I put the following question to the Microsoft Copilot :
There is a thing which puzzles me, and which you might be able to rationalise. In my 850cc Norton Commando engine, the compression ratio is about 10 to 1. Because I use methanol fuel, I have advanced the ignition timing about 4 degrees ahead of the normal 29 degrees which is used for petrol, To optimise the mid-throttle jetting, I have made my own needle jets at a size which brings the 5 settings of the leanest Mikuni needles into adjustment. It means the throttle cannot just be whacked open, it must be fed on in a controlled manner. The effect is that my motorcycle accelerates much faster then many others. I have been watching Youtube videos about German fighter air planes, and those of the Allies. It seems that for cruising, they used lean mixture, and for a dogfight they used rich mixture. My work with motorcycles seems to indicate that the opposite might have been better. Petrol and methanol as fuel are jetted similarly, but because of the high latent heat of methanol - the jets flow twice as much as with petrol, However petrol has 30% higher calorific value.
Robot says---
1. Where power really lives: mixture vs stoichiometric
For any fuel there are three key mixture zones:
• Stoichiometric (chemically correct):
• Petrol: about air:fuel by mass
• Methanol: about
• Best power:
• Petrol: typically around (rich of stoich)
• Methanol: roughly (also rich of stoich)
• Best economy:
• Petrol: lean of stoich, say or more
• Methanol: similarly lean relative to its stoich
So: maximum power is not at stoichiometric, it’s slightly rich. Leaner than best-power gives better economy but less torque; richer than best-power just wastes fuel and cools things.
2. Why your “lean” midrange feels so strong
What you’ve done with the Mikuni needle/needle-jet combo is not “lean for the sake of it”, it’s correcting an over-rich midrange that most people live with.
• Stock or safe race jetting is usually:
• Rich in the midrange and on transient to avoid detonation and flat spots.
• That gives softness when you crack the throttle—fuel excess, slow burn, soggy response.
• Your custom needle jets:
• Bring the needle into a range where all 5 clip positions are actually usable.
• Give a clean, near-best-power mixture in the mid-throttle band instead of the usual “fat” safety margin.
• Result: when you feed the throttle in, the engine is right on its torque peak mixture instead of wading through a rich bog.
That’s why you can’t just whack it open: you’ve removed the “liquid cushion” of excess fuel. Now the airflow and fuel flow have to stay in step—if you snap it open, air gets there first, mixture goes too lean for a moment, and it stumbles. You’re doing with your wrist what an accelerator pump does in a car carb.
So your bike feels brutally quick not because it’s lean everywhere, but because it’s closer to best-power mixture where others are simply too rich.
3. Methanol vs petrol: why the big jets still make sense
You’ve got the essentials right:
• Methanol:
• Lower calorific value per kg than petrol (about 30–40% less).
• Much higher latent heat of vaporisation → huge charge cooling.
• Needs roughly twice the fuel flow for the same air mass.
• What that does in your Commando:
• The cooling effect increases charge density—more air in the same volume.
• High compression (~10:1) plus cool charge plus methanol’s detonation resistance lets you run:
• More ignition advance.
• More cylinder pressure.
• Net result: even though each kg of methanol has less energy, you can burn more of it safely, and with denser air, so brake mean effective pressure goes up.
So the big jets aren’t “waste”; they’re the ticket to more trapped mass and more safe pressure.
4. Aircraft practice: why they went rich for combat
The WWII aircraft story is about survival and reliability under brutal conditions, not just peak dyno power.
In combat they enriched the mixture because:
• Detonation margin:
Rich mixtures burn cooler and slower, reducing peak combustion temperature and pressure spikes. That’s life or death when you’re at high boost, high cylinder head temps, and can’t hear knock over the noise.
• Cooling:
Extra fuel acts as an internal coolant—especially with petrol. Rich of best-power is a crude but effective way to keep exhaust valves, pistons, and heads alive when you’re at full throttle for minutes, not seconds.
• Simplicity and safety:
Mixture controls were coarse. Pilots were trained to go “full rich” for takeoff and combat because it’s idiot-proof: you sacrifice a bit of theoretical power for a big gain in detonation safety and engine life.
For cruise, they went lean because:
• You’re at lower power settings.
• You want range and economy.
• Cylinder temps are manageable, so you can afford lean-of-peak or near-stoich mixtures.
So the pattern is:
• Cruise: lean for economy.
• Combat: rich for cooling and detonation margin, not necessarily for maximum indicated horsepower.
5. Why your experience seems “opposite” but isn’t
You’re comparing:
• Your Norton:
• Short bursts of acceleration.
• Carefully tuned, high-octane methanol, high compression, good cooling.
• You’re free to run near best-power mixture in the midrange and WOT, with just enough richness for safety.
• WWII aircraft:
• Long periods at high power.
• Petrol, high boost, marginal cooling, primitive instrumentation.
• They deliberately ran richer than best-power in combat to keep the engines alive.
So:
• You’re right that over-rich mixtures blunt acceleration—you’ve proved it with your needle work.
• They were right that extra richness in combat was worth the power loss because it bought them detonation margin and cooling.
Those two truths sit on the same curve: you’re operating near the peak of the power hump; they were sliding down the rich side of it for survival.
