Vic Doswells on tuning an R25/5 in 1967

Begonnen von Jollyjester, 27 Juli 2018, 09:13:22

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Jollyjester

62 Cressingham Rd

Berks
18/1/67
Dear Mr Freeman
         Sorry to be so tardy in replying to your letter- Christmas, New year and all that! However,  better late than never.
   I am most interested to hear of your tuning project, of course we are up against it with the 250 BMW as it is very heavy bike and you cannot lighten it a great deal.
   Starting with the cycle parts, I have removed the heavy steel mudguards and put on alloy ones, the large duel seat replaced by a little single seat, the enormous silencer by a small Burgess. The air filter is removed complete, the sidecar ball joint sawn off. The tank replace by a little one of a  50ccs (sic )  Motobi.
   This is pretty well as far as one can go without removing the headlamp which precludes it from normal production stuff. I also removed the hub covers to let the brakes cool better. Tyres are ribbed front and racing rear.
   Now the engine, ( R25/3), we had a 9:1 piston already, we turned 80 thou off the top of the barrel which theoretically gave us 11:1, I may add that on trying the fit at T.D.C was necessary, and due to the head coming down the valves hit the piston crown. So we had to take some of the valve pocket edges and the head itself to clear. I tried to get a good head joint but had to put in gasket as it would not hold the oil at the joint.
The head was carved a bit as remarked above, the inlet port opened and straightened ( 1 1/16th )the match the choke of the R26 Carburettor.  We removed most of the inlet valve guide in the port, but let a little and knife- edged the side facing the carb. The valves were polished, (also head and inlet) and the seat, width reduced and edges rounded off.

   I had a sports camshaft from Hoske, and a set of stronger valve springs, the rockers were just polished to remove all the rough ness in the castings. . Make sure the rockers are very free and well oiled – we seized the exhaust one 2 years ago, it turned bright blue !
(Note ) I also felt the engine tighten once, thrashing down the M1 Motorway. When I looked later, the exhaust rocker was going blue! (Robert )
An extra oil supply can be arranged by boring the head, feeding copper tubes into the rockers spindles ( Note, the R25 head has these , Robert ) and preferably fitting a restrictor needle to control the amount. Valve clearances I put at 8 thou inlet and 10 thou exhaust: it clatters but does not close up.
The camshaft can only be timed at the original factory settings, unless you can get more keyways in it; one chain sprocket either way and the valves hit the piston! The ignition was slightly retarded (!) as with the quicker flame propagation the original setting mad eit oink.
Strangely enough I cannot get it to take anything bigger than a 125 jet without over-richening. Needle in the 2nd notch from the bottom, you mat be able to vary these with your engine.
A normal R26 carb was used with an angled inlet stub to clear the R26 frame, with the R25 motor in – a long inlet at the joint.
The head was carved a bit as mentioned above, the inlet port opened  and straightened, (1 1/16th) to match the choke of the R26 Carburettor. We remove most of the inlet valve guide in the port, but knife edged the side facing the Carb. The valves were polished, (also the head and inlet ) and the seat width reduced to 1/16th
I had a sport camshaft from Hoske, and set of harder valve springs, the rockers were just polished to remove all the roughness in the castings. Make sure the rockers are very free and well oiled. – we seized the exhaust one 2 years ago- it turned bright blue! (Note, same thing happened to the Little Horror, Tightened but did not seize).
I had A sport camshaft from Hoske and A set of hard are valve springs, of the rockers were just polished to remove all the roughness in the castings.  Make sure the rockers are very free and well oiled – hui's seized the exhaust 12 years ago- it turned bright blue!  An extra Oil Supply can be arranged from the feed tube, by boring the head feeding copper tubes from here into the rocker spindles, and preferably fitting a restrictor needle to control the amount.  Valve clearances I put at eight thou inlet and 10 thou exhaust it clatters but it doesn't close up.
The camshaft can only be timed to the original touring settings unless you cut out more keyways in it, one chain sprocket tooth either way makes the valves hit the piston!  The ignition was slightly retarded!  As with the quicker flame propagation the original settings made it pink
.  Strangely enough I cannot get it to take anything bigger than a 125 jet without over-richening.  Needle in the second knots from the bottom you may but be able to vary these with your engine.
A normal R26 CARB was used with an angled inlet stub to clear the R26 frame with the R25 motor in – a long inlet ram pipe was used although I do not think this does very much.
Tyres were run at about 30-of 32 PS I and the suspension on 'hard'; and reversed gear change (a cut and weld job), also the brake pedal brought back to pivot at where the rear side call sidecar ball had been sort off (R26 frame)and the linkage made up.
That's about it; I used the original handlebars reversed in the clamps and a quick action twist grip and ball and levers.  I think the speed is about 90 plus; on the longest rate Silverstone club circuit can be lapped in 00:01:28 by scratching hard and wearing out the inside of your boots.  I speak for myself!  others may go faster
This I personally believe is the art and lot of the particular motor but of course it may be possible to get a little more; I intend to process an R26 motor in a similar way and turn the flywheel down a bit.
Oh- to prevent clutch lip I cut the clutch plate like this, Note; illustration of plate cut through from the outside that to the holes ) the spring them separate portions alternate I the way a few sent this is very effective net about the special components that address is;

Ernst.. Hoske3251 Gr- Hillingsfeld, Hameln. W Germany
They do camshaft for R25 for 40 DM, a set a spring's 10.40 DM; and about £1.00 for post and carriage, exchange write 10.6 DM equal £1.  You should get an international money order and have it enclosed with the postal check account number which is page HAN  1234 06
This firm is very quick to send and most obliging; warning; but they will rise back in German which didn't worry me but I don't know your situation!
But it also do petrol tanks, sidecars, rev counters, ( not for singles I believe) clip on bars, valves, pistons etc.

I hope this is of some help to you and would be interested to hear how you get on; I have never had A rev counter but I would say that we get about 7000 RPM at which point the vibration is considerable.  Down the straight towards Woodcote at Silverstone I found it got maximum about; ¾ way along and one can scratch a bit against the faster two fifties by breaking late- the brakes are very good, and you can outbreak in fields and cetera I would say Woodcote ( for me) is about 55 MPH, Copse 65, Beckett's 30, and Maggots C:urve flat out
I am 42 and a reasonably terrified but I find a lot of the rides in these high speed trials are all either more terrified all just not trying as a lead me on the straight bike at some on the corners

Yours sincerely
V. S. Doswell

Jollyjester


4Taktix

Hi Robert,
thanks for sharing this article, very interesting and good to know.
Now I need someone to translate the last sentence. Anybody here which grew up with both languages and knows "slang" ?

I am 42 and a reasonably terrified but I find a lot of the rides in these high speed trials are all either more terrified all just not trying as a lead me on the straight bike at some on the corners

Cheers,
Sascha
Think outside the box !

† Will

Zitat von: 4Taktix am 28 Juli 2018, 10:11:03

I am 42 and a reasonably terrified but I find a lot of the rides in these high speed trials are all either more terrified all just not trying as a lead me on the straight bike at some on the corners

Cheers,
Sascha

I think that possibly the word rides is typo and should read as riders.
The same may apply to all which could be intended as or. The sentence would read -

I am 42 and a reasonably terrified but I find a lot of the riders in these high speed trials are all either more terrified or just not trying, as a lead me on the straight at some on the corners.
R25/3 + R65LS. The R27 has found a new home.
I was not lost, my GPS just found a different way back.

OldsCool!

English is my everyday business, but still I can't figure out what Sascha wants to tell us  ;D
Ich bin auf dem Dorf aufgewachsen. Ich wurde nicht erzogen, ich habe ÜBERLEBT!

4Taktix

Me auch nich - Weil Vic Doswell that vertellt hat, not ich  ::)
Think outside the box !

Jollyjester

Will has it correct, just a typo;

" I am 42 and a reasonably terrified but I find a lot of the riders in these high speed trials are all either more terrified or just not trying, as they lead me on the straight at some on the corners."


But I  catch them on the corners !



It is good to think Vic Doswell is still relevant to 2018

His most famous quote, I am sure you know.?
asking a BMW Head Man, ' How do I cure the vibration ?'

"Buy a Twin"

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