Grey Flash: Motorcycles that Matter

timetraveller

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David's experience with Mk 2 cams mimics my own. When I used to sprint my twin I tried revving to 6,000 rpm and changing gear twice on a quarter mile or revving to 7,000 rpm and changing gear once. It was repeatedly faster over the quarter mile with the extra gear change. Lacking access to a rolling road in those days my conclusion was that one is far over the peak of the power band at 7,000 rpm with a Mk 2 cam.
 

vibrac

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Some years ago, I had a friend´s Comet for some time. It was an original "C"-Comet. When it topped at 75mph, I asked a friend, to follow in his car and tell me his speedo readings afterwards. He confirmed my
readings, stating, that his Moto Guzzi Falcone did the same speed with a sidecar on!
So, I remembered, that this speed was also achieved by my first motorcycle, a 1952 NSU Max, 250cc, 18hp.
And I decided, to find out why.
Next step was, to fit a bicycle speedo, carefully adjusted. The Comet´s engine was in good trim, everything
well tuned, nice plug face, etc. So I started on a sunny afternoon on a flat, free motorway nearby and revved it
in 1st, until it did not accelerate, anymore, remembering the speed, the same in second and third. In fourth,
it topped at 75 with me sitting up. I stopped and noted the figures on a sheet of paper and went back home.
With the figures from Paul Richardsons book, I calculated the engine speeds in each gear and found out, that,
with works sprockets, it stayed 1000rpm below max. in fourth gear, which is app. 17 - 18 hp at the rear wheel.
So I fitted a 52 teeth sprocket, which was 4 teeth over standard.
Same day, same road, same conditions, and it achieved 90 mph, exactly works figures!
I do not torture my bikes, but they must deliver works standards!
So you geared it down and it went faster.......
 

davidd

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VOC Member
Tim,

I am not sure how well the 1-7/16 works as a down draft. It clearly had problems that the Factory had to fix due to the angle. With the replica I ran I did not have a problem, but I did not run it because I did not have enough time to do a lot of dyno runs to dial it in. I chose to run the 32 mm instead.

I think the big carb will fit under the tank, but it does "dead end" the bell mouth into the tank more than I liked. Because I could, I made a special tank cover to allow air to the bell mouth.
100_0271.jpg

Coburn Benson ran the Flash with the big port head and an 1-1/2 GP in all of his races. The GP does not do downdraft well (except for the GP2) so he made a manifold to hold it.
IMG_2118.JPG

This is probably the best way to run a vintage carb. Kurt Fisher ran 113.851 mph to qualify for the Daytona 100 miler in 1968. Coburn figured 38 MPG at full throttle and ran the race without a pitstop. Coburn got up early and grabbed one of the original 20 sheets that Daytona copied.
Daytona Qualifying 1968 1.jpg


David
 
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