Why is my Commando Reving so high relative to road speed ?

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Hi I have finally got my 750 MkV commando (Interstate back on the road) however I am struggling to understand why the bike is reving so high ?. I have an 18inch rear wheel fitted but the Diameter and circumference is almost identical to the 19 inch, I have been told to return to a 19 Inch but can that really make such a difference ?.

At 70 mph it is doing 5100 revs , I have a standard Engine and Clutch sprockets , the bike is on Chain drive no Belt conversion . It had a 21 tooth Gearbox sprocket which I changed for 22 this did little to improve the situation just made pulling away more of an art so I will return it to 21 .

I cannot feel any clutch slip at all and I have had the bike up to 80-90 mph .

My speedo is very erratic above 50 mph however I have followed my wife on her Bonneville at 70 mph and my bike also indicates 70 mph, So I am lost , I have one more weekend trip planned before I send the rear wheel off for rebuilding has anyone got any ideas what may be wrong ?

I last seriously rode the bike in 1982 but I do remember 70 mph being closer to 4000 revs and a bike that would easily rev to 7000 , though I spent most of my time below that.

My 850 always showed 4,000…

My 850 always showed 4,000 rpm at 70mph. I can't remember what sprockets but think it was 22 - but 22 to 21 wouldn't make that much difference. 19" rear wheel, again wouldn't account for your figures. Roughly you'd see 22/21 * 19/18 ie around 10% difference from the combination.

What makes you think the rev counter is correct, having checked the speedo against another bike as you have?

Sounds to me like you're…

Sounds to me like you're in third gear?

Hi John Have you considere…

Hi John

Have you considered getting you rev counter checked, after a 30 year layup it maybe a tad flakey

Regards

Katherine Scott

Previously Bob Matthews wr…

Previously Bob Matthews wrote:

Sounds to me like you're in third gear?

no def 4th gear

Previously Katherine Scott…

Previously Katherine Scott wrote:

Hi John

Have you considered getting you rev counter checked, after a 30 year layup it maybe a tad flakey

Regards

Katherine Scott

Katherine

thanks I am looking at that will need to be later when off the road in the Scotland Winter

Previously ian_soady wrote…

Previously ian_soady wrote:

My 850 always showed 4,000 rpm at 70mph. I can't remember what sprockets but think it was 22 - but 22 to 21 wouldn't make that much difference. 19" rear wheel, again wouldn't account for your figures. Roughly you'd see 22/21 * 19/18 ie around 10% difference from the combination.

What makes you think the rev counter is correct, having checked the speedo against another bike as you have?

My wifes Bonneville is the new model with electronic Speedo and she will cruise at a steady 70 mph , I have been through a number of Speedos when I rode the bike almost daily from 1977 to 1982 ish so I question the Tacho more as it is the Original from when I purchased the bike in 1977 , just wondered if any other had encountered anything similar

I have an excel spreadshee…

I have an excel spreadsheet that works out theoretical road speed from wheel diameter and various rear ratios i can email you the sheet for info if you want.

Sounds very much like a du…

Sounds very much like a duff rev counter. Have had to replace rev counters a few times over the years on a couple of Commandos when they became unreliable or packed up altogether. The Smiths unit was always more reliable than Veglias.

For reference the Mk 3 850 I used to have in Kenya was originally on a 20 tooth g/box sprocket (for police / presidential escortduty) and showed 70 mph at 5,000 rpm in top. I changed the g/box sprocket for a 22 tooth which then showed 4,000 rpm at 70 mph.

Previously Alan Blackhurst…

Previously Alan Blackhurst wrote:

I have an excel spreadsheet that works out theoretical road speed from wheel diameter and various rear ratios i can email you the sheet for info if you want.

Alan thanks but I have that Excel sheet

Rgds John

Previously james_fanning w…

Previously james_fanning wrote:

Sounds very much like a duff rev counter. Have had to replace rev counters a few times over the years on a couple of Commandos when they became unreliable or packed up altogether. The Smiths unit was always more reliable than Veglias.

For reference the Mk 3 850 I used to have in Kenya was originally on a 20 tooth g/box sprocket (for police / presidential escortduty) and showed 70 mph at 5,000 rpm in top. I changed the g/box sprocket for a 22 tooth which then showed 4,000 rpm at 70 mph.

James

Thank you as I said I will sent the Tacho off for checking when the Scottish Winter arrives, Mick Hemmings has given me a name and I found their web site, I do have a Smiths Tach fitted.

Just to share for referenc…

Just to share for reference, my MK2A that I've owned 20 years has Veglia clocks, both of which continue to function as they did when I bought the bike â maybe I'm lucky! â and a 22-tooth gearbox sprocket: the tacho has always shown a gnat's whisker over 4,000rpm at 80mph, with standard size rims and Dunlop K81s/TT100s.

My 750 Roadster with 19in…

My 750 Roadster with 19in rims and TT 100 tyres shows 17 mph per 1000 rpm as specified in workshop manual. 3000 rpm means 50 mph.(allowing for slight needle fluctuation.)

Ditto (as per Ian's mail)…

Ditto (as per Ian's mail) on the A1 on Sunday 50mph was 3k rpm (checked against my Garmin Zumo) - 42 year old Veglia speedo reading a tad low following removal & reconnection of speedo cable (previously exactly the same as satnav); 70 mph was 4k rpm

My rev counter is electronic (Smiths) with 19" wheels &Avon 100/90 RoadRiders front & rear)

Mike

Mk III ES 850 Commando (with standard gearing)

Norton Owners Club