Carburettor Sizing

[GNWT]

I am tying to get a bead on sizing a carburettor to an engine.
The engine is a 500cc single four-stroke cycle reving to 8200 rpm.
Currently it is fitted with a 41mm Gardner devise but a change to a VM 40 Mikuni has been a recomodation. Unfortunately the VM40 is no longer made and the next size is the VM44.
I did a few sums on gas speeds thro' the carb.,(89.5 m/s at 8200 rpm)
I have trawled thro' various books and papers but do not seem to find any suggestions. I have got a copy of G.P. Blair's book but not found much on carburettors!
Am I looking at the wrong criteria on which to size the carb. and is there something else I should look at, or should I just 'suck and see' and keep the results as an unanswered question?
Any help would be of interest and perhaps generate a topic for discussion.
Gary

 

[Tony Eeds]

First ... welcome to DRN :)

Second ... we have a guru that can likely fill you in with what you need to know and the rest of us can learn from his answer.

I am going to pop this over to the forum on modifications.

 

[helio lucas]

gary, you´re welcome.

some years the jap´s state that the carb size was not a matter of cc´s but power, either a 500 or 125 engine if produce the same peak power need more or less the same carb size.

2005 kx450f uses keihin fcr 40
2005 kx250f uses fcr 37
husky sm610 uses fcr 41
kawasaki klx 650\klr600 keihin cvk 40
ktm 950 twin keihin cvr 43
yamaha ttr 250 teikei 30
yamaha yz250f keihin fcr 37
yamaha yz450f fcr 39
yamaha xt 600 uses two teikei 27mm each if i correct remember. it´s a single and revs easly to 7000rpm.

the carb size is largely dependent on the performance of the engine, not only the displacement (wrong word employed here) and rpm. the type of the carb itself has a large influence on the choice. not to mention the engine type and even it´s flow capacities and the use.

it seems to me that engine is from a vintage bike. a vm mikuni is a old carb by todays standards. if you look for power and response the keihin fcr might be the way to go.

a very nice book to that kind of help you look for is "the four stroke performance tunning" from Bell. is a great book and not expensive at all with great sugestions on performance modifications. i own a copy of this book among others by the same author. it was his book on two strokes that started my real interest for engines.

http://www.amazon.com/Four-Stroke-Performance-Tuning-3rd-practical/dp/1844253147/ref=pd_sim_b_2

BTW, i am not the guru that tony eeds was talking about ;)

 

[helio lucas]

now that i am in home i give a look to the book and it states for semi-race of 500-650cc engine a 35mm mikuni carb is fine and for the same engine but with the capabilities to spin at 8500 rpm a 37mm is the ideal