I am tempted to go down the same route as Romus and Higgy with the modification to the Weber on my 906.
I am trying as many places as possible to track down the Auxiliary 4.5 venturi with no success to date. I have tried the potential suppliers mentioned in the article and also any other Weber dealers that I can find.
Does anyone know a dealer that might still have a pair stuck away in a cupboard gathering dust.
I could try to modify the emulsion tubes using the existing venturi's but I am not confident that it will give the improvement that I am hoping for !

The 4.5 auxilliary tubes came standard in many Maserati and Ferrari and yes they are very hard to find
I think at this point you would be better served by doing the pump cam profile and then just bumping your jetting to work better with todays fuels

Higgy when you say "doing the pump cam profile are you suggesting this".

"Accelerator Pump jet can be dropped down from 40 to 35, so that it will drop down to mid 11s to low 12s.
That is a lot better. It does not supply less fuel but squirts it as a smaller jet over a longer period. To go lower
than a 35 jet will require changing the pump cam".

I was amazed at how much fuel is pumped when you twist the throttle.

My bike at the moment seems good above 3000rpm but hesitant and even jerky below 3000rpm which I thought would be related to the idle jet circuit and also the size of the venturis.

No there is an article in the FAQ on modifying the cam profile to lessen the pump action if I remember correctly it was J's work Try doing a search on the weber what if ?



thread starts here


Long interesting read may save you some trouble in the long run

Try here: http://www.carburettorspecialists.com/

They are located at WITS-END, ULVERSTON ROAD, ASHINGDON, ROCHFORD, ESSEX. SS4 3JT

Their web site has a lot of parts listed for a variety of carburetors, including Weber. Under the Wber tab on the online Stock List, a quantity of (10) 4.5 Aux Venturis for Weber DCNFs are listed (P/N 70503.45O), but it would be best to e-mail to verify that they actually have them in stock. Note that their P/N ends in "O" not "0", which is not a correct number, but important when you request that that part number from them!

Thanks for the help. I tried the carburettor specialists a few months ago but I didn't have a part number so he wasn't sure if he had them. He asked me to measure the venturi which I didn't have access to, so left it there. I looked at his part list a few days ago and found it a little confusing but after your heads up I perservered and found the entries you were talking about. I have sent an email and will wait for a response.

You will only find them using the Ferrari part number which I have here some where just have to find it




Found it
4.5 tall auxiliary venturi you must look for Ferrari part number 31966.008 which came fitted to a 40DCNF Weber. I found my ..

Higgy, I assumed that the part for the 44 DCNF would be different to the one for the 40 DCNF ?

auxilliary tubes and chokes are common to most DCNF's

Now I am confused. The part number for the 40 DCNF aux venturi appears to be 71107.450 whereas the one that keeps coming up for the 44 DCNF is 70503.450. There does not seem to be a problem getting hold of the 71107.450. There must be some difference otherwise wouldn't they have the same part number ?

I recently managed to pick up a pair of 4.5 Aux Venturi's cheap, which is always good.

Is it worth just changing the standard ones for these taller ones without changing anything else, because in the document written by Romus and Higgy "Weber mod and jetting for the Ducati 906" in section 4.2 they state that......

4.2 The Auxiliary Venturi (smaller upside down T-shape venturi slotted into the big carb intake venturi)
increases sensitivity to the vacuum based atomization process from air flow. It controls the rate of fuel
removed from the float chamber at the ET having the same effect as the diameter of the ET. Changing the
3.5 to a 4.5 leads to a significant improvement on the 906. It will take out transition stumble on its own as
long as other jetting is not badly out.

Has anyone tried just changing the Aux Venturis and did it give the suggested improvement ?

The idea of the 4.5 venturi was borrowed from Weber DCNF's used automotively,Maserati,Lancia and Ferrari all had used them very successfully. Difference being their engines mainly peaked around 5 to 6000 rpm. The ducati on the other hand spends most of its time between 4 and 8000rpm.
Initially the 4.5 provided much better response around the idle transition with the appropriate jetting changes

It also worked well at small throttle openings all the way to redline.
At large throttle openings with speeds slightly over 100mph it became apparent it was near impossible to calibrate the main jet.

It was about that time the idea was presented that modifying the pump circuit went a lot further in helping the low rpm transition stumble caused by modern fuels. I would suggest starting there first m2cs


There is a lot of variance in the DCNF main bodies relative to the transition circuits. Unfortunately changes in this circuit are more than most can handle as it requires very delicate removal of body plugs and drilling and assumes no air leaking from the throttle plate shaft bushings

OK Thanks for the advice Higgy. I think for the moment I will just get used to the bike and enjoy driving it about, despite its low RPM stumble. Perhaps I will just avoid towns! At some point in the future I will have a go at modifying the pump circuit, always assuming there is not a new theory for tuning the Weber by then, and at least then I will have had enough time riding the bike to be able to decide if my modifications make an improvement.