Fuel Flow Rate for a Commando 850

Hey Mick,

I agree with your logic, my 850 is pretty standard, sans JS pistons & rods. I am running a 21 tooth sprocket, the bike had a 20 fitted originally and I find the 21 pretty tall! Granted I mostly carry a pillion, but even on solo days my gearing feels more than enough.
I have seen the cycle world reprints, the Trident was a stand out in that test, the Z1 Kwaka brochures listed 82HP but the dyno listed 58HP

Cheers Richard

I remember the test in that issue well. It was the last year you
could really take a brit bike seriously. The Trident had close ratio
gears and was VERY carefully tuned. It was curtains for the brits
after this test. I had a Trident and loved it but it was a constant
headache compared to the jap fours. You could trash them or
tour with them and they didnt need constant attention.
The only thing the Norton had was the isolastic suspension but
that was it compared to the other bikes.

rick in seattle said:

I'm not surprised that the Mikuni float valve reduces the flow rate... and will result in an empty bowl in no time...A single Mikuni is suitable for generating a sustained 35 HP or so, but not much more than that. Anyone else out there run this test?

Click to expand...

I made the test and yes it will starve for fuel at WOT. I don't see a solution other than short bursts of WOT. You make the test by shutting off the petcock until it begins to starve - then open the throttle & go WOT to see if the fuel can catch up at high RPM (it does not). There are twice as many intakes when using a single carb and the float bowl is not designed for the fuel required at high RPM.

Jim

I just tried this calculator plugging a very bad realistic Combat 1/4 mile ET as 12.5 sec and bike/pilot mass of 550 lbs.
"Your HP is 55.66 computed from your vehicle weight of 550 pounds and ET of 12.5 seconds. " My Combat may do 13 if I dared to rev it that hard. http://www.wallaceracing.com/et-hp-mph.php

Im a long way from my spread sheets and calculation files to independantly check but Im sure if you followed Kevin Camerons calculations your conclusions will be correct.

I have read and personnally confirmed that fuel flow can be insufficient for continuous running at peak rpm in many, probably most, standard fuel delivery setups. My friends and I have worked through this on our race bikes. On a 500 ccc single cylinder using methanol the flow rate needs to be 2.3 times the volume of petrol. This can be very diificult to acheive.

With Amal concentric carbs it requires the use of large industrial sized taps, and lines - I dont have them in front of me but I would estimate a good 5 mm ID. Any junction pieces need to be the same and then the carb bowl and float needle needs to be for alchohol with larger internal diameters.

I also had to go through this exercise to work out whether my gas tank was large enough to run 15 laps racing. You can use a hell of a lot of methanol in 20 minutes racing.

Even for petrol for racing we dremel a slot in the side of the tunnel that holds the float needle and check the delivery rate - with the tank caps closed.

But for the street I could hardly imagine this will be an issue unless you are on derestricted German autobans !!

On power measuremnt on intertia dynos. Yes not as good as a brake dyno but still very helpfull if you dont have a choice.

You can turn the mph data into rpm on a spreadsheet if you work out the gearing. Or on some dynos if you do a run on say 50 % throttle and read an rpm and mph data pair the operator can then reset the x axis to rpm on the computer.

I agree that anything near 50 bhp is very good for a street Commando. There is a shop in Upper Hutt that has many dyno runs pinned on the wall. The only Commandos over 50 bhp are race tuned.

The old original published data will be crankshaft. Plus they will be bikes in very good condition and properly tuned.

Proper tuning makes a huge difference. I have gained 7 bhp on an inertial dyno by working with cam timing, ignition timing and carb set up.

The guy that got that 12. something time in California was Norman White. Peter Williams mechanic, a Norton race shop tester and still in business today.
http://www.normanwhite.co.uk/profile.htm