The ton plus 10, what could be the problem?

ashman said:

On my orginal commando in the early days at 4,000 rpms my speedo would be between 60 to 65 mph

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my 74/850mk2 showed ~ 70mph at 4k rpms

Well it was a long time ago and depends which way the wind is blowing, how good your speedo is as well as your speedo cable is and I had a 19 tooth front sprocket, the only time I care about my speed is when the cops are around or know where they hide, another thing moden tyres are lower in profile, it all plays a part in speed readings and how honest the rider is about it.

Ashley

If you are looking for more top speed, I can think of one thing which might help. If you have not got rubber mounts for your carbs - buy some. I use 34mm Mk2 Amal carbs in rubber mounts with the port in the head tapered from 2mm per side for the first 12mm, back to the standard 30mm. 34mm carbs might be a bit too big if you are using petrol.
The usual Commando has a relatively low balance factor with the motor gearbox assembly rubber mounted. When you are riding at high speed, the vibration of the motor could be causing frothing which causes the float level to rise - that can richen the mixture.
With my bike, it is mainly about acceleration, so I very carefully jet the needle and needle jet. With the methanol fuel I use, getting that right is critical - slightest bit rich takes the edge off the motor. You are obviously using petrol, so your situation is twice as critical - methanol hides up tuning errors. If you are doing high speed runs, doing plug chops is important. With my bike, the main jets are always slightly on the rich side. In road racing, it does not matter, because you are never on the mains long enough to do damage. For sustained high speed, the situation is different. I don't know what the guys who ride commandos at Bonneville do.

Craig said:

roughly how fast should a '72 Commando be going at 5000 RPM ..... standard size tires both ends K81 4:10 X19" , stock rear sprocket , with 21 tooth counter shaft sprocket , 180lb rider ..... Big Thanks !!! ....

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Not quite answering your question as mine is an 850 with a 22t gearbox sprocket, but as best as I can tell, 5000rpm equals about 97-98mph. It’s difficult to be exact when the speedo goes in 10mph increments, and one is watching the road (private road of course).

Tacho and speedo are both Smiths digital jobs.

Fast Eddie said:

Not quite answering your question as mine is an 850 with a 22t gearbox sprocket, but as best as I can tell, 5000rpm equals about 97-98mph. It’s difficult to be exact when the speedo goes in 10mph increments, and one is watching the road (private road of course).

Tacho and speedo are both Smiths digital jobs.

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According to charts published by the INOA, a 22T with a stock primary should give 122 at 7000 rpm or 87 at 5000 rpm.

Forgot to mention, I have a Norvil belt primary, which I understand raises the primary gearing, but I’ve forgot by how much.

I also make no claim on my speedo accuracy, digital or not!

FWIW this is what I get on my bike using:

21 tooth front sprocket, standard rear w/ 100/90-19 RoadRiders

17 mph for every 1,000 rpm verified by GPS
102mph @ 6,000 rpm also verified by GPS
Bike tach accurate against digital induction tach up to 4,500 rpm

I don't have much idea about what speed my bike is doing anywhere. I only know it is usually too fast to fall off comfortably. It is amazing how fast peak revs in first gear actually is with a close ratio 4 speed box, usually you don't even think about it. A start-line crash can be very bad - a 30 MPH crash can be fatal, let alone 60 MPH + . It is not the crash, but the sudden stop ?

I made simple excel spreadsheet with a few formulae to work out speed vs RP< with various gearing. It's dead easy.

Engine RPM x primary ratio x rear ratio = rear wheel rpm :

Example
21/42 primary = 0.5
20/42 rear ratio = 0.476

7000rpm x 0.5 = 3500
3500x 0.476 = 1666

So the rear wheel is spinning at 1666 RPM

Our tyres are roughly 80" in circumference so we are doing 133280 inched per minute ( 1666x80 )

Another simple reduction gets us from inches per minute to miles per hour: 126MPH at 7000rpm

Once you enter the formulae into the spreadsheet you can change any parameter and see what happens when sprockets or tyres are changed.

Thanks PomJohn .... if I have worked this out correctly your formula gives me 94.696 mph ..... with my bike configured as I mentioned above ..... drivetrain set up stock except the 21T counter shaft sprocket .... ?

Everyone who has read and answered my question has provided excellent info which I appreciate a lot , looks like Monday or Tuesday this wk. I will have a go with the phone speedo app. and see how close I am to the various methods of working the math and my rebuilt speedometer .... had not really asked 'bout tuning and going faster but still enjoy the reading/knowledge passed on , thanks to everyone !!!

Some police speed cameras might be calibrated to a reasonable level of accuracy.

We are not very advanced here in Nova Scotia , no cameras yet , but they have talked about them in legislature also no ethanol gas available pumps across Province have not been modified yet although they do have 10% ethanol stickers maybe on some....

Craig said:

I will have a go with the phone speedo app. and see how close I am to the various methods of working the math and my rebuilt speedometer ....!

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I'm riding Nova Scotia in 2 months, if you'd like I can bring my Stalker II RADAR gun. You provide the "closed course"...

Michael

That is really cool Capt. M. ... as time gets close , we should make contact once you near our paradise !!

Can’t believe I never posted up my results , re: May 19,2018 .... anyways it was beautiful Saturday morning and I had gone to usual Ducati Club breakfast on the Commando .... had my phone with speedo app and had mounted the phone holder to bars earlier ... no one seemed interested in a ride so struck out on my own .... ended up on fave stretch of road and opened her up , did not take long to achieve the Ton .... not really prepared for wind blast with open face helmet and sunglasses ... held it momentarily and then let off ... seemed like it had some more to give but had already broken my never over 5000rpm rule , so took the long way home .... between the bike and me , we are well over a hundred years .... prolly won’t be doing that again , still nice to know the old bike can still stand on her toes .... had not intended to do my big test that day just way things played out .....the speedo app supposed to be very accurate ....
Craig

And you still haven't posted your results ;-)

Yakatak said:

And you still haven't posted your results ;-)

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"...did not take long to achieve the Ton .... held it momentarily and then let off ... "

Sorry , phone app was showing exactly 101 mph when I let off ... and nope not quite sure on rpm as was not planning speed run that day , a little over 6000 .... like I said wind blast had my glasses askew and helmet was trying to lift off .. there was still some go left in bike just not me ...

Craig said:

..... .. there was still some go left in bike just not me ...

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sort of unrelated but remembered this as I read the above......when I was in my 20's I had the last year CB750F SOHC, put the "big bore 828 kit" in it, balanced and bluprinted, Kerker header, mikuni smoothbore carbs with K&N filters and a big ole' oil cooler out front - no fairing and a tank full of beautiful blue racing gas. Did an impromptu speed run on the way to the texas world speedway with a few friends. At 125mph I backed off, although the bike was chompin' at the bit with still more left in it.

in fact I had put in new champion plugs and was running 75/25 114 race gas and pump high test .... amazing how clean the plugs were ..... since then have scaled back to a 50/50 mix ....