Author Topic: Fulton's Mastiff  (Read 1311 times)

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Offline Patrick

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Fulton's Mastiff
« on: 06 Jan 2018 at 16:41 »
The two surviving sons still own it and I am their mechanic for this remarkable, “One Man Caravan”, Douglas Mastiff. Since my past Forum postings, 2005(?), the bike was inactive then the engine was removed and sent to Dewey for overhaul. I reinstalled it this past winter and the bike has now accrued over 150 miles on local sorties including the Independence Day parade in Aspen. In August, the gearbox suffered a “no go” condition and I discovered sheared Woodruff keys on both primary and secondary sprockets. Full entry into the gearbox revealed that the bearings, gear sets, and shifting cam barrel were all in fine shape. My next perplexing objective is to fix or find suitable alternate carburetion to make the bike more rideable. Peculiarities of this installation include:
•   Side draft configuration. 22mm carb throat.
•   Possible re-clocked (30 degrees) and re-brazed carb flange to body.
•   The brass throttle body and slide are so worn and leaky it won’t allow a slow idle. The engine remains running even when the louvres are closed on that Mikuni air filter.
•   Our home elevation is 7500’ and our area of operation reaches 12,000’.
Why do you think they fabricated this side draft configuration for such a purposeful build in 1932? We are interested to acquire any historical tidbits to further document this bike’s history. Some of you met this bike, and Mr. Fulton, in Bristol and Kingswood in 1992. I trust all of you have read his fascinating chronicle of his trip. We’re sincerely grateful for you sharing your anecdotes and wisdom related to this bike and Robert Fulton.

Offline eddie

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Re: Fulton's Mastiff
« Reply #1 on: 06 Jan 2018 at 17:47 »
Hi Patrick,
               The sheared key problem may be caused by each sprocket having 2 keys. If the sprocket is correctly lapped onto the shaft, the taper alone should transmit the drive - with a single key preventing movement whilst tightening the nut. Adding a second key means the keyways need to be perfectly matched - if not, the taper will not seat correctly and therefore not be capable of transmitting the drive - so a correctly lapped taper with a single key should transmit more power than the same setup with 2 keys.
   With regard to the carburettor - that side draught arrangement was only used on the S6 derived 750's - so is a rare item. To get more useful life from your carb, insert a thin wire down the side of the choke block to measure the depth the slide drops to, then have the body bored from the top to that depth, to clean up the worn areas. Then turn up a piece of bar to be a neat fit inside the carb slide, and fit the slide on it, and retain it with a pad and a tailstock mounted revolving centre. The slide can now be skimmed down to clean it up. Then turn up a thin wall brass sleeve to fit on the slide and be a sliding fit in the carb body. Soft solder the sleeve onto the slide and carefully cut and file out the guide slots. When reassembled, the carb should then function correctly. Having said that, the horizontal slide arrangement is more demanding of the fuel level being correct than the normal vertical slide arrangement.

  Hope this is of some help,

Offline hoejmark

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Re: Fulton's Mastiff
« Reply #2 on: 07 Jan 2018 at 10:43 »
Hi Patrick
I fully agree upon Eddies advice.
I did the same modifications on my T6 carburetter a few years ago with exceptional results, and last year I also modificated a carb. similar to the one on the Fulton machine.

Following this thread you will see photos of my set up in the lathe when I modified the T6.

Good luck with the Fulton machine.

Regards hoejmark

Offline Patrick

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Re: Fulton's Mastiff
« Reply #3 on: 07 Jan 2018 at 17:43 »
That’s an interesting and credible perspective about the tapered shafts with doubled up woodruff keys. Of course that provokes the question, and one wonders, why was it engineered and manufactured that way?
Thank you two for your welcome news about the reparability of this old carb. The scarcity of that side draft configuration must relate to its performance & popularity.