Modern fuel and jetting...

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Fast Eddie

Fast Eddie

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I know a number of people have alluded to this before, and I’m of the same opinion, I’m pretty sure that modern fuel requires richer settings than it used to.

My T140 has brought this home to me, it definitely wants to be richer than I used to run ‘em years ago. needle in the middle and 200 mains is all I ever recall using on stock road bike motors, but I’m up to 220 on mine now and am about to test rating the needle one notch.

So, is it true that modern fuel requires richer settings? Or am I kidding myself ??
 
Not sure but formula varies from country to country and there are also seasonal variations too. Makes you want to fit an ECU.
 
Fast Eddie said:
I know a number of people have alluded to this before, and I’m of the same opinion, I’m pretty sure that modern fuel requires richer settings than it used to.

My T140 has brought this home to me, it definitely wants to be richer than I used to run ‘em years ago. needle in the middle and 200 mains is all I ever recall using on stock road bike motors, but I’m up to 220 on mine now and am about to test rating the needle one notch.

So, is it true that modern fuel requires richer settings? Or am I kidding myself ??
Click to expand...
True.
BTU’s per volume is less.
 
An old Norton service note says what octane rating the Commando was designed to use. In the UK standard unleaded is 91 octane and premium is still only 93 octane.
So power is bound to be reduced. It also indicates that the main jet should be changed with reduced octane.


 
SteveC said:
In the UK standard unleaded is 91 octane and premium is still only 93 octane.
Click to expand...

Standard UK octane is 95 (RON). Premium Super is 97- 98 (RON).
www.jct600.co.uk

Fuel Types Explained – UK 2020

Are you confused by all the different car fuel types there are to choose from? In this article, we explain all the main fuel types available in 2020, from petrol through to electric. Get all the information you need right here.
www.jct600.co.uk www.jct600.co.uk

bp Ultimate Technical Details | Products and services | Home

www.bp.com www.bp.com
"BP Ultimate Unleaded has a higher octane (97 RON minimum) than the minimum 95 RON required by law for petrol in the UK."

The quoted fuel octane numbers in the Norton Service Release would be RON.
 
Last edited:
EstuaryBoy said:
Metric petrol..... Never as good as the old imperial stuff :)
Click to expand...
Exactly, I can go a lot farther away on a gallon than on a litre.:)
My CB250 racer have same jetting on Shell V-power (99 octane, 5% ethanol) as it has on Aspen+ (98 octane 0% ethanol).
Found no difference in AFR readings nor in lap times.
 
I haven't changed jetting in the 45 years I've been riding my Norton. Leaded, unleaded, unleaded with ethanol... always seemed to work the same.
 
Fast Eddie said:
I know a number of people have alluded to this before, and I’m of the same opinion, I’m pretty sure that modern fuel requires richer settings than it used to.

My T140 has brought this home to me, it definitely wants to be richer than I used to run ‘em years ago. needle in the middle and 200 mains is all I ever recall using on stock road bike motors, but I’m up to 220 on mine now and am about to test rating the needle one notch.

So, is it true that modern fuel requires richer settings? Or am I kidding myself ??
Click to expand...

If you have some mikuni needle shims handy they can effectively double the needle position range, I've found this helpful for fine tuning between cylinders
 
Differences in octane rating of modern petrol should not affect the jetting. The thing which has the biggest effect is different latent heat of vaporisation. If the evaporating fuel causes the carb to be colder, the air in the incoming charge is denser and the mixture becomes leaner. Alcohol in the fuel can cause this to happen, but there would need to be a fair bit of it.
If you use leaded Avgas or Elf racing fuel , you would probably not need to change jetting between the two.
 
You don't usually jet lean enough that you get knocking. Best power is slightly richer than that. If the fuel has a very low octane rating, you might get near to detonation, but most modern petrols are above 90 octane. Elf racing fuel is RON 102, and is probably subject to better quality control. - Important if you are racing a two-stroke.
 
I'm sure this varies country to country, but I've certainly found all my Amal equipped bikes need richer settings than the factory specs. They also need choke to start cold nowadays, which they didn't need back in the 70s.
 
A few observations here. I used to get the odd bit of pinging with my standard motor but when I had the rebuild with a Webcam 312 (B?) That went away, never to return. In the mid 70s when I had my first 850, the bike used to run hot and would "tink" like crazy as it cooled down. None of that these days. All very civilised. I suspect that most Nortons are run quite rich these days. Since I've run my O2 sensor and AFR gauge, I've leaned it off quite a bit. Over 60mpg (Imperial) with a range of 160 miles to reserve. It runs sweetly everywhere now. The plugs are whjte with a nice tan colour at the base of the centre section, which, I believe it shoulb with modern fuels. I changed the last set of plugs at 15,000 miles, not because of problems, but because I thought I should.
 
It's not necessarily about octane. Modern vehicles are made to run hotter than anything made in the 70s for emissions control reasons. Modern fuel, contains with oxygen and/or ethanol in many places which causes leaner burning which can influence jetting on older vehicles.

My '72 is running on the edge after I rebuilt it, dark brown plugs are now a lighter tan colour, even in the cold climate I ride in - I may revisit jetting to suit.
 
concours said:
True.
BTU’s per volume is less.
Click to expand...

As Concours stated and put another way it's heat content. Straight ethanol has an AKI of about 110, R + M/2, but you need almost twice as much to equal the heat content of gasoline, 2,2,4,4, tetra methyl pentane. So E-10, local to men New England has a maximum octane (AKI) of 93, R + M/2; the blenders adjust the alcohol percentage based on average ambient temps.

So, yes, larger main jets may help you get back to the heat content you had when ethanol content was on the label of your favorite spirit as it's proof number.

When you change compression ratio, cam profile, port shape, yada, yada, increased ethanol content may be your friend. My son built up a 4.6 liter twin cam in his '03 Mustang Cobra which produced 720 RWHP while running on a chassis dyno using 93 octane E-10. The tuner told him that he could get about 100 more horses on E-85 by increasing boost and increasing the duty cycle of his monster fuel injectors supplied by twin fuel pumps. The point being that he could tap the potential of the E-85s AKI.

For a stock Norton engine you can safely increase the main jet sizes by 10 (220 becomes 230, 230 becomes 240, etc.) If you have changed engine tuning you can still benefit from increasing jet size, needle setting/profile/needle jets but the parameters of your mixers will surely require more trials.

I'd strongly suggest that you study Fullauto's posts involving the use of a wide band O2 sensor,or be prepared to spend gallons of money on dyno (eddy current) time which will give you a clear picture of your A/F ratio of a broad range of load. Eddy current dynos are fairly rare compared to the dynos that give you data at maximum RPM; good for drag racers not real useful for street or even road racing.

Best.
 
bsaboss said:
I'm sure this varies country to country, but I've certainly found all my Amal equipped bikes need richer settings than the factory specs. They also need choke to start cold nowadays, which they didn't need back in the 70s.
Click to expand...
That's because of global cooling
 
Fullauto said:
A few observations here. I used to get the odd bit of pinging with my standard motor but when I had the rebuild with a Webcam 312 (B?) That went away, never to return. In the mid 70s when I had my first 850, the bike used to run hot and would "tink" like crazy as it cooled down. None of that these days. All very civilised. I suspect that most Nortons are run quite rich these days. Since I've run my O2 sensor and AFR gauge, I've leaned it off quite a bit. Over 60mpg (Imperial) with a range of 160 miles to reserve. It runs sweetly everywhere now. The plugs are whjte with a nice tan colour at the base of the centre section, which, I believe it shoulb with modern fuels. I changed the last set of plugs at 15,000 miles, not because of problems, but because I thought I should.
Click to expand...

Ken, did you weld O2 adapters into the headers, or stick probes in the end of the peashooters ?
 
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