Weber 40's to 46mm?

EarlySport · 06-18-2009, 05:18 PM

I have heard bits and pieces in the past about opening up Weber 40's into 46mm carbs. I was having a look at a spare Weber 40IDA body yesterday, tape measure in hand, and noted the entire upper half of the carb, including the bellmouth stacks, are 46mm ID. The carb steps down to a 40mm ID near the throttle plates, right below the venturi's.

So my question is, is the only difference between a 46mm Weber IDA and a 40mm IDA the bore size below the venturi? And therefore by boring that area to 46mm and fitting suitable venturi's a 40mm carb could be converted into a 46mm?

I have never seen a 46IDA in the flesh, and maybe a 46 has a 50mm or larger upper body internal diameter..

Your expertise and opinion is appreciated.

docrodg · 06-19-2009, 04:08 AM

you can do this, but a 46 weber is only good for really large displacement engines that run at high rpm. A larger carb can actually kill performance on an engine by reducing air velocity causing more reversion and less mixture volume getting to the cylinder due to the low velocity. Case in point... in college we had a nice dragster, 77 el camino with a 540cc chevy and flow bench ported heads etc. with only a 800 4 bbl it ran great, any more carb flow and it lost HP as shown on the dyno.

Steve@Rennsport · 06-19-2009, 08:17 AM

Just remember, venturi size plays the biggest role in airflow at all throttle openings. This allows you to tailor the carb configuration a bit to each engine's displacement, camshshaft profile and RPM operating range.

The main limitation to using a 46mm Weber (with smaller venturies) on smaller engines in relatively mild tune are the somewhat crude, idle and progression circuits. This is another place where PMO's are FAR superior,.....

ajwans · 06-19-2009, 04:06 PM

Doesn't a 46IDA also have the taller secondary venturi inside each barrel?

andy

EarlySport · 06-19-2009, 05:37 PM

The carbs are destined for a 3.2SS 10.5.1 motor with 3.2 heads and DC62 cams that will mostly see track duty, hence my desire to maximise upper rpm horsepower, and hence why 46mm carbs I beleive would be the correct choice.

If I do bore the lower carb bodies to 46mm, and given the above engine specs, what would be a good starting point for venturi's? I have a set of 36's - should I go bigger?

The carbs will be on PMO tall manifolds.

My reasons for modifying these carbs are also driven by the fact that I have to fully rebuild them, including oversize throttle shafts. the 46mm mod is a while-I'm-in-there type of thing.. :-)

Steve@Rennsport · 06-19-2009, 05:53 PM

I'd agree,....46mm carbs are the correct choice for your motor.

One would need to know something about your local temperatures, humidity, elevation, as well as whether you have close-ratio gears to offer specific configuration and jetting information,...

38mm venturies would be one place to begin, contingent on the above.

Eagledriver · 06-20-2009, 11:48 AM

Yes they can be modified. I had a set that had been modified. They actually overbored it to about 50mm and then sleeved it back down to 46mm. They drilled the progression and idle holes in the sleeves. I think Beiker (sp) can do this.

-Andy

jsoderbe · 07-31-2010, 11:27 PM

Hi,

I found this thread while searching the answer to the same question. Did this bore-out work? I have a set of 40's that are toast, especially the lower half. Either I throw them away or I bore them out. I don't have any use for them right now, but maybe down the road.

It would be a great help if someone could tell me what parts need to be machined, and to what sizes? I have access to all possible machines....

Is it hard to find the venturis and jet's to 46:es?

Thanks,

Johan

Walt Fricke · 08-07-2010, 08:54 PM

Johan

It is not hard to get Weber venturis or jets for the 46s.

tknobby · 08-16-2010, 02:06 PM

Bieker does not answer the phone... out of service.

Anybody have updated info on them?

Thanks,

Tom...

jsoderbe · 08-16-2010, 09:51 PM

Hi again,

I've made some effort to find more information;

My 40:s are actually 46 mm in the upper half, but then maybe 46:s are larger in the upper half?

Also, I think it is the idle holes that need to be adressed, which can be made with sleeves as stated above.

The thing I don't get is how to make the inserts where venturis and jets are mounted? That hole is impossible to machine with those sharp corners. They have to be "sparked" (I have no idea what that machining is named in the US, where you use current and a mirror image part to machine).

I'm giving my one more try to fix them up. I managed to get the axles out, but they were corroded beyond repair. Also, I dented two of the throttle body discs, but they can be purchased new. And I can't get the last 4 venturies out, but I'm sure some diesel will loosen them up.

I'm just thinking it might be too much work, even though I have all nessecary machines....

Thanks,

Johan

1QuickS · 08-18-2010, 06:08 PM

I make 46's from 40's as well as make 42's. I have a method for reliably plugging the progression port left when boring to 46mm and when I locate the progression holes I do so by indicating off the throttle shaft on each side of each throttle bore. Here is a link to a previous post showing a set I made:

46mm Webers for 68S Historic Racer

Paul Abbott
Performance Oriented