to a duel carb set up, and expecting a large increase in performance, may be a little unrealistic - I have run 2 mikuni flat slides on my 850, and found that versus a single TM - there is little difference (except for gas consumption) that being said the PWK twin carb setup that Jim sells is a nice bit of kit and they are easy to set up.

maybe you just need to ride the duc more or live within the limits of the commando

If you can't get to 100 just by opening the throttle, then you do have a problem.
And as any road jockey knows, drop back a bit and get a bit of a runup to overtake when its clear.

What is the aircleaner behind that Mik - washed it lately ?
Those pipes look rather blue - running lean, with no get-up-and-go ?

HTH.

pkeithkelly said:

For example, yesterday, I wanted to pass a truck and trailer in front of me, as they were blocking the view of the oncoming roadway, and i was getting impatient too. On one of my other bikes, or even in my Toyota, I have enough acceleration to pass quickly and be done with it. The Norton, despite all its other remarkable virtues, is lacking in that dept.

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How fast were you going?
What gear were you in?
What RPM were you at?

If you're dropping it into 3rd and can't pass anything I'd be concerned. In top gear and a low rpm (say 3500) and then wacked it I'd probably not be surprised.

If that thin pancake air filter has a flat plate behind it, then I'm surprised that there is enough surface area to even let it tick over. That said, the downpipes do look, as others have mentioned, to be seriously lean (or timing retarded which will cause it to hold back too).

What are the silencers like internally ? If they're not straight through with flutes then they'll cause problems.

An 850, even a Mk111 and even with a Boyer should fly in the mid range, especially if the the original air filter has been replaced. They do run out of chuff above 100mph but at 70mph - 80mph, it should just howl and shoot forwards.

21t is a nice compromise gearbox sprocket.

This Tripe should be good for 10/5 with a sticky 18 in rear .http://commando.yolasite.com/performance-mods.php

The things probly got a complex with a mikuni and thinks its supposed to sound as if its going twice as fast as it actually is .

Might want to try 40s if you want to blow of yr 999 , or flog the carbs of that .45 DCOE Weber or DHLA Delorto or pumpers .

2S or 4S or a good American Cam , Decent valve spring set up ,and keep on a full time tuners . :wink:

What are the silencers like internally ? If they're not straight through with flutes then they'll cause problems.

79x100, How do you tell if the silencers do have mutes/flutes, apart from shoving a broom handle down there? ? Can you still see day light through them? 'Straight through' pea shooters clearly still have some silencing....at least off the shelf... :mrgreen:

And how are these mutes removed?

Matt Spencer said:

This Tripe should be good for 10/5 with a sticky 18 in rear .http://commando.yolasite.com/performance-mods.php
The things probly got a complex with a mikuni and thinks its supposed to sound as if its going twice as fast as it actually is .
Might want to try 40s if you want to blow of yr 999 , or flog the carbs of that .45 DCOE Weber or DHLA Delorto or pumpers .
2S or 4S or a good American Cam , Decent valve spring set up ,and keep on a full time tuners . :wink:

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You tried any of these ideas Matt ?
A near std Commando should do whats asked here.
That air filter is looking like the culprit - well spotted 79x100.

P.S. It was previously discussed that a DCOE 45 won't even fit between the framerails, so can that idea.

Mutes and flutes are 2 very different things.

Some series of peashooters had mutes - these were little bits of thick pipe that bolted into the exits of the peashooters. With them fitted they were reasonably quiet - and passed noise regs.

Flutes were the internal construction of the peashooters - if you looked through the muffler you could see straight through, but the walls were lined with little 'flutes' - triangular sort-of metal sticking out, that did the silencing.

Other peashooters (Campbells ?) have complex internals, with all sheet metal and tubes and view blocking stuff. Not nearly as good in the performance stakes ? - and no quieter either ?? Rust out good though....

HTH.