40 DCOE with 36 chokes

Competition engines and ancillaries - general discussion
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GregS
Posts: 3
Joined: July 13th, 2009, 6:51 am

40 DCOE with 36 chokes

Post by GregS »

Hi,

Does anyone one have experience with 40 DCOE Webers fitted with 36mm chokes, the largest available for this size.

My concern is that the 4.5 auxiliary venturis have a long, thick walled body with an internal diameter of about 37mm, being only 1mm larger than a 36mm choke (main venturi), I have doubts about how the venturi can work effectively to draw fuel out of the nozzle. There is also a big step from the 37mm bore in the aux venturi to the 43.8mm diameter of the choke entry radius.

Do you know if there are auxiliary venturis specifically made for use with large chokes, with a body internal diameter of about 40mm or more. (the bore in the carburettor itself at this point is 45mm dia). Even better would be ones that have a relatively thin narrow band that holds the auxiliary venturi, like the ones in the 45 DCOE. I find it hard to believe that this is how they are meant to be run with the 36 chokes, I wonder why Weber changed the auxiliary venturi design from the 40 to the 45 DCOE.
If I can’t find any, I may have to machine out the bore of the ones I have, but it will be very fiddly, and I will have to be careful not to make the wall too thin as they are a die casting, not metal the full dia.

In hindsight, I should have bought twin 45 DCOEs with 36 chokes, but I thought the 40s would work better on my 1500 X1/9 engine and give good torque and flexibility. The intake port is only about 31mm dia, so why use a 45mm dia carburettor. Anyway, I have the 40s now, and I don’t really want to get smaller chokes, as 36 seems about right for a 1500 that I am aiming to develop 130+ bhp at approx 7,300 rpm, (Big inlet valves, Piper BP300 cam, 11:1 C/R ) it will be used mainly on race circuits and hillclimbs.

thank you, Greg
Mine don't have the outer end machined like these to fit ram tubes
Mine don't have the outer end machined like these to fit ram tubes
40 DCOE aux vent.jpg (44.83 KiB) Viewed 4596 times
Std 45 DCOE auxiliary venturis
Std 45 DCOE auxiliary venturis
45 DCOE aux vent.jpg (42.11 KiB) Viewed 4596 times
Attachments
40DCOE aux venturi and 36mm choke (parted slightly to see internal diameters)
40DCOE aux venturi and 36mm choke (parted slightly to see internal diameters)
36 choke+Aux Venturi.jpg (40.01 KiB) Viewed 4596 times
Guy Croft
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Re: 40 DCOE with 36 chokes

Post by Guy Croft »

Some good reasons there for not going bigger than 34 or 35mm on 40s...

G
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