1/4 inch fuel line?

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Apr 15, 2004
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I'd like to redo my fuel lines to eliminate the crossover. With that goal in mind, I procured a pair of double-outlet banjos for the Amals. They turned out to have 1/4 inch outlets not 5/16 inch. Another example of a question I didn't know I needed to ask :roll:

1/4 inch spigots are available to fit the petcocks and the cost is quite reasonable. Should I go ahead and use my new undersized banjos, or should I get the larger ones? Just wondering if the 1/4 inch line will flow enough fuel to keep the carbys happy...

I can't really comment on the original 750 set-up, mine came with an SU fitted.

I have always used the later Mk111 setup which uses double unions under each carb. My PW3 fitted motor with 33mm bored out carbs has never objected to this system.

All of my of unions and tap spigots are 1/4" (with an I.D. of approx 5/32")

Bearing in mind also that the later 850s are supposed to be thirstier than 750s, I don't imagine that fuel starvation would be a problem.

That said, there were horror stories of Tridents in the seventies seizing and holing pistons if the reserve taps were not opened for high speed runs which suggests that fuel supply lines can be a limiting factor, but their consumption can be in the range of 30 (imperial) miles per gallon. I hope my Commandos never do anything like that :)

I think that you should be OK but if you're doing track days or trying to cram more than 120 miles into an hour :evil: it might be worthwhile opening the reserve tap just in case. Otherwise, simply be on the lookout for any suspicion of leanness the first time that you give it full throttle.

You could check the bore through your petcocks (fuel taps) as they could be even less than 1/4", if so, then changing to 1/4" pipe fittings isn't likely to have any effect on the rate of fuel delivery?

My own 850 MkIII has the 1/4" pipe and fittings and I've not had any starvation problems.
Sounds like it should work then. Cool! I'll keep that in mind about opening the second tap for high speeds. Still running it in - 600 miles on the motor now - so I won't be doing the ton for a bit. I did run it up to 80 mph briefly yesterday though - whee!

Thanks guys :D

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