Author Topic: Mk 5 petrol tank  (Read 247 times)

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Offline Rex

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Mk 5 petrol tank
« on: 10 Oct 2017 at 13:50 »
Some bikes with two petrol taps have a deep top tube tunnel meaning the tank, for all intents and purposes, is actually two tanks joined only at the top, but the Mk 5 seems to be so low at the back end as to be virtually "joined" for all but the lowest fuel level.
Anyone know roughly how much would be contained in each half when it drops to the cut-off point and becomes two separate quantities of petrol....if that makes sense.
I'm guessing less than a pint per side. 

Offline eddie

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Re: Mk 5 petrol tank
« Reply #1 on: 10 Oct 2017 at 14:25 »
On a Mark tank, the back of the tunnel tapers down to the base level of the tank, so no petrol will be trapped on either side of the tunnel, especially when the bike is being ridden and the fuel is slopping about!

  Eddie.

Offline Rex

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Re: Mk 5 petrol tank
« Reply #2 on: 11 Oct 2017 at 09:25 »
Which makes me wonder why Douglas didn't just fit  main and reserves taps and Tee the lines together...?

Offline eddie

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Re: Mk 5 petrol tank
« Reply #3 on: 11 Oct 2017 at 10:46 »
Mark series machines were fitted with an independent supply to each cylinder, each tap having a 'Main' and 'Reserve' position. A much better system than having the 2 feeds tee'd together - if you have a problem that affects one cylinder, you can turn the fuel off for that cylinder and still limp home on the other one.

  Eddie.

Offline Rex

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Re: Mk 5 petrol tank
« Reply #4 on: 11 Oct 2017 at 10:58 »
To be fair that's something I would never do on any bike. I'd check the spark and the petrol feed to that carb, and I might (depending on where I was) strip and check the carb, but further than that would be a take-me-home job via Mr Nokia...

Offline eddie

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Re: Mk 5 petrol tank
« Reply #5 on: 11 Oct 2017 at 11:11 »
I don't think Mr Nokia was around in 1952!!

  Eddie.

Offline Hampshirebiker

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Re: Mk 5 petrol tank
« Reply #6 on: 11 Oct 2017 at 20:54 »
I can confirm that one pot is an adequate get you home. I did it once in the 60s, but can't remember if I turned off the other tap. :frown: The downside is that with two old double taps there's twice the grief.

Offline douglas1947

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Re: Mk 5 petrol tank
« Reply #7 on: 12 Oct 2017 at 12:44 »
I had finished a MK1 this summer. I can tell you the MK1 has only 1 tap and I went home with only 1 cylinder (oil problem on the right cylinder with oily plug).
So I can`t stop the petrol to the fault cylinder!
Not so good for the engine.
Michael