Weber 40IDF carbs - best choke size

[vcg]

Hello Guy,

Some people claim that 40IDFs with 36 chokes and modified aux venturi (cut in order to look similar to the 44IDF aux one), flow significantly more than with the 34 chokes on. On the other hand, other people claim 36 chokes don't work on 40IDFs, and the best flow is with the 34 chokes on.

Have you experimented on that? Does it work? If yeswith what emulsion tube?

Thanks,

[Guy Croft]

The carb flow will obviously be higher with a 36mm primary choke than a 34mm, bigger diameter flows more air.
I would not make any changes to the design of the chokes primary or secondary without flowbench testing, and I'd certainly be concerned about the 'signal' in either case, ie: what the level of vacuum was in the smaller secondary choke, because if alterations succeed in reducing the vacuum, the fuel is not going to come out or atomise so effectively. It is well known that having two chokes staked up increases the depression in the barrel, and the orientation of the two chokes is critical.

Ultimately whether a 34 or 36 primary choke is better on a particular enginehas to be dyno tested, and yes, I do know Weber list 36mm chokes for the 40 DCOE so there is no reason in principle why they should not work in the 40IDF. Be aware that the bigger the chokes the more 'aggressive' the cams have to be (lift, LATDC, overlap) to keep high airspeed thru the carb, and if the exhaust and inlet lengths are not right you won't get the best anyway.

If anyone says 'do this or do that..' ask to see their dyno results before/after.

GC