Comet Top End Speed

[Bill Thomas]

Morning, Just standard, With a big hole, L/ning Grey flash. It's the bike that I call the Special, A mix of D top frame, C rear, Norton forks and disc brakes. The Comet engine is Grey Flash spec', Mk2 cam, 9 to one piston which is not too high tune for the road. Cheers Bill.

 

[redbloke1956]

Hi Bill, just for interest sake, I hear some talk about the Mk2 cam so how much difference would a Mk2 cam make to say, a bog stock Comet engine.
Would the carb/head/exhaust/cr need to be tweeked to make a noticeable difference?

Regards
Kevin

 

[Bill Thomas]

Hello Kevin, I have only run Mk1 cams in my twin in 1965 with a sidecar !! and that was not for long, They wore out, I was putting bigger main jets in the carbs trying to make it go faster, Then I checked the valve clearance to find the valves were just jigging about ! The valves were only opening about 1/8th of an inch, It went very well when i think about it. The thing with Mk2 is the tickover won't be as slow, I have heard some Comets very slow running. As with any tuning it has to be all done to get the best, The biggest thing for me is the exhaust, It is no good getting more petrol in if the gasses can't get out. You have to make your mind up if you want top speed or miles per gallon. Good luck, Bill.

 

[Bill Thomas]

Sorry, Don't forget you will have to take the Cyl' Head off to take a bit off the top of the lower valve guide when fitting Mk2 cams, Cheers Bill.

 

[Kansas Bad Man]

Someone,a long time ago when the dinosaurs roamed the earth, I was told that a Comet was supposed to go better with a rear head that a front one.He ran with a 229 mounted sideways and the correct float chamber to go with it..John

Lets see , how could that come to be? Could it be a snow job on a guys buddy that had a Comet, the guy needed a front head for his twin to go racing? Lets trade, im doing you a favor ol buddy!

 

[nkt267]

Lets trade, im doing you a favor ol buddy!

Yeah no problem.I'll certainly swap a front head for a rear one, so long as it's attached to the rest of a 1000cc vin engine..John

 

[timetraveller]

My two pen'orth regarding MkII cams for road use is dont. I have run the ex-Cecil Mills bike for years on MkIIs and many other people have also used them on the road. However, recall that these cams were meant for racing, the bike will not be tractable at low engine speeds. One can always tell if MkII are fitted when a bike is ticking over and the throttle blipped. You will see a blow back from the carburettor for 9 or 10 inches which will only disapear when the engine revs at over 2,000 rpm. It will also run better above 4,000 rpm than below that. However, if you want to go faster, rather than have a flexible road bike, then there is much that you can do to the engine, but standard compression ratio, carbs. etc. will not make best use of the cams.

 

[Kansas Bad Man]

I guess if you spend your time blipping your throttle at the stop sign or you spend most of your time running down the road at 80 knots another opinion could be had.

Max

 

[Howard]

Someone,a long time ago when the dinosaurs roamed the earth, I was told that a Comet was supposed to go better with a rear head that a front one.He ran with a 229 mounted sideways and the correct float chamber to go with it..John

Yes John, I seem to remember discussing this while eating a Brontosaurus steak. I think the idea of a rear head being better than a front one was down to "tuneability", there's more meat around the inlet tract so it can be opened out more. I don't have facts to support that, just the ramblings of an old memory.
H

 

[timetraveller]

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.