SOHC.co.uk Forums > CB350/400
Need larger main jet?
K2-K6:
--- Quote from: Mikep328 on September 09, 2024, 10:43:08 AM ---" Max. speed testing years ago involved involved such things as unbelievably high tyre pressures, rear chain de-greased and given a light dose of 3 in 1 oil, the rider wearing tight leathers and laying flat on the tank, feet on rear footrests, left hand holding top of left fork leg, and peering between the clocks."
I guess this would do it...https://silodrome.com/wp-content/uploads/2018/07/Rollie-Free-Vincent-Black-Lightning.jpg
--- End quote ---
Theres a book about the man in the photo "Rollie" in going into that speed attempt .... review here https://thevintagent.com/2017/09/02/flat-out-the-rollie-free-story/ that seems an interesting read.
My parents were personal friends with Vincent head of engineering and his wife when the bikes were contemporary, with many stories around them and racing them too.
McCabe-Thiele (Ted):
I am not advocating this as an answer Mike - if you are running the standard air cleaner system there are adjustments that you could make with the needle rather than changing any jets - raising all four needles on the sliders would richen the mixture at any given throttle opening. (i.e.one groove lower than the mid point effectively raising the needle in the jet)
There must be a reason why they put five grooves in the needle - might be for those living at high or low altitudes.
K2-K6:
--- Quote from: Mikep328 on September 09, 2024, 08:43:25 AM ---Thanks! Good info! Maybe I'll hop back on my 900 Kawasaki to go up that hill! ;)
--- End quote ---
As the American saying goes "there ain't no substitute for cubes" as a certain amount of torque will only be able to overcome a set amount of resistance.
You're just seeing the finite tail out of tge 400 here in this example, the 900 and anything else will do it too, just at much higher velocity.
Also a characteristic of slide type carbs, in that if the torque available can't pull it of of the "hole" then further opening of the slide ultimately decreases vacuum in the carb throat, such that they would be forced lean under these circumstances.
The engine rpm " following" the slide up, think big torque motors, is keeping that equation in check. As soon as that breaks down and the engine cannot keep adding torque, then the "gap" between the two gets progressively wider, dropping the absolute vacuum as the hole is bigger in the venturi but air speed through it is not increasing, in reality air speed there drops under this scenario. Bernouli principle I believe .... http://hyperphysics.phy-astr.gsu.edu/hbase/pber.html :)
CV carbs seek to avoid this, their biggest attribute in this circumstances as properly designed they should keep the vacuum level to match the intake "suck" the engine is presenting to them.
When an engine is run on a dyno too, they don’t use a "terminal" gearing ratio generally, for example 3rd gear often engaged for a recorded pull as it then shows in graph terms the whole power curve, up to peak output and then on past that to observe the decay of dynamics as it goes over peak efficiency and starts to drop again.
K2-K6:
--- Quote from: McCabe-Thiele (Ted) on September 09, 2024, 12:08:08 PM ---I am not advocating this as an answer Mike - if you are running the standard air cleaner system there are adjustments that you could make with the needle rather than changing any jets - raising all four needles on the sliders would richen the mixture at any given throttle opening. (i.e.one groove lower than the mid point effectively raising the needle in the jet)
There must be a reason why they put five grooves in the needle - might be for those living at high or low altitudes.
--- End quote ---
Its indolvable Ted as its not the jetting doing this.
Its too high a ratio for the torque available to pull, but if you sent it downhill in that gear (removing some resistance effectively) then it would carry on rising in revs.
Raising the needles will just make it rich for all of the general running scenarios other than this one. It's set correctly as it is.
Needle adjustment is there for scenarios you described though, primarily altitude. It would also be unacceptable to produce a carburettor with no adjustment too, then expect a long service life out of it. It just must have some level of compensation built in to cover reasonable eventuality in its usage.
McCabe-Thiele (Ted):
Not sure where Mike lives - not on top of K2 for sure.(pun?) ;D ;D ;D
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