Help identifying a part, Dellorto throttle plate in Weber carb?

[Colin Byrne]

So I've always had problems getting my car to idle slow enough, I've gone through checking all the spring returns, accelerator pump springs and re positioned the throttle plates many times, but without much improvement.  I decided to try swapping over the throttle plates, so purchased a set of new ones and very carefully installed them, being very happy with the final fit.  I also replaced all the idle adjustment screws, as the old ones did not have the correct o-ring and washer arrangement. 
Anyway I started the car and let it warm up, and after idle screw adjustments I \ had the car running at around 12.8 A/F on idle on all 4 and could adjusted the RPM down to whatever I liked, to the point of stalling if I wanted (so massive different).  I also noticed the mixtures had a much more defined transition from the idle jets to the progression holes that I'd seen with the previous plates
When comparing the parts, the old throttle plates have 2 machined grooves on the edge of the plate, that correspond to the position of the idle jet transition holes similar to the attached picture, the newly installed plates did not.  After a little bit of googling the only plates that I could find that have this groove was a dellorto.  Could a set of dellorto plates have found their way into my webers? Does anyone know what the point of these grooves is?

[LaStregaNera]

They're intended to speed the transition off of the idles I believe - I know people who've machined the throttle plates on big downdraft webers the same way (quad 48s on a 253) - it's supposed to improve the way they behave off idle when the engine is on the small side for the carbs (This is from memory of a conversation back in the late 90's).

[Colin Byrne]

Thanks, yea I figured they were there to influence the transition of idle jets, but they seemed to be doing more harm than good in my case (crazy fluid dynamics!) so do we think this was a Weber option? Because these ones are definitely done from the factory.

[1750GTV]

Do they have a number stamped on them?

They look very much like Dell'Orto butterflies.

Chris

[Colin Byrne]

Hi Chris, they ones I have have the angle stamped in them, which leads me to think they are webber parts. From what I've found on google the dellorto plates have a part number stamped into them?

[1750GTV]

Yeah - I guess that is why I asked.

My experience isn't vast, but as you say Weber plates have the chamfer angle marked on them whereas Dell'Orto plates have a part number stamped on them.

Also didn't realise that they were interchangeable.

Chris