Fuel consumption

Previously guy_manning wrote:

I have a MK2a Commando with a single Mikuni VM34 Carb. I am getting 45mpg riding it like a nun, is this right? I also own a Yamaha XJR1300, 4 carbs, bags of power and i ride it like i stole it, i get 50mpg. I was thinking of changing to a single premium Amal, what do you think? Will it improve my fuel consumption?It should be better than this shouldnt it?

thats about right my 750 with a single 34mm VM mikuni dose about the same,but if i hold it at 85-90mph it drops to just under 35mpg ,i am suprised your xjr is that economical my fj12 would only do around 35mpg i once fitted it with a single CV mikuni on a home made manifold but it still never did more that 40mpg!!,if you really want the best mpg from your Norton you want an SU conversion some people claim over 70mpg with this setup,,,,,,,,,,,,,nick

Many years ago I replaced the worn Amals on a 750 Commando with a VM34 Mikuni, fuel consumption went from 30 mpg to arround 55. Starting, idleing & general tractability were also improved. I would have thought an 850 would give similar results. If you want most miles for least fuel, try to find a Pheonix SU conversion, if you ride it like a nun then, you'll nudge 100 to the gallon. I regularly average 80 mpg when touring fully loaded with luggage & 'getting on with it', so to speak.

Regards, Tim

Hi Guy,

I would have expected much better m.p.g considering a single carb and nun like riding. I get about 50 mpg from twin amal mk1's on my mk3, PW3 camshaft and engine regularly revved to 6,500rpm on twisty B roads. Carb bodies have been accurately bored and slides sleeved with brass to originally 0.001" clearance and still minimal wear after about 20,000 miles. By all accounts Premier Mk1 Amals are excellent,

Simon.

yes i am thinking about an SU myself now!,some people say they can be difficult to start and hard to fill the dashpot and you lose a bit of top end but if you got 80mpg surely its worth it, i once owned an 850 mk2a with a single 32mm mk 1 concentric this would do about 60mpg with a carefull throttle hand,

Previously nicholas_marshall wrote:

yes i am thinking about an SU myself now!,some people say they can be difficult to start and hard to fill the dashpot and you lose a bit of top end but if you got 80mpg surely its worth it, i once owned an 850 mk2a with a single 32mm mk 1 concentric this would do about 60mpg with a carefull throttle hand,

I don't think you would need to bother with an SU, I have twin Amal concentrics fitted to my MK3 Commando. I don't ride that slowly cruising around 70mph and with bursts up to 90mph and I get between 50-60mpg. I have a PW3 cam fitted which gives lively acceleration and it will get up to 60mph from standstill quick enough to beat most cars from the lights. I also have a BMW S1000RR which also averages around 50mpg ridden within the speed limit and about 30mpg on trackdays. My Norton 961 will give easily give 50mpg at legal speeds. Even my Triumph T160 will give up to 40mpg on a run and that is with 3 Amals.

Cheers, John

Previously john_mcnicoll wrote:

Previously nicholas_marshall wrote:

yes i am thinking about an SU myself now!,some people say they can be difficult to start and hard to fill the dashpot and you lose a bit of top end but if you got 80mpg surely its worth it, i once owned an 850 mk2a with a single 32mm mk 1 concentric this would do about 60mpg with a carefull throttle hand,

I don't think you would need to bother with an SU, I have twin Amal concentrics fitted to my MK3 Commando. I don't ride that slowly cruising around 70mph and with bursts up to 90mph and I get between 50-60mpg. I have a PW3 cam fitted which gives lively acceleration and it will get up to 60mph from standstill quick enough to beat most cars from the lights. I also have a BMW S1000RR which also averages around 50mpg ridden within the speed limit and about 30mpg on trackdays. My Norton 961 will give easily give 50mpg at legal speeds. Even my Triumph T160 will give up to 40mpg on a run and that is with 3 Amals.

Cheers, John

at the moment i am commuting 74 miles a day i tend to ride at about 90mph this is 5500 rpm on my present gearing i have a screen and panniers fitted i rarely get more than 40mpg ,maybe without the screen/panniers and slow down a bit it would improve dramatically?........nick

Ah Nick, I think it all becomes clear now. If you ride at 90mph with a screen then I don't think you should expect much better! Fuel consumption increeases somewhat exponentially with speed so the best thing you can do is drop to 80mph and maybe change the gearing on the bike. Mine does 80mph at about 4500rpm and returns more than 50 mpg with twin Mk1 Amals. I think your gearing is the issue not the carbs.

David

Previously nicholas_marshall wrote:

yes i am thinking about an SU myself now!,some people say they can be difficult to start and hard to fill the dashpot and you lose a bit of top end but if you got 80mpg surely its worth it, i once owned an 850 mk2a with a single 32mm mk 1 concentric this would do about 60mpg with a carefull throttle hand,

I have two 850 Commandos fitted with SU's, one Roadster, 22 tooth g/box sprocket, 80mph = approx 5000rpm, cruising with luggage & pillion etc, returns 75-80 mpg, the other an electric start Interstate with 24 tooth g/box sprocket, 80mph = approx 4000rpm, same scenario gives almost identical fuel consumption figures, maybe just a tad better, but much more relaxed. Both will start first kick provided there is no restriction in the fuel supply & both will do 1500-2000 miles before the dashpots need topping up provided I use 20/50 oil & not the thin stuff we all tended to use in SU dashpots, this can be done quite simply with a syringe & a length of washer tube. Performance is difficult to comment on as on the Roadster the SU replaced a pair of worn Concentrics 15 years ago, which although they ran ok, I'm sure they weren't giving their best, certainly they didn't give very good fuel consumption, the main reason behind the conversion, & the Interstate I built from scratch & hunted arround untill I found an SU kit, as I was so happy with that fitted to the Roadster, & have never tried it with anything else to compare, but both bikes have tons of bottom end grunt & excelent mid range, by which time I'm well past legal speeds & that's quite fast enough for me now!

Comparing a forty year old two valve pushrod twin with a modern lean burn four valve, twin ohc four cylinder is pointless IMHO. But if you are brave enough to consistently cruise at 90mph on a Commando with a screen and panniers you should be more concerned with when the whole lot will "let go" I don't see many bikes cruising anywhere at 90 let alone a Norton. It's not april 1st is it?

My Mk3 is on standard gearing and gave 45mpg on originaltwin Amals. I changed to a single 32mmMk 1 Amal and now get 68 mpg.

The only problem with single carb on a Commando is that you have to take the carb off the manifold to change things. My advice is try and set it up with advice from an expert before you fit. 260 main jet and 3 1/2 slide for starters. John Dunn was my expert here.

High speed cruising is possible but it does hammer the fuel consumption. At 90+ speeds, even without screen and panniers, 40 mpg is as good as you can hope for. But of course we should all be a bit more considerate to the old heaps now really. At least most of the time. At 37 miles each way, I would think that the bike isn't really getting properly warmed up before being stretched a bit. When I clocked up impressive mileages, I took it pretty easy for the first 10 miles, used abit more performance for the next 40 miles than used what was available.The bike seemed to like that regime - i.e. it didn't blow itself apart. That came later. Gordon.

Previously david_evans wrote:

But if you are brave enough to consistently cruise at 90mph on a Commando with a screen and panniers you should be more concerned with when the whole lot will "let go"...

Dave,

the engine revs at these road speeds are not what I'd call extreme. Fair enough they are long stroke engines so piston speeds are relatively high. Utilising parts made with modern materials and production methods and with an engine correctly put together there is no reason why riders shouldn't still thrash their bikes. That's what they were originally built for,

Simon.

P.S While some owners still insist on using beef dripping, or whatever was recommended when the engine was designed, engines will benefit from fully synthetic 20/50.

Before you go changing carbs I'd look at the other likely reasons for increased fuel consumption. For instance, experience tells me that a worn camshaft will sap the bike's power and make you ride everywhere with more throttle to compensate.

Henry

Beef dripping does it for me. When these bike were built I remember that fuel was 7s 6d a gallon but then my first weeks wages as an apprentice was £7 10s and that was for a 40 hours week. "You try telling kids that today and they don't believe you"

I would try a Stromberg CD 125 carb which I had 2 of on a 998 Sprite that once did 520 miles on 10 gallons. Very similar to the SU but with a rubber diaphram.Come to that any modern CV carb.

Previously david_evans wrote:

Comparing a forty year old two valve pushrod twin with a modern lean burn four valve, twin ohc four cylinder is pointless IMHO. But if you are brave enough to consistently cruise at 90mph on a Commando with a screen and panniers you should be more concerned with when the whole lot will "let go" I don't see many bikes cruising anywhere at 90 let alone a Norton. It's not april 1st is it?

i do run my norton hard but so far i have had no problems with it i dont think 5500 revs is that bad?,after all is said and done i built the bike from scratch and if anything goes wrong it will be my fault and if it blows up i'll build it better next time i would like a bit more mpg out of it but i cant junk the screen because i have a neck injury but i may try an SU just to see!,,,,,,,,,,,,,,,,,,,nick

John Hudson, former Norton Service Manager stated that the 750cc motor will last forever, as long as you keep it below 4,000 rpm. As they say roundhere, "Ye've been telt". Gordon.

Previously Gordon Johnston wrote:

John Hudson, former Norton Service Manager stated that the 750cc motor will last forever, as long as you keep it below 4,000 rpm. As they say roundhere, "Ye've been telt". Gordon.

I find it impossible to believe there is absolutely no friction between components at below 4,000 rpm and yet at 4,100rpm, for example, the engine starts to wear? This is engineering nonsense no matter who is claimed to have stated it.

I have to say the majority of the 30 odd thousand miles I've covered on my Roadster has been done cruising at arround 5000 rpm/80mph & other than the odd oil weep on the cylinder head (now cured by adding a breather to the inlet rocker cover) it doesn't seem to have suffered too many ills. I think it is about ready for a valve guide or two, but can't really complain in that mileage, other than usual maintenance, it hasn't been touched since I replaced a leaky head gasket before riding down to St PomPom in 2001.

Regards, Tim

Previously guy_manning wrote:

I have a MK2a Commando with a single Mikuni VM34 Carb. I am getting 45mpg riding it like a nun, is this right? I also own a Yamaha XJR1300, 4 carbs, bags of power and i ride it like i stole it, i get 50mpg. I was thinking of changing to a single premium Amal, what do you think? Will it improve my fuel consumption?It should be better than this shouldnt it?

I get about 60mpg on my 850 mk3 with a new set oftwincabsdoing a steady 70mph.Paul

Previously simon_ratcliff wrote:

Previously Gordon Johnston wrote:

John Hudson, former Norton Service Manager stated that the 750cc motor will last forever, as long as you keep it below 4,000 rpm. As they say roundhere, "Ye've been telt". Gordon.

I find it impossible to believe there is absolutely no friction between components at below 4,000 rpm and yet at 4,100rpm, for example, the engine starts to wear? This is engineering nonsense no matter who is claimed to have stated it.

It's not just friction which destroys engines. It is high levels of repetitive stress which result in strain - fatigue in fact. What Mr Hudson was trying to put across is that at higher rpm the rate of fatigue of components increases rapidly. Now we all know that different metals have different fatigue properties and the one that concerns us is aluminium alloy. There is no safe limit above which aluminium alloys begin to fatigue, unlike steels. The use of aluminium alloy for conrods for instance does have its downsides. So continuous use of high rpm uses up the fatigue life of components surprisingly quickly. John Hudson wasmaking a broad generalisation - but nonetheless a fair comment. Gordon.

Previously Gordon Johnston wrote:

It is high levels of repetitive stress which result in strain - fatigue in fact. What Mr Hudson was trying to put across is that at higher rpm the rate of fatigue of components increases rapidly. Now we all know that different metals have different fatigue properties and the one that concerns us is aluminium alloy. There is no safe limit above which aluminium alloys begin to fatigue, unlike steels. The use of aluminium alloy for conrods for instance does have its downsides. So continuous use of high rpm uses up the fatigue life of components surprisingly quickly. John Hudson wasmaking a broad generalisation - but nonetheless a fair comment. Gordon.

I think I'm correct in saying any material subjected to cyclic loading within the materials elastic limit is eventually subject to failure due to propagation of cracks. So unless a component is operating below it's point where it becomes elastic, then it will fail. So, no, it's not a fair comment.

Er... I thought a Commando was supposed to be safe up to 7,000 r.pm. I regularly ran my 750 up to 6,000. If you are going to potter along at 4,000, why buy a Commando in the first place?

You can ride your bike any way you wish. The longer you spend at high rpm, the faster you use up the fatigue life of components. At 4,000 rpm, the fatigue life of your engine is pretty much indefinite. At 10,000 rpm the fatigue life is measured in seconds. It's an interesting subject well worth discussion. A perfectly designed machine would work perfectly at maximum output until it reached its design lifeat which point every component would simultaneously turn to dust. On a strictly practical note, my son cruised his Atlas everywhere at 85mph with no problems for ages until a crack propogated in a piston (original Hepolite dished one) and the top detached. Would the piston have failed if he had kept the speed down abit? Discuss. Gordon.

Gordon,

It depends, as I mentioned previously, if the component is being subjected to forces loading the component within it's elastic limit. The cyclic stretching and shrinking eventually creates a crack to form at the components weakest point, be it a design or material flaw, which leads to failure. e.g your sons piston. A component subjected to forces which exceeds the materials elastic limit and into the plastic limit will fail immediately. I agree the failure is accelerated the higher the cyclic frequency i.e rpm, but I would expect that a 750 con-rod at 4,000rpm is subjected to bending and strain within the materials elastic limit and therefore will fail, albeit later than an engine revved at 7,000rpm, but will not 'last for ever',

Simon.

Aha Simon, I see you do not care for sweeping generalisations. The point being made by John is that the number of cycles required to produce a failure at 4,000 rpm is many orders of magnitude of the cycles required to produce a failure at 7,000 rpm, with which I think we are both in complete agreement. The problem with aluminium components is that they do not have a clearly defined elastic limit, unlike steel ones. Aircraft mainspars are invariably given a fatigue life because of this. Despite making the case for fairly gentle treatment of Norton engines, I haven't revealed just how I treat mine. You might be surprised.

Gordon.

Young's modulus (E) allows calculations to predict deformation of a material under load. So I disagree with, '....do not have clearly defined elastic limit'. I think every load bearing component on an aircraft is given a fatigue life.

Anyway, I hope, and suspect, you thrash your bikes, Gordon. People plodding around at 60mph wearing dayglo will only give other Norton riders a bad name,

Simon.

Previously simon_ratcliff wrote:

People plodding around at 60mph wearing dayglo will only give other Norton riders a bad name,

Simon.

Well, you wonât catch me wearing âdaygloâ, but when Iâm out on my model 19 outfit, Iâd love to be able to plod along at 60mph

Regards, Tim

Generally..............most Atlas owners will tell you that it wasn't the engine that was the problem when riding long distances at 80mph plus. My experience of such rides was being unable to walk,talk or write for quite some time until the numbness had subsided. Blurred vision and chipped front teeth added to the physical stress. Once a reasonable level of normality had been resumed a visual inspection of the bike was made in case the usual suspects......battery box lid, horn, rear light etc had done a runner.

The smartest move for my Atlas was dumping the twin carbs for a bigger single Concentric. My average fuel consumption then dropped from 45mpg to a regular 60mpg. My present 750 Commando has an SU which gives an easy 80mpg if I cruise at 50mph. A steady 70 mph gives 65mpg. Starting and driving off immediately is no problem compared to someMikuni shod Commando bikes in my area. The dashpot top-up is a pain to do and quickest if the tank is taken off first.