TheSamba.com :: View topic - Dellorto venturi O.D. sizes? 36,40,45mm interchangable?

Sun Apr 22, 2007 9:23 am

Anyone know the outer diameters of Dellorto venturis? Are they different for each size of Carb? (36,40,45) It seems to me that 36, and 40mm carbs had interchangable venturis but wondering about the 45's. What about the aux venturis, are they specific to the carb size? Was looking at the CB Performance update kit for the 45's but 38mm vents would be too large for my application and was hoping the update kit for the 40's (34mm vents?) would fit in the 45's instead. Teach me.

Squarebob.

Sun Apr 22, 2007 12:40 pm

36 and 40 venturies are interchangeable.(the only minor difference is a small step near the butterfly on the 36, but there is no difference when you use it)
i have never tried to use a vent for 45 in a 36 ou a 40, but the outside diameter is the same in my memory.

what is sure, it's you can't use a 40 update kit in a 45mm carb(the auxiliary vent is different).

about the body of the carbs, between the 36, 40 and 45, the difference are the machinig for the butterfly and the depth of the transition holes, above the butterfly.

Wed Apr 25, 2007 6:56 pm

What is different with the aux vents that makes the update kit for the 40's unusable in the 45's? Is the outer diameter here different? Still a little confused. Maybe I should just talk to the guys that sell the kits!

squarebob

Wed Apr 25, 2007 7:08 pm

The bore is larger, and the aux vent is winged instead of barrel style. Totally different setup.

Wed Apr 25, 2007 7:14 pm

Vents interchange from 36's-40's-45's..... 48's are bigger. Huge.

The C.B. upgrade kits are actually a downgrade in quality. Cheap Charlie.

They fit 36'-40's same (european 36 and 40's are the winged style)

45's are bigger (winged style).

Wed Apr 25, 2007 8:08 pm

Makes sense, thanks!

squarebob.

Sat Dec 27, 2014 12:07 pm

Sorry to dig up a 7yr old thread but....

What are the differences between an auxiliary venturi for a drla 36 and drla 40? Are they interchangeable? I have 36's with that CB update kit that I want to return to stock, but I'm having a hard time finding auxiliary venturies for the 36's.

Sat Dec 27, 2014 12:10 pm

They are the same...

Email me via one of my ads if you want a set of Aux venturies.... I think I have a few extra sets.

Sat Dec 27, 2014 2:10 pm

There are three types I know of, round, round with most of the round part cut off, and one winged like 44 idf that fits into slots in the carb.

I expect all 36 drla use the round ones!

I have two spares but sorry not four

Sat Dec 27, 2014 3:07 pm

The American Imported 36's and 40's (CB Performance) had the full round style.

The European OEM factory Alfa Romeo style had round bores with squared broached ends that took the winged style (with an o-ring).

If your bore is round, it takes the full round style.

Sat Jan 28, 2017 9:22 pm

1200RS wrote: 36 and 40 venturis are interchangeable.(the only minor difference is a small step near the butterfly on the 36, but there is no difference when you use it)
i have never tried to use a vent for 45 in a 36 ou a 40, but the outside diameter is the same in my memory.

Here in this photo of the Dellorto DRLA 36 is a CB-Performance 28mm Venturi that I just installed. The 30's I took out covered this step. Does anyone Know if this will be a problem.

Looking Down at the CB-28mm venturi with the auxiliary venturi removed:

Sun Jan 29, 2017 12:37 am

If the trialing angle should be 5-7 degrees
I haven't done the math, but probably, they have exceeded 7 degrees.

Another check:
also set the aux vent on top of the venturi, and kind of slide it over to the side, the bottom of the aux vent should be at the even or 1mm below the smallest point of the venturi. They might have got that wrong too

Sun Jan 29, 2017 4:02 pm

If I understand this correctly the small venturi tube in the center of the throat of the carb is the Secondary Venturi. It extends quite long. I would say 6mm or 7mm further down below the smallest dia inside the 28mm choke. The problem as I see it here is that the CB 28mm choke is tapered out all the way to the bottom where it's ID becomes too large. Hinse the step in the picture above.

Looking Down

Looking Up

Sun Jan 29, 2017 4:11 pm

Wow that's very far from right. It will flow bad AND have weak signal.
Don't run those.

The old CAST venturis CB sold before were butt ugly, but a lot better than this.

Sun Jan 29, 2017 6:09 pm

somebody asleep in the QC department.
I just checked, the stock Dell 28mm vent doesn't even come close to the bore of a 36 DRLA; it easily covers where you have the step showing. The bottom of a 34mm vent is close to the bore where the step is, it ends almost flush where you have the step. Have you measured? What is the inside diameter at the bottom of those 28's?

Sun Jan 29, 2017 6:40 pm

esde wrote: What is the inside diameter at the bottom of those 28's?
They are 40mm inside the outside is 41.8mm and a loose slip fit, looser than the Auxiliary Venturi fits.
Yeah, these are what I ordered. Maybe I have them upside down? That would move the 28mm choke well below the Main fuel Nozzle and the Aux vent too.
I will call CB-Performance tomorrow. They will probably try to sell me the Up-date kit with the horizontal discharge tube, but that seems to only be available for the 40mm carbs.
Dellorto 36mm carburetor venturis:
The original 30mm venturi's have the choke at 12mm down from their top.
The CB 28mm venturi's have the choke at 8mm down from their top.
The Aux Venturi extends 15.5mm down.

Sun Jan 29, 2017 7:22 pm

I gotta be honest, I don't know it how far down the constricted part of the Venturi is supposed to be. I do know that it's the highest velocity point for the incoming mixture, and the lowest pressure spot, so it's relationship to the air horn on the auxiliary vent is what draws fuel from the main circuit. Install it upside down and watch the engine fall flat on its face above idle. And, the curve of the venturi matters. It's not just a tapered part with a rounded edge, there's a formula for the taper, much like an airplane wing cross section.
But yeah, those aren't going to work.

Sun Jan 29, 2017 7:52 pm

Danwvw wrote: Maybe I have them upside down?
no, it's just totally wrong.
Been like that for some years now.
I've bought some for 40 and 45 dells, but I order 2-4mm smaller and machine them correctly myself.
Sorry you were not aware of it.
I assumed it was a temporary goof, and they'd get it fixed.......any day now, any year now?

Tue Jan 31, 2017 1:34 pm

Here is my thread on the engine I put these carbs on, I did install the 28mm venuris and fire it up last night. Single Zenith NDIX 32 to Dual 36mm Dells

Sat Feb 18, 2017 8:52 pm

Ok Here are some photos of a real Dellorto DRLA 28mm venturi and a CB-28mm venturi "front": The choke of the real Dell 28mm venturi is 12.5mm down from it's top!
Apparently Not all Dellorto Auxiliary extend the same!
The auxiliary venturi shown here extends 16mm down.

and
Here is the Dell 28mm venturi and a auxiliary venturi together.

There is no step left where the venturi sets with the Dell 28mm venturi: