Fuel crossover pipe

Why does my bike have a crossover fuel pipe linking both taps to both carbs.?
Would it be wrong to fit a pipe from one fuel tap down to the carb on the same side of the bike, and the same for the other carb.??
If your bike was to start running on only one cylinder, surely it would be ok to shut the fuel off for the one cylinder, stopping the fuel washing the cylinder that was not firing, thereby allowing you to ride home on the one cylinder just to get you home.?
As with most people I’m sure we never let the fuel level get down so low as to need to go onto reserve.??

The same system is present on the ‘big blocks’ - my T3, for example. The ‘cross-over’ ensures supply - from a single tap, if you prefer - to both carbs. I presume equally and evenly, but am ignorant of the engineering design reasons. Some people find these a faff to fit and maintain - when renewing aged/damaged fuel lines, for example.

There are a few commonly deployed alternatives/mods, which achieve the same ‘cross-over’ mechanism:

~ T-pieces in line - components available from, for example, eBay (I search for: “fuel in-line t”) - I’ve no photos of this mod, personally, but have heard it described. Personally, it sounds almost as ‘fiddly’ to me, but your bike…

~ Dual outlet/inlet banjos - example at Eurocarbs, here: 7890 Fuel inlet banjo - Twin - 8mm - Eurocarb - one of these on each carb, lines to fit and job done. The example I included here is for the VHB carbs on a T3.

As for your other concerns: on “running on only one cylinder”, it feels (to me) like you’re possibly over-thinking a possible problem, but whatever you fit, I’m sure you can imagine a way around this, should the need arise. And as for “we never let the fuel level get down so low as to need to go onto reserve”, well… Yet, sir. Yet. I know I have, and that’s when I found out about the two level filter and pipe assembly insde my tank, which had come adrift. I got a lift home though, with a fellow motorcyclist in a van, and who had the compassion to stop and ask if I needed assistance!

Edit: note that one distinct advantage of the ‘cross-over’ pipe mechanism is this: when you go onto reserve, in a moving lane of traffic, you only have to reach down to one tap, and switch that to the reserve position, not both. If you’ve never had that happen, it’s an experience that you should have, at least once in your lifetime ((o;

Thanks Barry for your reply.
What I was thinking was to do away with the crossover completely, and just have a pipe from the left hand fuel tap down to the left hand carb fuel banjo, and the same for the right hand side, going from the tap down to the carb, no crossover or T piece at all.?
2 weeks ago myself and my mate were 100 miles from home, one of his coils failed on his 650, only took his spare out of his tank bag that morning…:weary:

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What I was thinking was to do away with the crossover completely, and just have a pipe from the left hand fuel tap down to the left hand carb fuel banjo, and the same for the right hand side, going from the tap down to the carb, no crossover or T piece at all.?

I don’t see a problem with that, personally, after all many other twins (presumably) do just that with no apparent downside - but at the same time, I don’t pretend to fully comprehend just why MG opted to do this in the first place.

The only downside that I see (speaking as a mechanical numpty) is that you would then have to remember to always operate two taps: fuel on (both), fuel off (both), switch to reserve (both).

As for switching off the fuel when you’re only “running on one”, I imagine that running a twin “dry” on one cylinder for 100 miles might incur other forms of mechanical damage. (see “mechanical numpty”).

Believe me, I am Also a mechanical numpty,
But the oil would still be circulating to the non firing cylinder, providing lubrication to the moving parts, but being able to turn off the fuel going to that cylinder would prevent the fuel going into the cylinder and washing the oil out back into the sump, resulting with fuel in the sump oil.? Which would then affect the lubrication going to both the cylinders…?
If this was possible at the time when my mate broke down, it would of prevented a 2 hour pillion ride back home, 2 hours back up with a van, and then another 2 hours back home with the bike and van.?? Spoilt the day really…
But then again if he’d left his spare coil in the bag for that one extra road trip, everything would of been fine…:anguished:

Have you tried riding a Moto Guzzi on one cylinder only? Doesn’t work very well! I did for a while have only two hoses, one on each side. However in practice it was a pain switching to reserve. Usually I couldn’t let go of the twistgrip or it will stall completely, so can only tun on the left tap ‘for now’. Once it’s going again properly, then can let go of the twistgrip ‘at some convenient point’, to turn on the right tap. So it helped a lot to have reserve from one tap feeding both. I ended up using a ‘X’ type crossover union which was easier to arrange than 2 separate ‘T’ joints. I agree there’s not a lot of room in there to play with.

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My t3 came to me with the crossover in place. However I found that when it got down to the point that reserve was needed, it would cut out completely before I could get one tap switched to reserve. I took out the crossover leaving the two carbs independent of each other. Then, when it was getting low on fuel it would stumble along one one cylinder while I switched both taps over to reserve, one at a time. It would take a little while but the bike kept up with traffic rather than slowing down like it did before.

Hi I would leave the cross over pipe in place so that the carburettors are getting the same amount fuel you can then turn to reserve from either side as designed


OK, my 3p here: yes, using V50 as a commuter a decade ago, I had a missing spark problem, but as I needed to get to work on time, I rode one cylinder for couple of miles - hence some smoke from it ever since… defo petrol washing issue - poor lubrication. But it happened just once. And crossover is useful at ALL times! I’d rather carry a tiny clamp to stop the fuel by squeezing the fuel line, a clamp used to stop brake fluid leaking. You can hide it in your fist. Weighs much less than a spare coil!:slight_smile:

Why is a fuel crossover pipe useful at all times, compared to individual fuel lines.??
I just don’t get it

when the machine is fitted with two fuel taps, it helps to balance and equal the flow of fuel, most motorcycle tanks are saddle tanks, ie once the fuel drops below a certain level, it cannot flow to the other side, unless you corner hard, then it sloshes over, under hard riding, one side running out of fuel can cause a weak mixture and pop a hole in a piston, or a misfire under a light load
it is always advisable to change over early to reserve if you are getting low, but with the link pipe a balanced flow of fuel will be delivered