SOHC.co.uk Forums > CB750
Carb jets
Spitfire:
I don't know if you have seen this but it may be handy,
http://www.motorcycleproject.com/motorcycle/text/specs.html
Cheers
Den
K2-K6:
Den, that's a really useful collection of information and helpful to quite a lot of the people on here.
Greame, that could be arguably the best way to set them (using a gauge) intially as it is physically correct and will not take into account of any other irregularaties so assuming the carbs are near enough correct it then allows a good platform to work through any other stuff.
The slight increase in idle can have another benefit whch is often overlooked with these bikes which you'll feel when you ride it more. When you close the throttles completely i.e. slowing into a corner, with the carb slides right down at the bottom and engine revs up then the vacuum in the carbs makes it quite difficult to smoothly roll the throttle back on. The effect you get is it feels unduly resistant then, as you put more effort into it suddenly you snap to more opening, it's not dangerous but a bit difficult to get it smooth. If the idle is slightly higher it minimises this feel and allows a smoother operation just not enough to feel it's pushing you when closed, at a guess 1100/1200 for low speeds maybe 1300 if you used it faster for a longer period. It has a much cruder action than some modern CV carbs which natuarally damp this feeling for you.
Graeme77:
Good point about the idle speed.
I have another pair of carbs of a slightly different design. I believe they may be later F2 carbs. Not sure if there 100% complete, might be missing jets. I have 3 brand new genuine floats for these too. My other set are earlier. Ill get the numbers off the carb bodies later and check.
I suppose what im getting at is, are the later design carbs any better?
K2-K6:
The F2 carbs step to a different strategy for running the motor at varying speed and load.
They are set to run as lean as possible in a steady state constant speed (probably this era saw the first real steps on bikes to clean up emissions) and are supplemented by an accelerator pump that operates as you move the throttle open more. It just pumps more fuel in to accept the load you are trying to put out so instead of going lean as the slides come up a spray gets it temporarily rich to facilitate the acceleration phase.
They also have a cable operated choke that gives rich run to start but pushed in half way will run the motor at about 2000rpm with mixture backed off to avoid making the bores too wet (you can see the change in intent from the old set-up and overlaps what we were saying about contaminating the engine oil) and also helps to get the engine hotter faster, Has benefits to the exhaust too as it runs dryer like this and gives less chance to help rot the pipes from the inside.
The F2 motor has bigger valves, wilder cam and higher redline so the spec jetting for the F2 carbs may not be entirely accurate for previous engines, guess you'd have to run a set-up to correct if not ideal.
Probably not much different from a rider point of view, maybe marginally snappier if you wack it open quickly.
Graeme77:
Thanks, ill no doubt see if there complete when i get a chance, whether ill ever try them or not i dont know.
Maybe one day, out of curiosity :)
I also have perverse thoughts about fuel injection though ;D
All a way off though, first things first, get it running and ride it for a good while. Got to get it reliable as we've just booked our IOMTT 2012 and the cb will be our transport.
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