Fuel Flow Rate for a Commando 850

I'm concerned about the fuel delivery rate on my mk3, and thus read with interest the Kevin Cameron article (TIOC June 2008 ) on tuning Mikuni carburetors referenced on a post here a few days ago. First off, I'm a big fan of Kevin Cameron, and have read his TDC column since the early seventies; I even bought his book. So I was really excited to find this column on calculating the required fuel flow rate for 4-stroke engines. Kevin Cameron has forgotten much more on the subject of motorcycle engines than I will ever know. However, this article was not his best effort. He appears to get lost in unit conversions (one gallon comprises 36 fluid ounces??) and winds up with a fuel delivery requirement of 45 oz/min at 60 HP, which would drain our 2.5 gallon tanks in 7 minutes (or 10 minutes, if you remove his 50% margin).

Starting with his basic assumption, likely correct, that a 4-stroke gasoline engine needs 0.5 pounds of fuel per horsepower per hour, and working the units through, I find that a 60 HP engine operating at WOT requires 11 oz/min of fuel flow. I confirmed this number using results from the sailing community and their engines.

I'm running a single Mikuni VM34 (rebuilt to Stan Smith/Rocky Point specs) on my rebuilt Mk3, and decided to run the Cameron test. Removing the bowl plug, I was discouraged to find flow rates of 5 oz/min on either the main or reserve sides of the tanks. Hmmmm. At the petcocks themselves (new BAP units) I get 21 oz/min each, more than enough to satisfy the engine at WOT. Dropping the bowl entirely gives a slight improvement to the initial test, giving 6 oz/min on the main side, and 7 oz/min on the reserve (the reserve has a larger hydraulic head pressure helping it).

I'm not surprised that the Mikuni float valve reduces the flow rate, even when opened, but 5 oz/min seems to me to be way low, and will result in an empty bowl in no time. I'm still breaking in the engine, and haven't run at WOT for very long, although the bikes runs and starts fine. One conclusion I can draw is that a single 34mm Mikuni is suitable for generating a sustained 35 HP or so, but not much more than that. Anyone else out there run this test?

I'm a whole lot less scientific about the process, but have OEM petcocks and FCR 35mm Keihin carbs. One petcock opened provides enough fuel to run both carbs WOT indefinitely (okay, until I hit reserve...). So I find it hard to believe the petcocks are truly a limiting factor.

Yes well it can be confusing because he is talking about the weight of the gas in oz not fluid oz.
A 100 HP engine uses about five ounces a minute. Adding 50% safety factor gives us a need for fuel delivery of 7.5 ounces (240cc) per minute, per 10 horsepower. So a 50 hp motor needs 1200CC's and that gives a 50 percent extra factor that's at peek power. Now that does make a little more sense. It’s using about 2.5 OZ of fuel by weight per minute and the petcocks need to over run that.

Thanks for your responses, guys.

However, I believe that I got it right. Cameron gives the base line at .5 lbs per hour per HP. Weight is converted to volume at 6 lbs per gallon, the rest is just simple volume and time conversions. The problem I see is NOT in the petcocks, which pass loads of fuel. It's in the internal float valve and related piping of the 34mm Mikuni. If you had TWO Mikunis, say, then everything would be in balance, with a flow rate of 5 oz per min for each carb, or 10 oz/min total, exactly what's needed at 60 HP.

And an 850 with a single 34mm makes 35 to 38 horse on my dyno. Jim

comnoz said:

And an 850 with a single 34mm makes 35 to 38 horse on my dyno. Jim

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How does that compare to stock 850 with dual carburators :?:

Jean

Most stock 850s run between 42 and 45. The best stock I have seen was 48. A bike with a single Mikuni makes about the same peak torque around 4700 so it feels about the same for most uses. Getting good horsepower readings requires a lot of RPM . Peak horsepower usually comes around 6200 rpm and by that time most single carb installations are well beyond their prime. The problem isn't with the single carb but it's flow problems caused by the commonly available single carb manifold. Jim

One of the reasons the multicylinder engines use a carb for each cylinder is for exactly the reasons noted above. If Honda (for example) could have used a single carb for their 4 cyl engines, and got the power they wanted, they would have done so from day one.

It's not that it can't be done engine-wise -car engines do it as normal - it's that it can't be done effectively on a bike due to the space that a large enough carb and manifolding would require.

comnoz said:

Most stock 850s run between 42 and 45. The best stock I have seen was 48. A bike with a single Mikuni makes about the same peak torque around 4700 so it feels about the same for most uses. Getting good horsepower readings requires a lot of RPM . Peak horsepower usually comes around 6200 rpm and by that time most single carb installations are well beyond their prime. The problem isn't with the single carb but it's flow problems caused by the commonly available single carb manifold. Jim

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Both results are still a long ways from published figures :lol: and shows that most users run their bikes well below their capabilities, so no need to worry about fuel flow rates for petcocks for street use. As for a carb per cylinder, I think marketting may have more to do with it than performance, just look at the old KZ1300 six, it had three carbs if I remember right, but with 300cc more than the CBX, it could have been enough to make it win the power war of the time.

The simplicity of a single carb may win out over duals in the end, not to mention that torque figures are probably very close and that is what the SOTPD (seat of the pants dyno) feels the most.

Jean

My 750 makes 49 1/2 HP on the dyno. Yes 6000 and above. To get within range so it's on tap you have to be doing about 5000 RPM. So enter no corner at under that RPM if you’re meaning it, needing it. Yes you will need mark two Amals or such, 4S cam or such, flowed head or such and pistons that fit up at .0025 total clearances if you’re going to have it live as a street bike for any length of time. The worst part is you can't get there just by buying the parts and putting them in. It took many years of playing with petcocks and everything else to get to the point that the bike would do a steady 118 MPH not lying down on the tank in fourth gear. 100 MPH was easy everything after that was earned. The bike is still fun down low, runs right with the stock 850's, it just out runs them as the RPM's go up. To ride it regularly and have it perform day in day out is the trick. Many long hours on a job that never ends you got to want it. Fuel flow is just one thing to get right than there’s spark at the plugs, jets, wiring, sprockets, weight reduction and on and on.

Thanks again, guys,

Life is sweet when it makes sense. Comnoz, your dyno data tells me that the Mikuni's fuel delivery and performance are in balance, and I don't need to worry about running the bowl dry under more spirited riding conditions. I'm strictly a recreational rider, and am quite happy with the performance of the Mikuni on the mk3. Above 50F it's a single kick (or button push) machine. Now to put the carb back together; our local vintage motorcycle club (VME) meets tomorrow, and, in the absence of rain, I plan to ride the Norton.

rick in seattle said:

Thanks again, guys,

Life is sweet when it makes sense. Comnoz, your dyno data tells me that the Mikuni's fuel delivery and performance are in balance, and I don't need to worry about running the bowl dry under more spirited riding conditions. I'm strictly a recreational rider, and am quite happy with the performance of the Mikuni on the mk3. Above 50F it's a single kick (or button push) machine. Now to put the carb back together; our local vintage motorcycle club (VME) meets tomorrow, and, in the absence of rain, I plan to ride the Norton.

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I have wrung a few Nortons' necks a time or two before, and I run a single 36mm Mikuni on my street bike, if that means anything.

I'm happy Jim posted realistic HP numbers. Sometimes I have to look away and plug my ears when I hear guys talking about the power their bikes have.

-Kenny C.
NYC

Holmeslice said:

rick in seattle said:

I'm happy Jim posted realistic HP numbers. Sometimes I have to look away and plug my ears when I hear guys talking about the power their bikes have.

-Kenny C.
NYC

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Click to expand...

Maybe they are just quoting the HP with AC and power steering disconnected. :mrgreen:

I too run 36mm Mikuni on 750 with Combat head and 1.5 exhaust. 300 Main

Instantaneous horsepower measured at the crankpin...

" our local vintage motorcycle club (VME) meets tomorrow'"

A vintage motorcycle club actually allows a Norton in there with a Mikuni Carb? Scandalous!

Next thing you know people will be installing Holley double pumpers on their old Ferraris!

More likely enriched fuel. Not in the gas tank either. Jim

Well it was a few years ago But here's the sheet from that day. Only as good as the dyno but it was rear wheel.

MexicoMike said:

" our local vintage motorcycle club (VME) meets tomorrow'"

A vintage motorcycle club actually allows a Norton in there with a Mikuni Carb? Scandalous!

Next thing you know people will be installing Holley double pumpers on their old Ferraris!

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VME is pretty laid back, besides, there is no such thing as a stock Norton.

And here is the best from Fuelie on a pretty conservative Superflow eddy currant dyno. Jim