What effects does the size of the choke have with twin 40 dcoes

In simple terms

Small chokes, hi gas speeds good lowdown torque, poor top end.

Large chokes, low gas speed, poor low down performance, not necesarily good top end.

With multi dcoes there is no substitute for a rolling road setup by a weber specialist, who has spares and can experiment with choke, jet, emulsion tube etc. settings.

Also DCOE bodies come in many types with different progression holes to suit different applications.

Google DCOE Choke and Jet size, you'll start to understand the minefield.

[Edited on 31/5/13 by r1_pete]

Pete is correct - the main thing is to get the right choke size to match your particular engine set up/state of tune etc. Probably as Pete says by consulting a good rolling road tech.

Would a difference of 1 mm make much effect?
I've been told 36mm is best but I have a set of 35's

Choke (venturi) size also has an effect on the engine speed the carb transitions from idle jet (really slow running jet) to main jet. Normally the transition from the what Weber call the idle jet to main jet takes place capproaching 3,000 RPM.



A small change in venturi size really dosen't make a huge difference unless you are after ultimate power it is better to err on the smaller size.

If you are skint Weber DCOE main venturis can be bored out on lathe ---- something to watch out for when dealing with carbs you don't know the history of as the markings may not match the hole size.

Also the closer the venturi gets to the nomminal carb body size the less the gain in potential gas flow ie changing from a 32mm to 34mm venturi has more effect than changing from a 34 to a 36 on 40DCOE carbs.

[Edited on 31/5/13 by britishtrident]

The Venturi effect is critical on webers, and there is a maximum choke size for them, any bigger and the gas speed over the main fuel feed is reduced
I believe that the maximum Venturi size recomended is 7mm less than the DCOE carb number.
40DCOE = 33mm max
45DCOE = 38 max
48DCOE = 41 max

The reason why 40 DCOE carbs are only good for a standard Pinto and 45's are recomended when it has been modified with bigger valves and cams