Commando 850 petrol tank capacity and mileage

Over the years I have seen a few reports on gas tank size and mileage but last week I measured mine and found as follows.

The bike is a 1974 Mk2 850 Roadster built August 1974. Definitely an original tank because I have owned or known it almost 50 years. Bike has a pair of near new 32mm Amal Mk1 Premier Concentrics. I spent a lot of time setting them up and they now seem to be in good tune. Setting up consisted of very careful cleaning of swarf from passages, setting fuel bowl height, checking all jet sizing and needles. Currently using 100 % standard Mk11 850 settings except raising the needle on the left carb one notch due to a very lean plug appearance on that side. Motor has about 1500 km after a rebuild with new pistons valves etc. Head is RH10. Cam is standard. Using NZ 95 RON pump gas. No ethanol. 21 teeth gearbox sprocket. Air cleaner is a standard hamcan but reduced in diameter to fit above a CNW starter.

I drained the Roadster steel tank to empty both sides. Both taps are reserve taps so the only gas left in it was less than 10 mm below the tap base.

Volume to refuel the drained tank was measured with a calibrated cylinder at 12.4 litres.

That contrasts with 11 litres recorded in both the Norton Commando workshop manual 1970 on and the Clymer manual.

I used litres sorry because I can't be having with this US versus Imperial gallon ancient history malarkey.

I then ran the bike 211 km total and refulled. It used 10.2 litres. The mileage was pretty mixed NZ legal and certainly not pushing it. It included two runs over the Rumatuka hill in 2nd and third gear but also around 40 km of 80 kph restricted road. Rest was 100 kph and some town running.

So that comes out to 20.7 km per litre or 4.8 km / 100 km.

Potential range for a full Roadster tank of around 256 km to bone dry. (160 miles)

To be kind and against my better judgement I also worked this out to be 49 miles per US gallon

Or 59 miles per UK gallon.

This range and mileage is a bit better than I expected but probably due to new carbs, correctly set up and pretty quiet legal NZ riding.

In contrast back in about 1979 with some very very very worn Amals I ran out of gas after 140 km riding up the Parapara road from Wanganui. Possibly going a little faster than 100 kph.

It only matters if you don't have enough.

concours said:

It only matters if you don't have enough.

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I'm a facts guy.

Prefer to plan ahead rather than fix after.

I would be interested in any mileage noes others have measured.

Strictly ballpark measuring but I would say routine riding gives 60 miles per imperial gallon. Or 50 USA miles per gallon. Non scientific but with the huge Interstate tank I haven't pushed it home yet.

I don't really care so I fill it every 100 miles and don't bother to calculate MPG. I've noticed it always takes over two gallons.

johnm said:

Currently using 100 % standard Mk11 850 settings except dropping the needle on the left carb one notch due to a very lean plug appearance on that side.

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If you drop the needle, the carb will get more lean. Could you possibly have meant that you dropped the "clip" one notch on the needle??? That would raise the needle and make the midrange richer... right?

I would bet someone living in a flat terrain, rural area would get the best gas mileage, and someone living in the mountains or stop and go suburbia would get the worst mileage. I agree that it's a good thing to know your bike's fuel consumption rate. The only time my bike ever left me stranded on the side of the road was because I ran it out of fuel I eventually went to an interstate tank because I fill up with non-ethanol fuel only and the big tank makes planning refills less of an issue and so computing the mileage is less of an issue too.

johnm said:

I'm a facts guy.

Prefer to plan ahead rather than fix after.

I would be interested in any mileage noes others have measured.

Click to expand...

The only mileage that matters is YOURS.

Parade riders get 55mpusg

Spirited riders get 35 mpusg

Only YOU know how you ride it.

I have drained the tank in less than 100 miles while showing the tail light to my Hardley Ableson buddies.

Being a facts guy, acknowledge there is a very wide window of fuel consumption data points based on variables.

And then there are headwinds. The mileage on my Vincent might be around 55 imperial most of the time but with a passenger and luggage on it drops to high 40s.
Add in a strong headwind fuel mileage drops down to low 30s. Big climbs have a similar effect, maybe not quite as much as a really strong headwind, probably low 40s when loaded.
Once in awhile you get big hills and headwinds. That really gobbles the fuel.
So even though I'm a numbers guy, there's no set number for fuel economy on a trip, even when it's always the same bike and rider.
I have learned to fill up early and often!

On a long trip with the MK3 850 I averaged just under 60 mpg imperial. That was down the coast, not many hills, solo but with luggage and relatively slow as the highways there are mostly are 60 mph speed limit.

Glen

Difference btwn 1970 book capacity and the measured capacity of a 74 roadster tank might be down to the fibreglass/ metal tank different shapes/volumes.

When I rode from Ohio out west to the Oregon rally on the MKIII Interstate, I was getting between 50-55 mpg. Riding between 65-70mph, one up, Craven luggage, Mikuni carb, 22 tooth sprocket. Ran 300 miles on a tank one day without going to reserve. I don't remember what the real capacity of my tank is exactly, but when I ran out of gas once and pushed it to a station, it took well over 6 US gallons.

With my SU carbed Mk3 with the small interstate tank my usual mpg (imp) is between 65 to 70 and regularly achieve 280 -300 miles before getting near reserve. This is not riding like a granny, more like a hooligan. Riding like a granny i can easily achieve over 80 mpg, and once in the swiss alps i calculated i was hitting 90 mpg! I was riding like a granny then as the scenery was stunning.
A question, Has anyone tried a modern CV carb? i know the SU works great and is so easy to live with but it is a lump. A newer CV carb would surely look better and fit better under the frame

o0norton0o said:

If you drop the needle, the carb will get more lean. Could you possibly have meant that you dropped the "clip" one notch on the needle??? That would raise the needle and make the midrange richer... right?

I would bet someone living in a flat terrain, rural area would get the best gas mileage, and someone living in the mountains or stop and go suburbia would get the worst mileage. I agree that it's a good thing to know your bike's fuel consumption rate. The only time my bike ever left me stranded on the side of the road was because I ran it out of fuel I eventually went to an interstate tank because I fill up with non-ethanol fuel only and the big tank makes planning refills less of an issue and so computing the mileage is less of an issue too.

Click to expand...

Of course. Your exactly correct. I used a lower notch which raised the needle. Getting old.

Thanks for the correction.

concours said:

The only mileage that matters is YOURS.

Parade riders get 55mpusg

Spirited riders get 35 mpusg

Only YOU know how you ride it.

I have drained the tank in less than 100 miles while showing the tail light to my Hardley Ableson buddies.

Being a facts guy, acknowledge there is a very wide window of fuel consumption data points based on variables.

Click to expand...

Very true.

But that's exactly why I specified the riding style and speeds I had used. To give a calibration point and somewhat constraining the variables.

And then said I only got 140 km from a tank when young riding hard with worn carbs.

I thought the mileage was pretty good considering it included twice crossing the Rumatuka hill.

dobba99 said:

With my SU carbed Mk3 with the small interstate tank my usual mpg (imp) is between 65 to 70 and regularly achieve 280 -300 miles before getting near reserve. This is not riding like a granny, more like a hooligan. Riding like a granny i can easily achieve over 80 mpg, and once in the swiss alps i calculated i was hitting 90 mpg! I was riding like a granny then as the scenery was stunning.
A question, Has anyone tried a modern CV carb? i know the SU works great and is so easy to live with but it is a lump. A newer CV carb would surely look better and fit better under the frame

Click to expand...

Yep "bikes by Brian " on YouTube uses CVs on some of his commando's
He was in here recently
He modifies the taper on the needle

I generally get 11 miles per litre with my single carb set up
But it really does vary depending how I ride

baz said:

I generally get 11 miles per litre with my single carb set up
But it really does vary depending how I ride

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17.6 km per litre.

Around 15 % less than my test. Thanks . That's interesting compared to the twin Amals. Does that include UK motorway riding at 70 mph or so?

milfordite said:

When I rode from Ohio out west to the Oregon rally on the MKIII Interstate, I was getting between 50-55 mpg. Riding between 65-70mph, one up, Craven luggage, Mikuni carb, 22 tooth sprocket. Ran 300 miles on a tank one day without going to reserve. I don't remember what the real capacity of my tank is exactly, but when I ran out of gas once and pushed it to a station, it took well over 6 US gallons.

Click to expand...

That's a single Mikuni ?

Very good mileage for that kind of usage I think. 22 sprocket may help I imagine.

johnm said:

17.6 km per litre.

Around 15 % less than my test. Thanks . That's interesting compared to the twin Amals. Does that include UK motorway riding at 70 mph or so?

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I use motorways a fair bit but I'm always north of 70mph
Interestingly when I had twin amals I'd get roughly the same mpg
I have a TM 40 pumper fitted at the moment
I'll have to try one day riding slowly and not accelerating hard just to see what it will do
Many years ago I was an impoverished apprentice my only transport was a mk2a
This was back in the late 70s ,I fitted it with a Norton atlas manifold that I cut and welded for the commando
I fitted one of the amals to it with a pancake filter that I indented the top to clear the frame gusset and that was about it
I don't remember re jetting it or anything
I was immediately getting 65 mpg
The bike was a little over geared and it lost some pep it had a dunstal 2-1-2 fitted at that time

baz said:

I use motorways a fair bit but I'm always north of 70mph
Interestingly when I had twin amals I'd get roughly the same mpg
I have a TM 40 pumper fitted at the moment
I'll have to try one day riding slowly and not accelerating hard just to see what it will do
Many years ago I was an impoverished apprentice my only transport was a mk2a
This was back in the late 70s ,I fitted it with a Norton atlas manifold that I cut and welded for the commando
I fitted one of the amals to it with a pancake filter that I indented the top to clear the frame gusset and that was about it
I don't remember re jetting it or anything
I was immediately getting 65 mpg
The bike was a little over geared and it lost some pep it had a dunstal 2-1-2 fitted at that time

Click to expand...

Back when I was a student money dictated I stayed with worn out original Amals. The slides almost went down sideways so I jetted rich as

That's when consumption was 30 mpg. Plugs were very very black.

I had absolutely no idea what I was doing but lived on about 15 quid a week so new parts were off the table.

Unfortunately North of 70 mph is licence losing in NZ. Our limit is 100 kph or very rarely 110 on a motorway. So mostly just over 60 mph.

I think at 128 kph ( 80 mph) your into automatic loss of licence for one month. Very heavily policed.

This MKIV has the stock cam with 32mm amal premiers tuned in. It gets a shade under 60 miles per imperial gallon. That is mostly cruising 100 or 120 km/ hrs with small towns scattered here and there with speed limits. And it is an average, so wind etc. is factored in. It is the most economical vehicle, other than a bicycle, that I have ever had on the road. Couldn't be more happy with the operating cost