Any Production Racer's out there for sale?

jimbo said:

read somewhere there were 33mm concentrics, but that's the problem there seems to be alot of misinformation out there

Click to expand...

I have also heard of 33s, but can’t remember what they were fitted to.

I think that in the UK, production racing happened in the late1960s. The theory seems to have been 'race on Sunday, sell on Monday'. We had the Castrol 6 Hour Race in Australia. The bikes were carefully checked before and after racing, so they were as sold to the public. In the UK, I think more modifications were allowed. But in Australia ways were always found to get better performance. It is all silly stuff anyway. A bike which is good for racing is not necessarily good on public roads. Amongst the photos in this thread, there is a Gus Kuhn Commando - probably a Seeley. That would be difficult to beat in a road race, but useless on public roads. I would buy a genuine factory production racer, just for what is - not what it can do. I would not buy a genuine Manx Norton - they come with a responsibility. It saddens me to see the way most of them have been modified. That might be called genuine racing history - it will get to the stage where nobody will ever know why a Manx was so good. They are worth riding just for the experience.

jan nelder said:

I have also heard of 33s, but can’t remember what they were fitted thello Jan, they were fitted to the 750 short stroke as mentionned in the 1973 brochure

Click to expand...

marinatlas said:

https://www.classicbike.biz/Norton/Brochures/1970/1973NortonLineUp.pdf

Click to expand...

thanks....

Pretty sure they’d be bored out 32s rather than purpose made 33s.

Fast Eddie said:

Pretty sure they’d be bored out 32s rather than purpose made 33s.

Click to expand...

a fair assumption.
I saw a pair of 1000 series 34mm Concentrics on a Rickman 8-valve Bonneville. Serious looking carbs!

They were bored out 32s. The factory (and others) had been doing this for some of the race bikes well before the short stroke came out. That was as much as you could bore them out and still have the standard slides still work. I did that to the stock 32 mm concentrics on my PR. Worked well, but not sure it made much difference.

Ken

lcrken said:

They were bored out 32s. The factory (and others) had been doing this for some of the race bikes well before the short stroke came out. That was as much as you could bore them out and still have the standard slides still work. I did that to the stock 32 mm concentrics on my PR. Worked well, but not sure it made much difference.

Ken

Click to expand...

Another half mm is possible Ken, I’ve had ‘em out to 33.5mm.

Fast Eddie said:

Another half mm is possible Ken, I’ve had ‘em out to 33.5mm.

Click to expand...

I'll take your word for it, Nigel. I never tried going past 33. It was a long time ago, but I think I was probably unwilling to risk ruining a pair of carbs to find out. That was back in my early race days, and I hadn't yet accumulated boxes of spare parts I could experiment on. I knew 33 would be safe because others were doing that successfully.

Ken

My Seeley 850 has two 34mm Mk2 Amals on 30mm ports which are carefully shaped to match diameters. You are dealing with two things - sound waves and vacuum. At TDC when both valves are open, the exhaust system interacts with the inlet tract - there is probably a kadency effect - similar to what happens with a two stroke. However an expansion chamber on a four stroke would probably need to be huge. Loss of vacuum makes mixture too lean and can cause the motor gasp. On a motor in which the ports are too large, smaller carbs might give more vacuum. In my motor, I keep the ports small to get more vacuum, and keep the carbs big to supply more mixture. The internal shape of the ports is important. Have look at the shape of a jet plane and turn it inside-out, and back to front.