Carb for Comet

[davidd]

Thanks for the kind words, but you have great experience with street singles. Very few owners use a dyno to check their tuning. I think Redbloke can get good answers on this forum.

Your picture is interesting and I have not tuned my intake tract length for lack of time. With the exhaust system I am running the engine does not like to run under 4300 RPM. I think I could have lowered it a few hundred rpms by lengthening the intake tract.

David

 

[nobby]

When I saw this picture again, I realised there is one very special modification visible. I wonder why nobody asked what it is.
Here it is again, c'mon gents!

A tip: Hartmut Weidelich and Sid Biberman made me do this...

 

[nobby]

Did I ever put up the video of the dyno test? Do not remember. But here it is, two runs, including the fresh hon. member Volkers!
[video=youtube_share;4oayTBZ1Dwc]http://youtu.be/4oayTBZ1Dwc[/video]
I am not on the video, coz' I was 'filming'. The guy sitting on the bike, helped tuning several twins before.
No better way to adjust your Supertrapp!

 

[redbloke1956]

Still no definitive answer to this question though.
What carb? what slide?? jets etc
By the way Kevin, the twin brigade didn't get in early with the 'Buy a twin' quip. John

This little pocket rocket has cost me a fortune so far....a twin would have cost me double

 

[redbloke1956]

When I saw this picture again, I realised there is one very special modification visible. I wonder why nobody asked what it is.
Here it is again, c'mon gents!
View attachment 1110
A tip: Hartmut Weidelich and Sid Biberman made me do this...

Gotta be the inlet tract extension Cornelis? Have read about them somewhere (maybe Tuning For Speed) but don't quite understand the physics yet.
Kevin

 

[Howard]

Seeing as we're talking inlet tracts. Here's a quick question - can't see it being a quick answer.

I've tinkered with exhaust lengths and inlet tracts in the past on a racing comet with relative success, but recently I read an article that says (if I'm reading it right) that the tuned inlet tract length is measured from the valve head to the end of the bellmouth. The question is, if that's right, wouldn't it be easier to make up a range of bellmouths - or a telescopic one even?

H

 

[davidd]

Howard,

That is correct. Stuart Hooper, World's Fastest Velocette, told me he started with a telescoping bell mouth that was 22". Of course, you need an eddy current dyno to do that type of on the fly tuning. The inlet tract is measured from valve to bell mouth and the shape of the radius on the bell mouth has some influence. As I understand it, it is similar to tuning the exhaust. You have to decide what "pulse" or wave you are going after and start with that legnth. The strongest pulse is the second, but it makes for the lognest bell mouth. I suspect most street bikes are tuned for the 4th or 5th pulse, which is the shortest. The calculation is also rpm dependent, so you have to add that variable into the mix. If someone says the best length is 15.25" the question is: "At what RPM?" You can use telescoping intakes and exhausts on a dynoJet, but you have to do separate runs.

David

 

[Bill Thomas]

Hello Kevin, You don't need a carb' All you need is a bit of pipe, A rotary valve and a butterfly valve + a brother named Ron to weld it up !! Just a bit of Fun, Good luck Bill.

 

[timetraveller]

Yes, I used Wal Phillip 'fuel injectors' as it was the cheapest way I could get an 1 3/8" hole. Not very good on the road but 135 mph at the end of a standing start half mile was not too bad for a road bike in the 'olden days'.

 

[Little Honda]

Hi, Chanky Bore,
I had Ernest Hegeler´s Comet for testing some months ago. I mesured wheel diameter and had a digital bicycle-speedo fitted, which gives a nice catalogue of information, especially top speed, when you are crumbling into the fuel tank and
cannot stare at the dial for minutes, as there might be other traffic around. It had a 30mm Concentric MK1 fitted and
Ernest´s homemade cams, which are more or less MKII timing figures, but with quietening ramps. Compression I did not take, it started first kick, every time. Ah well, it also had Ernest´s dyno with his own electronic ignition 32deg btdc.
I achieved 142 kph, which is some 90mph, in other words, works standard.
It was just to see, that it can do it, not to try it any time! 75mph upright seating, easily, anytime on the motorway, no
oiling, no seizing, safe running!
I think, a Comet, which cannot exceed 75, isn´t adjusted correctly. Remember, that was the topspeed of a 250cc NSU- Max of the early 1950s, so it wld b a shame for a Comet!
Good luck!