Author Topic: Both fuel taps on?  (Read 159 times)

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Offline Cheekywig

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Both fuel taps on?
« on: December 28, 2019, 02:25:37 PM »
Do both the fuel taps need to be on to run on both cylinders assuming theres fuel in both sides of the tank? mine fired up on two then went to one soon after but I don't think I had enough fuel in both sides. As the taps were leaking I plugged in another tank with only one tap on to see if it worked just in the LHS of the carb and still only ran on one cylinder... or possible rectifier problem? any suggestion most welcome thankyou.

Offline Paulval

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Re: Both fuel taps on?
« Reply #1 on: December 28, 2019, 04:15:26 PM »
The left tap supplies the left carb, the right tap the right carb, so unless the fuel pipe from one tap has been T'd to supply both carbs, then yes both fuel taps need to be on to supply fuel to both carbs

Offline restaurateur

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Re: Both fuel taps on?
« Reply #2 on: December 28, 2019, 09:50:01 PM »
Keeping one closed with running engine opens the possibility of burning a hole in one of the pistons.



Adrie

Offline guylr

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Re: Both fuel taps on?
« Reply #3 on: December 28, 2019, 10:57:45 PM »
Not really as no combustion happens with no fuel. Modern fuel injected engines cut fuel delivery on deceleration with no ill effects. It will make the other side work harder but I don't recall ever seeing a holed piston on account of one side not firing. 

Guy
Now Retired Yamaha guy. Cut me and I bleed "Cinquasia Red"

Offline restaurateur

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Re: Both fuel taps on?
« Reply #4 on: December 29, 2019, 08:38:11 AM »
I burned a hole in the piston of a Honda Elsinore, by having the tap closed.
your argument is correct, modern engines brake the fuel supply during decel, as well to regulate the speed when on the
cruise control.However the location of the injectors is extremely close to the combustion chamber , and the injectors do cut the
pulses sized to the combustion/lambda value=1; the cut comes before the next ignition, and an ecu is used to avoid deviating to far from rich/lean/ Lambda=1
This was not so for the first injectors that used one or two jets in the trajectory of the inlet manifold.This is not so close to the engine's intake=manifold sometimes filled with lean mixture.
With carb's however,the same happens; you will fill the manifold with a mixture that is way to lean if a tap is closed,and contains enough mixture to have a number of cycles.
Basically one can safely assume , that all these truths are context -sensitive.
Adrie

Offline guylr

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Re: Both fuel taps on?
« Reply #5 on: December 29, 2019, 12:50:58 PM »
Yes two stroke racing singles like your Elsi can lean out enough as the float bowl empties and combustion still happening to sieze or hole the piston. Four stroke twins like our 500s are not so sensitive and again I've never seen a holed piston on one due to leaving the petcock closed. Indeed most bikes in those days relied on the rider turning the petcock from on to reserve as the fuel level gets low in the tank and the engine begins to stutter. So let's not worry folks about a non proplem.

Guy
Now Retired Yamaha guy. Cut me and I bleed "Cinquasia Red"

Offline lumpy252

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Re: Both fuel taps on?
« Reply #6 on: December 29, 2019, 01:47:46 PM »
Hi all I agree not to be too concerned about the short spells of one cylinder running but the cause of the problem which is the fuel not getting to both sides needs sorting. Is there rust build up in the original tank which could be blocking one tap ? You mention a leaking tap which will require new seals or a replacement complete tap .

Offline restaurateur

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Re: Both fuel taps on?
« Reply #7 on: December 29, 2019, 04:04:39 PM »
Splitting the hose would allow to use one tap, but gasoline will partially remain in the tank.-

One can use a little extern tank with very clean gasoline for testing purposes.An old bidon?


Offline CrouchyUK

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Re: Both fuel taps on?
« Reply #8 on: December 29, 2019, 07:30:05 PM »
This does remind me of a very nice XS500B that I bought off a guy that said he couldn't get the engine to run properly?  We agreed an appropriate price to reflect the fact that some investigation and work would be required to try and get it running again.

When I got it out of the van I noticed just the LH fuel tap on- turned the RH one on and rode it around the yard sweet as a nut!

For the original post- the rectifier will have little to do with ignition......
1975 Honda CB200T (2) - 1975 Yamaha XS500B (2) - 1976 Yamaha XS500C
1977 Yamaha RD400D - 1977 Yamaha XS500D - 1978 Honda CB550K - 1980 Honda CB400N
1980 Honda CB900FA - 2000 Kawasaki ZX6-RG2 - 2002 Suzuki GSX-R1000K2

Offline mvluv13

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Re: Both fuel taps on?
« Reply #9 on: December 29, 2019, 09:52:19 PM »
More on the topic of running with one tap closed. A few years ago when I still had points, one side failed and I couldn't repair it on the road side so rather than waste fuel I turned off the tap and rode her 80 klms on one cylinder. Not best practice but got me home.

Offline berniebee

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Re: Both fuel taps on?
« Reply #10 on: January 15, 2020, 09:13:38 PM »
ONE more belated story: This spring I finally got my '76 XS500 ready for the road and had to get it inspected by an authorized mechanic before I could get a license plate for it. "Be ready right after lunch" , he said. When I returned, he said, "It's all fine, but it runs on only the left cylinder!".   He chuckled when I pointed out that there were fuel taps on both sides.  Of course, no matter what side he would have looked there was "the" fuel tap. He had never seen a bike with two fuel taps before. Come to think of it, neither have I.

"So when you switch to reserve, you have to switch for both sides?", he said. "Yep."  "Pain in the -" "Yep."

Offline CrouchyUK

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Re: Both fuel taps on?
« Reply #11 on: January 16, 2020, 01:22:18 PM »
I wonder why Yamaha came up with the 2 tap solution in the first place?  Their other bikes of the period tended to just have a short balance pipe between the 2 sides of the tank if needed.....
1975 Honda CB200T (2) - 1975 Yamaha XS500B (2) - 1976 Yamaha XS500C
1977 Yamaha RD400D - 1977 Yamaha XS500D - 1978 Honda CB550K - 1980 Honda CB400N
1980 Honda CB900FA - 2000 Kawasaki ZX6-RG2 - 2002 Suzuki GSX-R1000K2

Offline guylr

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Re: Both fuel taps on?
« Reply #12 on: January 16, 2020, 10:44:52 PM »
Pretty much all the 70s Yamaha twins, triples and fours over 400cc rhat I can think of have dual petcocks.

Guy
Now Retired Yamaha guy. Cut me and I bleed "Cinquasia Red"