If the petrol drain hole in the manifold is blocked 1) what would happen and 2) do you have to remove the 26/35 carburettor to access and unblock this drain hole?
My 2CV Special runs for 10 -15 minutes and then peters out. Could this be a worn needle valve or a worn out fuel pump and the engine is stopping because the carb is empty? The fuel filter at the carb and prior to fuel pump are clear likewise no crud in carb.
With the fuel pipe to carb disconnected and the ignition on petrol does gush from pipe. Just using the starter handle to tun engine over just a small trickle of petrol comes out.

Well, the fuel pump will be a lot less effective with you turning the engine at 1 rpm, compared to the engine at 3000 rpm!

If the pump was dead it wouldn't be starting.

What ignition are you running?

When you say runs, do you mean driving, or idling?

When the engine stops, are you able to take the top off of the carb and determine if it is full or empty.? This way you can eliminate, or confirm if its a fuel issue.

Bit of bent wire should clear the drain hole, it is quite small but fiddly to get at.

Others have suggested carb solutions, but a good clean out wouldn't go amiss. Are you certain the rubber pipe from the tank top the nylon section is sound? It's only about 8 inches long, might allow fuel to flow through gravity, but fail when the pump sucks. You'll get air into the line and starve the carb. You'll need to drop the tank to check - best done when near empty.

The ignition is points and condenser. Only happens when engine idling, not when driving, and I haven't checked the short pipe from top of tank. Did pump year old petrol out and put 2 gallons fresh petrol in. The vent hole (in manifold) not sure where it is to poke clear. Cleaned carb bowls which had negligible sediment and the floats were at correct 18mm. Needle valve appeared to be working.
Will take carb top off when engine next stops to see how much fuel it contains. Assuming carb is full of fuel, how long would the engine run on the fuel contents if the fuel flow stopped? Thanks for advice so far

Fault finding is often a process of elimination, as much as it is of finding the fault. I was thinking, if the carb is full of fuel after its stopped, then its not likely to be a fuel issue.

If you take the carb off you should be able to see and clear the drain hole from within, it's in the base of the heat exchanger below the carb.

hypno,
nobody yet seems to have questioned why there could be any possible connection between the fuel drain drilling in the manifold and your engine cutting out after idling for 15 minutes.

Does the engine restart easily after it has stopped?
If it refuses to start, maybe check if there's an HT spark or substitute that coil with a known good one...

Hi Ken. 2CV doesn't restart easily after cutting out after idling. Usually after some considerable time, about an hour starts easily. When actually driving no problems at all. Just when idling. Will check HT and substitute coil.

Thanks Derek I was wondering how to access this drain hole. This particular 2CV is the first since owning 2CV's from 1990 that hasn't dripped fuel from drain hole.

Update. This morning at just before stopping at 18 minutes, engine idling tone changed as if fuel starvation. Lid off carb and fuel in two rectangular chambers approx 18mm from top. Needle valve seems ok, not blocked. Unfortunately on checking my spares I don't have any coils to substitute:-(
Cannot see any drain hole beneath the carb. It might of course be rusted solid. There is a groove that fuel could run in. Need to put new batteries in torch, rest my back as working in spurts between bouts of chronic pain.....