Here's a question for the carb gurus When you swap from 45s to 40s or the 40s to 45s do your jets etc change as well. I'm just thinking that the choke size could stay the same therefore the amount of air does as well or am I completely wrong? Not thinking of doing it, just a question???? :-D
The size of the throat directly before and after the choke will alter the rate at which the air pressure ( vacuum ) will alter and hence alter the venturi effect at the main jet resulting in a different rate at which fuel is drawn into the carb for a given engine speed. As the diameter of the butterfly valve is also larger or smaller the rate that which air is sucked past during idle will alter the venturi effect on the idle valve resulting in a different jet required. A different carb may result in plastic floats or brass floats and the need to use a different tube due to fuel height differences but you could alter that i suppose.I would think the pump jet could remain the same. Using different trumpets on a given setup will alter the way a carb works and move the output of the engine toward more hp or torque, short generates hp whilst long tends toward torque.Carbs while quite simple in operation but require years of experience to tune properly and are quite an art to get right i reckon, its why few people run them and most complain that the state to tune requires to be altered as the tune wanders to goes out?? not true if done right the first time. The lack of a connection for a laptop has a tendency to put people off.
I remember the time when tuners were put off by laptops and EFI! Once they discovered the benefits, they never looked back! Times do change!
I think as a starting point you would get away with them being the same Though as stated above you will possible need different air correction and idle jets I have a book on dcoe carbs somewhere though not seen it in years Some very good info on line if you look mostly people with old fords still use them I have a set of twin 40s stat in my old man's loft off a 1.6 crossflow
I did some research last night and looked at the old set up in my 45's It was fitted with 36mm chokes which I reckon are too small for a KR engined rally car but I'm no expert. I would think that i would be able to get the same performance out of 40's but i suppose i would only know with a bit of trial and error.
aye had a look at that before here is the dcoe one http://www.s262612653.websitehome.co.uk/DVAndrews/webers.htm
