Author Topic: 2 3/4 cams  (Read 8626 times)

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Offline ian scott

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2 3/4 cams
« on: 26 Mar 2008 at 08:46 »
hi,

i'm reassembling my 2 3/4.

i have a variety of cams. some of which are in good condition.

does anyone know which will give the best performance?

or perhaps compare the performance of different types?

thanks. ian.

Offline graeme

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Re: 2 3/4 cams
« Reply #1 on: 27 Mar 2008 at 22:36 »
Hi Ian
Do these different cams have any identifying marks? ie - do they look like factory items, or have they been modified by past owners. The factory may have offered different cams for different models - I presume they offered a sports cam of some sort. I would think that there are 2 3/4 owners that would have knowledge of this.
Cheers, graeme

Offline Chris

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Re: 2 3/4 cams
« Reply #2 on: 28 Mar 2008 at 06:54 »
        For a long time I have wanted to produce an authoritative article on cams used on 2.3/4 hp models. The problem is lack of information and perhaps this thread will bring together any knowledge that is out there. I also have several different cams and have attempted to tabulate them and assess their differences. Firstly there is usually a mark stamped on the cam in the form of a number or combination of number and letter. The most common cam seems to be one marked 5B. This is certainly the one used in most 1920s models such as TS,SW and CW. Earlier machines often had a plain No 5 cam. I have also seen unmarked cams and I have one that has distinctly different profiles between the inlet and exhaust whereas most have the same profile just set at different angles. There are rare examples of a No 5A cam supposed to be more sporty. I have only ever seen one and that had a tooth missing. I have no idea how they perform in a machine. I will try to dig out some measurements I made some years back to add to available knowledge but would be pleased to hear of any other contributions to this subject. Does anyone know what cams were fitted to the TT models?  Chris.
« Last Edit: 28 Mar 2008 at 10:21 by Chris »

Offline eddie

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Re: 2 3/4 cams
« Reply #3 on: 28 Mar 2008 at 07:48 »
Chris,
          I can't come up with any technical details at the moment, but the cam in LB1120 (a 1913 'TT' model) is just marked with a 'C'. Some years ago - when the bike belonged to Trevor Phillipps - I compared the cam to a 5B, and it had higher lift and longer opening.
  For those who dont know the history of this bike - it was bought (new) by the Stroud brothers of Reigate and raced on the board tracks around London. It was then laid up until Allan Phillipps bought it (from the Stroud brothers) in 1960. Allan rebuilt it and used it in numerous Club events until his untimely death in 1972 (including leading the parade at the 1967 Diamond Jubilee TT meeting in the IOM - where it was also sampled by the likes of Bill Ivy, Phil Read and Agostini) - then Trevor (and eventually, Janine) took over the riding. I wouldn't like to guess how many thousands of miles it has covered - but it is still a quick 2 - easily capable of 60 mph.
      Next time I have to look in the timing cover, I'll put a degree plate on the cam and expose it's darkest secrets!
                                                Eddie.

Offline ian scott

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Re: 2 3/4 cams
« Reply #4 on: 31 Mar 2008 at 11:30 »
hi,

i looked through my pile of cams. there are a number of 5Bs and one 5.

the 5 has noticeably more duration on both lobes and more overlap and a bit more lift.

i'll instal a 5B initially and see how it runs then i'll instal the 5 to make a comparison.

i will report back.

i will also plot the opening and closing angles of the valves and the lift.

give me a few weeks as i'm waiting on some parts.

Offline ian scott

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Re: 2 3/4 cams
« Reply #5 on: 31 Mar 2008 at 11:41 »
the thought crossed my mind that the different cams might produce power characteristics appropriate to the different gearboxes.

Offline nickcanfor

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Re: 2 3/4 cams
« Reply #6 on: 31 Mar 2008 at 20:34 »
I have one of the 2 3/4 hp TT models. It would seem that there are only 3 recorded in the club records.  I took off the timing cover tonight, in the interests of research and it is a 5A. Chris, you can measure it sometime if you like.

With regard to the racing cams used by Douglas I attach a copy of an article from the Motor Cycle dating to June 1913. You will see from the picture,  they used a very special cam. drilled for lightness and with a facility to have a separate cam for each valve. I doubt we will ever see one of these as I bet only a very few were ever made. It seems from my research that lots of different ideas were employed before the WW1 as one off's for races. These one offs were often only used by one member of the Douglas Works Team while the others tried different equipment. For example they used different exhausts, oiling systems, carbs (including twin carbs) and gearboxes.

I would like to hear more about these cams - keep it flowing.

Nick




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« Last Edit: 21 Jun 2010 at 07:12 by Dave »

Offline RobertC

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Re: 2 3/4 cams
« Reply #7 on: 17 Apr 2008 at 07:24 »
I found this timing diagram for the 5B cam in my "Handbook of the Douglas Motorcycle". Sorrry about the oily thumbprint - I think I urgently needed to refer to this page when I was trying to get the motor to start for the first time! I would be very interested to hear how this compares with some of the more lively cams that seem to be around.

Rob

« Last Edit: 17 Apr 2008 at 07:46 by Dave »