Mr Fiat do new carbs of both 26 & 28 mm size

1 way of sorting the warped base (if you haven't access to a 'flat-plate') is to get a sheet of reasonably thick glass, which will be absolutely flat, tape a sheet of 'wet-and-dry' (emery cloth) to it and very patiently sand the base flat--it is a slow and tedious job, but is cheaper than a new carb.

Wow man! That is pretty bad. A new one would obviously be better, but if you could find a machinist he might be able to take and shave some metal from the center to make it a bit flush. By looking at the picture it looks like a rocker. Take some material out of the center and the ends flush out.

I've used the glass and wet and dry method (lubricated with WD40) on things other than a Fiat carb and it is very effective but tedious. That one really looks too far gone. If the carb is otherwise good, you could build up one side of the flange using epoxy metal, file to rough shape and then use the glass method. Trouble is that you might block important little holes or passages and the carb would have to sit at an angle.
Most people aren't as mean as me and would just buy new or refurbished.

I'm going to give it a shot and try to sand it down a bit in the center. I'm sure I can get it to better than it is right now. Then I can double up on that gasket and hopefully that will take up any gap remaining. If not, I will just get a new or rebuilt one.
I bet this is the reason why I was having a heck of a time getting the engine to idle correctly!

I got it pretty flat with a file and some sandpaper. Doubled up on the gasket as well. I will follow up on how it worked.
I followed the gasket change procedure here and made finding all the gasket locations a breeze. Thanks for this thread!

Change of subject. Would a 126 carb be better than the standard 500 l carb if nothing else is changed on the engine? That is to say would I get any noticeable gain in power? I am asking because my mechanic just rebuilt my engine while I was gone on deployment. He brought everything back to original to include the carb. I have the 126 carb and was wondering if it would be worth refitting.

All good! No air leak or gas leaks. Purrs like a kitten. Idle is smooth now as well. Thanks for the advice guys.




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Luxe said:

Change of subject. Would a 126 carb be better than the standard 500 l carb if nothing else is changed on the engine? That is to say would I get any noticeable gain in power? I am asking because my mechanic just rebuilt my engine while I was gone on deployment. He brought everything back to original to include the carb. I have the 126 carb and was wondering if it would be worth refitting.

Click to expand...

Any thoughts on my carb question?

Re: Butterfly and spindle

Chris,


When you replaced the starter/choke cover, did you use a gasket between it and the main carb body? If you did, which did you use? The rebuild kit doesn't seem to be supplied with a gasket specifically for this.


Thanks,
Matt

Re: Butterfly and spindle

MattGibson said:

Chris,


When you replaced the starter/choke cover, did you use a gasket between it and the main carb body? If you did, which did you use? The rebuild kit doesn't seem to be supplied with a gasket specifically for this.


Thanks,
Matt

Click to expand...

I have stripped down , I would guess over 50 26 & 28 IMB carbs and had never seen a gasket on the fuel enrichment assembly then last year while refurbishing some used 26 IMB carbs I came across one that had a gasket in that joint. Never seen one in a rebuild kit or shown on a parts list so something of a mystery.

Re: Butterfly and spindle

Toshi 975 said:

I have stripped down , I would guess over 50 26 & 28 IMB carbs and had never seen a gasket on the fuel enrichment assembly then last year while refurbishing some used 26 IMB carbs I came across one that had a gasket in that joint. Never seen one in a rebuild kit or shown on a parts list so something of a mystery.

Click to expand...

Thanks Toshi. I noticed there was nothing in the kit, but it seemed unusual that a gasket wouldn't be used in a carb where two metal faces meet. I've just had an email from Ricambio also stating that it's a flush fit with no gasket, so I guess I'll just make sure the cover is tightened down!


Matt

Morning Matt;
Good to 'see' you on the forum again---how are you keeping? I have just been rebuilding a couple of "28 IMB" carbs for a project, and as 'Toshi' has mentioned, there is definitely NO gasket between the 'Fuel Enrichment Device' (choke) cover and the carburettor body. It relies on the close interface between the cover, internals and carb body for it to work correctly.

Hi Tom, I'm good thanks. I've been a bit absent from the forum over the last few months as I got bitten by the ultra running and iron man triathlon bug so I've been out running and cycling and neglecting the poor Fiat. I was building up to the Marathon des Sables which I managed to finish last month, so I now have some time to work on the car again. It's due for an MoT next week so just in the process of overhauling the carb, fitting electronic ignition and getting the timing done. Then I'm looking to finally fit the FZD and sort a couple of other bits out.
I hope you're well and your 500 is getting plenty of use now that the sunshine is finally out.
Matt

Hi

I know this is an old thread but an absolutely brilliant one which I keep returning to when working on the carb.

I don't have an issue (for a change )- more of a general question! The excellent photos here and parts suppliers show the main jet as a small "screw" that fits in the end of a "holder". Now when I dismantled my carb last year I tried to remove the small jet to replace it with the one in the refurb kit but just couldn't move it from the holder (and bu**ered it up as the metal is so soft so had to get a secondhand replacement). Even the main jet on the spare secondhand carb I obtained doesn't look like the end unscrews and I don't want to make a mess of that one too! I did ask a supplier if they also sell the "holder" and they said that they didn't (which seems odd).

So I was just wondering - is the main jet always a separate "end" or are there one piece screw/jets out there? If they are always two pieces, how do you unscrew the tip/jet to replace it when servicing the carb?

Just intrigued.

Thanks

As far as I am aware the main jet holder is regarded by the trade as an obsolete item even though they are common to other Weber carbs from the same era. To remove the jet from the holder I use two small socket sets. One for the holder then one with a tight fitting screwdriver bit that I tap into place to ensure it does not slip.

Read the majority of this thread and am impressed at the wealth of knowledge and expertise on what most would consider a simple carburetor....yet the array of problems we all encounter never seems limitless.

Question: have any of you ever come across a Cubit Feet per Minute (CFM) rating for any of the Weber single barrel carburetors?

I know the numbers must be embarrassingly small compared to counterparts....(I’m guessing somewhere in the neighborhood of 40-60 CFM).....but I have been unable to locate a single document confirming an official CFM rating on any Weber 24/26/or 28 IMB carburetor....

Anyone?

I also have never seen an official CFM figure for the Weber IMB carb, but if you are good at maths and want to work it out, look on the site:---'Stovebolt Forums'---'CFM Rating re-loaded'. The CI displacement of a 500 (110) engine is 30.5---max torque is at 3,500rpm and max power at 4,400rpm. The 110 engine peaks at about 4,500rpm. If you ARE able to work out the CFM of the IMB carb, please let us know, it will be another little piece in the information jigsaw!

the hobbler said:

I also have never seen an official CFM figure for the Weber IMB carb, but if you are good at maths and want to work it out, look on the site:---'Stovebolt Forums'---'CFM Rating re-loaded'. The CI displacement of a 500 (110) engine is 30.5---max torque is at 3,500rpm and max power at 4,400rpm. The 110 engine peaks at about 4,500rpm. If you ARE able to work out the CFM of the IMB carb, please let us know, it will be another little piece in the information jigsaw!

Click to expand...

Will do....

I suspect it’ll require the use of a “flow bench” to get a number of any accuracy. Will keep you posted.

Naples500L said:

Will do....

I suspect it’ll require the use of a “flow bench” to get a number of any accuracy. Will keep you posted.

Click to expand...

Not really......... it is basically as Tom describes...
an engine needs what air it needs, and it must suck it through the carb, as the volume of the cylinders is known, you can easily calculate how much air it MUST be able to suck through the carb at any rpm...

If an engine needed 25 CFM at 3000rpm, the only place it can get it is through the carb.... thus the carb must be able to flow at least 25cfm....
So in theory you can calculate the cfm for any rpm even if the engine runs out long before.......


there is a basic formula for calculating cfm requirements for an engine:
CFM = Cubic Inches x RPM x Volumetric Efficiency ÷ 3456.
Any ordinary engine will have a volumetric efficiency of about 80-85%.
(obviously you need to convert CCs into Cu
Race engines have a volumetric efficiency of about 90=110%
these calculations are based on a well maintained and not a worn out lump...
If you know the cfm the engine requires then a stock carb will or should be able to supply this...

If you want to know the max that the carb will flow then sure it would need to be done by other means....