Author Topic: Doublefly Query  (Read 5355 times)

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Offline Rickchat

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Doublefly Query
« on: 21 Aug 2017 at 11:45 »
Hi,
I am intrigued by a picture I saw on the Internet of this machine and sent it to a long-time friend who owned two Douglas 350 motorcycles. He sent back more information he got from this site. I would like to know several things.
1. Who built the Doublefly (I thought Douglas went out of business in 1957)
2. Is this model going to become generally available and, if so, when?
3. how much will it cost?
I hope you can answer the above questions as the Doublefly is a machine I would dearly love to own, once the bugs (carburation, etc.) are sorted. I have been motorcycling since 1958.
Thank you for your consideration.
Regards,
Rick Chattaway

Offline eddie

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Re: Doublefly Query
« Reply #1 on: 21 Aug 2017 at 17:52 »
Hi Rick,
            The Doublefly is a one-off special that I built between 10 and 20 years ago. I had been passing a critical eye over the Dragon Four that Don Brown built in the early 1990's, and had asked why he did this or didn't do that - only to be told that if I thought I could do better, I should get on with it! The Doublefly is the result of taking on that challenge. I wanted it to look as if it could have come from the Douglas factory, but with more modern refinements, such as indicators, disc brakes and electric start. I decided to base it on the earlier engines as there had been some correspondence in the early 50's between Eddie Withers and the Factory suggesting they could produce a 4 cylinder unit from already available parts from a 350 twin, for a very modest outlay. Stacking 2 flat twin engines threw up some unexpected problems with carburetion and oiling (which I eventually solved by producing 4 special carbs and an auxiliary oil pump to lubricate the valve gear). The bike now runs reliably with an exhaust note not unlike an Ariel Square Four.

  Regards,
               Eddie.

Offline Hampshirebiker

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Re: Doublefly Query
« Reply #2 on: 21 Aug 2017 at 18:12 »
Are there any pictures of it on the site Eddie? I did have a quick scan; honest.
Dave.

Offline eddie

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Re: Doublefly Query
« Reply #3 on: 21 Aug 2017 at 18:43 »
Hi Dave,
             Here are 4 photos to whet your appetite. The first is the Doublefly and my standard Dragonfly. The second shows the Mark crankcases 'cut and shut' ready for welding - followed by the completed bottom end of the engine. The last photo shows 2 of the 4 special carb bodies canted over at 10 degrees to give access for the throttle cables.


Offline Hampshirebiker

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Re: Doublefly Query
« Reply #4 on: 21 Aug 2017 at 19:02 »
Thank you and a very good job you've made of it. I don't know if development is ongoing, but have you considered a single modern carb. on each side? Or even a single carb. for the lot. I appreciate that the "plumbing" might get complex and too bulky.

Offline eddie

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Re: Doublefly Query
« Reply #5 on: 21 Aug 2017 at 20:13 »
Dave,
         I originally set it up with twin carbs, but when you stack 2 flat twins, the firing order always ends up with the 2 cylinders on one side firing one after the other, then the cylinders on the other side firing one after the other. This results in each carb getting 2 sucks then a pause while the other one gets 2 sucks - this plays havoc with the carburation. I even tried a balance pipe across the carbs, but that just resulted in an almost continuous suck that stuck the slides to the back of the carb so that they wouldn't drop the last 1/8", resulting in the engine refusing to drop below 2,000 rpm!

Eddie.

Offline Hampshirebiker

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Re: Doublefly Query
« Reply #6 on: 22 Aug 2017 at 11:36 »
Most odd and that's not an expression of any doubt. Thanks for posting.
Dave.

Offline Pensioner73

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Re: Doublefly Query
« Reply #7 on: 22 Aug 2017 at 14:44 »
Hi Eddie.

The VW Beetle is a flat four with a two throw crank and fires 1-4-3-2 which is right, left, right, left firing and not two on one side and then two on the other.
Would this not work on yours as bottom right, top left, top right, bottom left?
Would need a four cylinder distributor which might not be possible on a Mark engine.
Regards,

Ian

Offline eddie

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Re: Doublefly Query
« Reply #8 on: 22 Aug 2017 at 15:21 »
I'm afraid not, Ian. Looking from the back of the engine, to get equally spaced firing strokes, the firing strokes have to alternate between the 2 cranks, starting bottom right, the next firing stroke is top left, then bottom left, top right (then repeats BR, TL, BL, TR, etc.)(a sort of horizontal figure 8). (Turning the top crank one full turn just results in a reversal of the figure 8!).

 Eddie.

Offline Pensioner73

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Re: Doublefly Query
« Reply #9 on: 22 Aug 2017 at 16:56 »
Okay Eddie.
http://1967beetle.com/firing-order/
In this diagram, if you imagine it is two Douglas engines in tandem, you can see the rear engine (upper) is 180 degress out of phase with the front (lower) engine which gives the four cylinders
firing within the four stroke cycle, 2 revs. One of the pistons comes into compression every half rev as with any four cylinder engine. 1(BL), 4(TR), 3(TL), 2(BR) looking from the saddle.
Surely that would work!

Regards,

Ian

Offline eddie

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Re: Doublefly Query
« Reply #10 on: 22 Aug 2017 at 17:14 »
Ian,
        It's not possible to have 2 consecutive firing strokes on one crank (top left following top right, and bottom left following bottom right) - the firing strokes have to alternate between the 2 cranks as they are timed 180 degrees apart.

Eddie.

Offline Pensioner73

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Re: Doublefly Query
« Reply #11 on: 27 Aug 2017 at 21:57 »
Eddie, I see the error of my ways and I'll stick to a flat twin.  :?

Ian