I'm sure this is addressed somewhere but couldn't find it... My 500 has been running flawlessly at all rpms except after sitting and idling for longer than a minute or so when warm. The idle is very smooth, but then when I give throttle it chokes and refuses to rev. I have to blip the throttle several times, give it a second, and blip it a few more times before it will start revving again, at which point it continues as though nothing happened. Config is Dell'orto DHLA 40 to a Panda head on a 695 build with 123 electronic ignition. Starts immediately even at freezing temps and goes like hell otherwise. I'm thinking a float or idle jet related issue?

Check that the (accelerator) 'pump' jet is clear and that there is no sediment at the bottom of your float chamber that is being pulled through it. The fact that it idles well and runs well would indicate that it could well be a fouled jet.

Just had another thought---it could be that the 'pump'jet is too big, and that suddenly wacking a big squirt of fuel into the carb after it has been idling is flooding it, supporting the fact that it goes well after you have 'cleared it's throat'

Many carbs have a certain amount of 'adjustment' on the linkage that operates the diaphragm that squirts the fuel through the accelerator pump jet - essentially a nut is turned to load/compress a spring on the linkage.

If this nut is slackened, i.e. the load on the spring is reduced by lengthening it, the rate at which fuel is injected by the acc. pump jet is reduced. conversely if the nut is tightened, i.e. spring more compressed, the fuel is delivered faster and may run out before the engine has started to speed up, i.e. a sudden big dump of fuel, which can cause the engine to bog-down, followed by nothing more from the acc. pump jet (at least until the accelerator is released allowing the acc. pump housing to refill).

I'm not suggesting that this will cure the OP's problem, but this adjustment is worth keeping in mind if you want to 'fine-tune' how the pump jet works. Iirc, some official carb literature gives a time figure for setting up this linkage spring loading, i.e. how many seconds it should take for 1 stroke/operation of the acc. pump to pass through the pump jet.

Also iirc, some accelerator pump housings have a circuit (sometimes a drilling sealed by a ball and small weight) to allow fuel to be returned to the float chamber and not exit from the pump jet/nozzle when the accelerator is opened slowly - worth checking this return/bypass if the acc. pump isn't working correctly.

Al.

Back in the day, some used to say that this hesitancy on pick-up was due to the heavy crankshaft counterweight i.e. inertia, therefore a characteristic of the model.

Did this engine always hesitate when you tried to rev it up or is it a recent development?

Sometimes, advancing the static/idle ignition timing a little can improve pick-up.

(Disclosure :- I'm not a stickler for sticking rigidly to timing marks/ignition settings decided long before there was unleaded fuel....).

Al.

Sorry for the delay, took a family trip to Pompeii. I'll be home to check these things tomorrow evening. Al- I'm actually not sure if it's a new development, the car almost never sits more than a short moment while driving. The timing could possibly use adjustment. When I first installed the 123 to this engine, I used a timing light and then adjusted by ear while running afterward.