Thanks and appreciate the info. What I’m really interested in finding is the “upper limit” of the carbs capabilities.

I’m going to experiment with bolting on one of those Volvo AISN-300 superchargers on the 650 I have, and wondered how much a 24-28 could have pulled through them before starving the supercharged-650.

It’d be easy to bolt on a bigger carb but the AISN-300 is a good size for this small of a carb...in fact probably a little too big.

Doing the math so far it’s looking like a 24-28 won’t work....but that’s still doesn’t stop me from wanting to know what Weber rated their CFMs at.

Long winded explanation but a healthy discussion...

If you want to use cubic centimeters rather than cubic inches to calculate CFM the formula becomes:


CFM = Engine capacity (cc) x RPM x VE / 56633.

VE is fractional (ie: 0.85 rather than 85%)
The calculation still results in cubic feet per minute

Chris

Naples500L said:

Thanks and appreciate the info. What I’m really interested in finding is the “upper limit” of the carbs capabilities.

I’m going to experiment with bolting on one of those Volvo AISN-300 superchargers on the 650 I have, and wondered how much a 24-28 could have pulled through them before starving the supercharged-650.

It’d be easy to bolt on a bigger carb but the AISN-300 is a good size for this small of a carb...in fact probably a little too big.

Doing the math so far it’s looking like a 24-28 won’t work....but that’s still doesn’t stop me from wanting to know what Weber rated their CFMs at.

Long winded explanation but a healthy discussion...

Click to expand...

Have you considered using an S.U. carb, lots of cars that were supercharged 'back in the day' in the U.K. used S.U. Iirc, you can either blow through or suck through these carbs and you only need to play around with the jet needle to get the mixture correct throughout the rev range (unlike fixed choke carbs with multiple jets for each choke). Plus being variable choke, they can't be 'too big' at low engine speeds.

(The 'BENTLEY BOYS' used S.U. carbs on their Blower Bentleys ).

Al.

If you DO decide to try a SU carb, the HS2 (1-1/4in semi-downdraught)can be fitted to the 500/126 engines by using the same manifold as the FZD Dellorto uses---same flange measurements and same angle of fitment. You will need a "right-hand" carb (float on left when looking at it fitted to the engine--on the 500, it will be at the front of the engine). A good stating point is a 90th jet and a M9 needle. This set-up will allow you to BLOW through the carb.

Hi,
I've recently been asked to look at a friend's 1962 Fiat Nuova 500.
They bought it 'fully restored' from Italy in 2014 but it kept breaking down and has sat ever since.
I've never dealt with a 500 before, and decided to start by stripping the carb which kept flooding (float full of fuel) .
It has a Weber imb 28, which I thought strange as the engine should be 500cc for this car.
So my first question is, will a Imb 28 work on the 500cc engine or do I need to change anything?
My second question is, how would I quickly check the size of the engine, as the number doesn't match the car so has obviously been put in during the restore.
Thanks

In addition to my previous post:
I noticed whoever did the 'restore', drilled a new hole to fit the air intake elbow, presumably from the smaller carb. They made a new aluminium gasket to cover the original hole.
Why would they do this? Just as a short cut as they had the smaller intake lying around? Or is it an attempt to adapt the imb 28 to the 500cc engine?
Also, do you reckon having an open drilled hole loosely covered by the gasket will affect the carbs performance?
Thanks,

Thanks for this thread. Very useful info.

I finally decided to tackle the carb since I was suffering from the car sputtering while accelerating.

My rebuild kit has this rubber ring; I can't find what I'm replacing. Also the idle mixture screw is different from the one in the pics. Does it possibly go with that?

Also, as many others, my carb is also pretty bent, with the new air inlet gasket, I can see light, however if I use the old one as well, it solves the issue. I might have to resort to doing the same with the carb top gasket, since I can see through it (I can't use it like that right??)

If it it the air-filter elbow that is warped, it is an easy job, with a sheet of 'wet 'n dry' on a flat surface (thick sheet of glass is perfect) to flatten it. If the float chamber is constantly filling with fuel, check (a) that the needle-valve is not sticking and (b) that the float has not got any fuelin it. If it has, renew it--they cannot be repaired. Has the engine still got the original mechanical fuel-pump? As forengine type,let us have the engine type---it will be on the machined 'flat', level with the fuel pump on the rear of the engine.

So it looks like it's not the elbow, but the actual metal top of the carb that's warped. There seems to be high spots in the areas surrounding the 2 bolt holes, while the areas around the edges have worn away. It's the same with the base of the carb, I wonder if the vibrations have caused the metal to warp...

As for the O-ring, can anyone confirm that it should go on the mixture screw? And it looks like mine doesn't have an O-ring at all...so I should fit it.

turbo500 said:

As for the O-ring, can anyone confirm that it should go on the mixture screw? And it looks like mine doesn't have an O-ring at all...so I should fit it.

Click to expand...

Are you sure about that? I think the current "O"-ring is there in a groove but deceptively compressed and not looking like a ring.. Try tweaking it out with a sharp point or fine blade.

Yes I think you're right. So I should remove it and use the new ring then I guess.

:thumb

turbo500 said:

Yes I think you're right. So I should remove it and use the new ring then I guess.

Click to expand...

...yes

ben456 said:

whoever did the 'restore, drilled a new hole to fit the air intake elbow, presumably from the smaller carb. do you reckon having an open drilled hole loosely covered by the gasket will affect the carbs performance?

Click to expand...

I guess that whoever put on the 28 didn't have the correct elbow for it. Or there might have been an issue having the correct pipes for connecting to the air-cleaner. I have a metal air-cleaner lid from a 499cc combined with the 28IMB plastic elbow and it can be done with a mix and match combination of standard parts.

I don't think the spare tapped hole will be affecting anything but an inflexible aluminium gasket might. I would want to fill the surplus hole, get a proper 28 elbow and set things up properly. I've found that any imperfect connections between the air-cleaner and the carburettor can cause running problems so it's an area worth giving some detailed attention.

fiat500 said:

I have a metal air-cleaner lid from a 499cc combined with the 28IMB plastic elbow and it can be done with a mix and match combination of standard parts.
.

Click to expand...

I have the same set up

Looks like someone has gone to a lot of trouble polishing that casting but neglected the functuality side.
Those little o-rings do flatten off and petrify in time. A little lube when fitting the new one will help.

Toshi 975 said:

I have the same set up

Click to expand...

This is mine....

What was the purpose of the pre filter you get on a lot of 126 650 engines, obviously a filter but it has a dirty big hole through the middle of it but has a gauze surrounding it. Not really sure how it would function?

Vitesse said:

What was the purpose of the pre filter you get on a lot of 126 650 engines, obviously a filter but it has a dirty big hole through the middle of it but has a gauze surrounding it. Not really sure how it would function?

Click to expand...

I've never seen a convincing explanation. I once read it was a flame trap but there's already one at the rocker-cover breather. I think it looks like a silencer and if so.....what's the point?

fiat500 said:

I think it looks like a silencer and if so.....what's the point?

Click to expand...

Yes, it can’t be a filter because it’s straight through and it does look like a silencer!!! Very odd. I have one on my 650 engine maybe I should take it off see if it makes a massive difference!

Not a resonance chamber to help reduce drone / noise ?

lazylobster said:

Not a resonance chamber to help reduce drone / noise ?

Click to expand...

....or maybe a silencer.

...reducing drone? I probably need one for my bees.