Comet Top End Speed

[Monkeypants]

That's right my hotrodded ex Sprint twin has two front heads for that reason. It is ported to almost 35 mm.
I prefer the other bike with standard 28 mm ports, it pulls harder on hills. Maybe the ported one might have more power over fivethousand rpm, but I try not to go there too often or for very long.
Glen

 

[Kansas Bad Man]

Gunga Din started life with a front a rear head , ALL other record breakers over the many years of guys trying to get the most out of there Vin have used modified standard front heads or factory big port fronts. Shouldn't that tell us something?

 

[Martyn Goodwin]

Update: Yesterday I did a 260 mile run on my Comet and have come to the following conclusions re rear sprocket size.

For running around town and short moderate speed jaunts into the countryside a 50 tooth rear sprocket is the go - give a more responsive ride, all be it with a reduced cruise speed - for the same revs.

For touring a 48 tooth sprocket gives a more relaxed ride and the potential for a slightly higher cruising speed, though you will find a need to drop down in the gears a bit more often then with a 50 tooth at the back.

Just my observations.

Martyn

 

[TouringGodet]

When we got the Comet many years ago, someone had recently converted it back from a rear head to a front head. I have the scars on the under side of the tank where the notch used to be to clear the carb, plus an inlet stub with a very odd angle (think about that). I did get the rear head too, as a spare since the bike had a front head, but likewise, unfortunately, the rear head was not attached to a twin motor.

 

[mercurycrest]

Oh Dear. No there is actually less metal around the inlet tract on a rear head. The danger point is where it gets close to the rocker tunnel. I speak as one who found out the hard way just how little there is. Years ago Neville Higgins made a double ended calliper so that the outside end moved exactly the same amount as the inside end for checking just this distance.

You can always weld up the rear head intake and notch the rocker bearing and more than

double stock flow. But in the end a front head will go even bigger.

 

[Howard]

Oh Dear. No there is actually less metal around the inlet tract on a rear head. The danger point is where it gets close to the rocker tunnel. I speak as one who found out the hard way just how little there is. Years ago Neville Higgins made a double ended calliper so that the outside end moved exactly the same amount as the inside end for checking just this distance.

Well, there you go. Proof Dinosaur meat is no good for the memory. Wish I could remember who told me, but he probably wouldn't remember.
H

 

[Chris Launders]

I have a "standardish" twin ex Chris Williams, 2 front heads, big ports (about 34mm) 32mm mk1 concentrics and mk3 cams also a Norvin with front/back heads 33mm ports (the rear one did break through into the rocker tunnel) 34mm mikuni's and mk2 cams, both on 8:1 pistons, both have straight through absorbtion type silencers, there is a big difference in the way they run given that the major difference is the cams.
The normal twin has steady even acceleration and really pleasant to ride, the Norvin is completely different,won't tickover as steady, pulls ok up to 3000 rpm then takes a BIG step in power, acceleration and noise, then another one about 4.500 rpm. This means that with the Norvin you are forever having to keep an eye on the rev counter and gears and really is harder and noisier to use normally and there are too many cameras etc to use it as it wants.

 

[Bill Thomas]

Hello Chris, I would have thought the differance was the carb's, There is only one way to find out, Swop them, Bad Joke, Have fun, Cheers Bill.

 

[Chris Launders]

Took me long enough to get the concentrics running right to risk that, tried Amal mk2s on the Norvin for the first couple of years but couldn't sort them, mukini's ran right straight out of the box.

 

[Monkeypants]

David thanks,

Thinking I would like to step it up to a 50t. What length chain would I need and do the spares company have them?