Need fuel tank measurment

jseng1 · Nov 29, 2012

Someone please give me the distance from the top of the Commando fuel tank (not including cap) to the top of the carb.

thanks

Jim

Hobot · Nov 29, 2012

I've got Trixie's Roadster tank off with Amals still on so can measure a bit later. What part of top of carb? Cap installed or bore edge? Which part of tank, the highest near cap? Aslo have loose empty IS tank handy if that's of interest.

A briefing on procedure to measure this would be helpful. Maybe my S.O. will hold a ruler/level on carb or tank to ease the accuracy.

jseng1 · Nov 29, 2012

tank up to but not including cap

carb at cap

BillT · Nov 29, 2012

With my '73 on its center stand, lip of filler neck to top of bore is 9". From lip of filler to top of fuel bowl is 12"

Rohan · Nov 29, 2012

Presumeably this measurement is going to be different for Roadster, Interstate, Fastback, Long Range and for ProddyRacer tanks ???

jseng1 · Nov 29, 2012

BillT said:
With my '73 on its center stand, ... From lip of filler to top of fuel bowl is 12"

thanks

Lip to fuel bowl is what I'll use - its close enough - I'm doing fuel flow through carb bowl measurements per minute. I'm seeing about 8oz per minute per carb on the 32 & 34mm JSmotorsport flatslide carbs when the floats are adjusted correctly (with full tank).

lcrken · Nov 29, 2012

Interesting. In theory, 16 fluid ounces/min total flow (two carbs) would suppport an output at the crank of around 90 HP, assuming a BSFC of .5 lbs/hp/hr. That should be good enough for most of our old Nortons.

Ken

Rohan · Nov 29, 2012

And, assuming an average speed of 120 mph, give about 20 mpg, give or take any decimal points anywhere.

What do Nortons give at race speeds ?

Hobot · Nov 29, 2012

I got called away till midnight so glad others got Raodster fuel head. A good tuned 750 gets mid 30 mpg at 120 mph sustained. Seems only maxed out 920 race engines reach into 90-100 rw hp levels. The gravity head of about a foot tall is something like .5 lb

Bernhard · Nov 30, 2012

Rohan said:
And, assuming an average speed of 120 mph, give about 20 mpg, give or take any decimal points anywhere.

What do Nortons give at race speeds ?

All this is of no significant if you fit too small a bore petrol pipes on the tank/carb starving the engine of fuel, don’t ask how the John Player Norton team found out……….

lcrken · Nov 30, 2012

Rohan said:
And, assuming an average speed of 120 mph, give about 20 mpg, give or take any decimal points anywhere.

What do Nortons give at race speeds ?

I checked race track mileage several times back in the day on my 750 PR, and got values anywhere from 20 to 25 mpg, depending on the track and other variables.

Ken

Rohan · Nov 30, 2012

Bernhard said:
Rohan said:

And, assuming an average speed of 120 mph, give about 20 mpg, give or take any decimal points anywhere.

What do Nortons give at race speeds ?

Click to expand...

All this is of no significant if you fit too small a bore petrol pipes on the tank/carb starving the engine of fuel, don’t ask how the John Player Norton team found out……….

The point being, Bernhard, that if race bikes could average 10 or 15 mpg on a track anywhere, and the petrol pipes can only flow a max of 20 mpg (theoretical), then the petrol pipes/taps are acting as the main jets. Have heard of this before, on a number of ocassions. Particularly where alcohol (methanol) was involved, which needs (much) bigger flows than with petrol.

Thanks for the numbers Ken, puts real values to it.
Don't often see these quoted, at all.

Rohan · Dec 1, 2012

P.S. Fuel flow potential with the tank near empty may be a more valid and more important value ??
And the engine will be good and warmed up by then too....

BillT · Dec 1, 2012

Back when AJS ran their Porcupine, the fuel tank hung down like a pair of saddlebags, almost obscuring the engine. The thing carried something like 8 gallons, and the rider would pump fuel from from the sides into an internal reservoir to get the fuel up above the carbs. I think the reservoir held about 1-1/2 gallons

marinatlas · Dec 1, 2012

Hi Jim, while we are here with measurment what is the height of your flat side carb , say from the stud hole line ?? are they higher than amals, just cause I had some clearance problem with the bottom of my Seeley tank............

jseng1 · Dec 1, 2012

marinatlas said:
Hi Jim, while we are here with measurment what is the height of your flat side carb , say from the stud hole line ?? are they higher than amals, just cause I had some clearance problem with the bottom of my Seeley tank............

From the center of the bore to the top of the cap is about 63mm and to the top of the cable adjuster is about 77mm.

Need fuel tank measurment

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