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.
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.
