Having looked again at your pic 10 of 12 and having consulted the official Fiat workshop manual (available free from the Downloads section of this website), some of what I said in my post #30 might not be exactly correct....

You asked if the pinhole in the carb bore should be covered by the throttle plate (or venturi flap as you refer to it)? I thought this was the idle mixture hole and therefore should be uncovered. However I'm now thinking that this is a Progression hole (or Transition hole as Fiat refer to it). Progression hole(s) should not be visible when the throttle plate is at the idle position (i.e. almost closed) - their function is to supply extra fuel as the throttle is initially opened. (so should only appear as the throttle is opened).

Your problems with idle, the mixture screw having little effect etc. remind me of what can happen when the throttle plate is not centralised >
throttle plate can't be fully closed - so too much air can pass,
progression hole is exposed - so extra fuel is being drawn in,
mixture screw has little effect - because it can't regulate the fuel from the progression hole,
engine bogs down on acceleration - mixture goes very weak because there is no extra fuel available from the progression hole (it's already being drawn in for idling).

Incidentally, having watched your video, I noticed you're a bit aggressive in operating the accelerator linkage. The 500 is an old design of car and carb and has a heavy crankshaft and flywheel (think inertia) - the carb doesn't have an accelerator pump jet/circuit, so for the engine to speed up from idle, it's relying on extra fuel being supplied from the carb progression hole, this'll take a little time, even the best tuned 500 engine of this type (no accelerator pump jet) will tend to falter a little if the throttle is opened suddenly, when testing the carb it's better/more instructive to slowly open the throttle.

Your plan to try to centralise the throttle plate sounds like a good next move
And, while you have the carb apart, double-check the float for fuel leaking into it (just shake it, any sloshing sound means fuel has gotten inside, this will cause the fuel level to rise in the float chamber) - I find it very suspicious that in your video showing your carb fuel leak, it takes 45 secs for it to occur and then it doesn't stop....

If you haven't already read it, there's a wealth of info in the Fiat workshop manual, including how the various sections of the carb operates, diagrams of the fuel circuits, plus all adjustments.

Al.

I tested the float, putting it in hot water and verified that it is fully waterproof ( fuel proof ). So the float now seems to be ok. Dented but floating with no leaks.

After some tries I finally got throttle plate to fit perfectly and cover the progression hole. I had to flip it remove all the screw, close it a few times and then fit the screws. I did not shift it when I cleaned it so the wrong positioning was done previously. But as F123C explained could justify

1. throttle plate can't be fully closed - so too much air can pass,
2. progression hole is exposed - so extra fuel is being drawn in,
3. mixture screw has little effect - because it can't regulate the fuel from the progression hole,
4. engine bogs down on acceleration - mixture goes very weak because there is no extra fuel available from the progression hole (it's already being drawn in for idling).

In terms of the top lid leak I have tried to get it flat on vice without success. Tried to sand it flat with very limited success. Not sure if I was able to get if flat enough and if cannot what options should I consider ?

In the fotos attached you can now see the progression closed when you have no acceleration.

If you are happy with the rest of the carb I could probably sort out and send you a replacement top (or complete carb) it would be of the original type with the filter on the inlet with the big brass cover.

I will try to test it early next week and see if it is OK. If its I will settle with not perfect but working

Thank you for all the help up until this point. Learned a lot!

Update.

No leaks now!

Started with no choke
Stable idle at a good speed.
Tinkering with the idle mixture screw has an impact (previously it did not made any diference)

New thing I need to test:

- When warm If i pulled the choke, even if just a little bit the engine would die. Should it ? It did not do that before.

Pulling the 'choke'into operation enriches the mixture, which will and can, cause the engine to run roughly. If it will start from cold (but how 'cold' is 'cold?) it probably means the mixture is a shade on the rich side. However, if it is running and idling well, leave as is.

Thank you for your reply. Cold is about 28 degrees Celsius

Now that you reference it, i did try a main jet that was 120 instead of the 112 that was there. But the main jet is not used when idling, or is it ?

To us, 28C is ruddy warm! We have had hardly any days this summer that have got that warm. You are correct in thinking that the main jet does not come into operation until about 1/2 throttle; 120 does sound a mite large. Unfortunately, I am well away from home at the moment (looking after grandaughter), but when I get home on Sunday, I will see what was in my IMB before I changed it for the FZD.

I had a great chart for motorcycle carbs that illustrated the various circuits in use during idle to flat out acceleration. Not directly applicable but the mains should only come into play in the last 1/2 to 1/3 of throttle. During idle it is obviously the idle circuit that should be doing the work and changing the mains will have no effect. If you find at full throttle it’s feeling fuel starved, larger mains may help but obviously only to a point on these tiny cars. It would be easier if they had a proper tach you could observe. I’ll try to find the link to the chart

Did about 50km this weekend. In terms of running and starting everything is working great.

Did not test top speed or long range performance. But everything is looking. Good.