Ian... The little hole in the jet block is there to drain excess petrol in the mixing chamber due to an overfilled reservoir, this prevents ignited petrol being blasted out all over the riders leg if a backfire should happen, the hole was only put there on the later 276's.
Tickling should stop when petrol is seen around that lower nut where the drain hole is, there is no need to hold the tickler button in till petrol gushes out at the top. The top hole in the reservoir is to allow air to escape out of the top when petrol fills the reservoir chamber, its not there for petrol gush out because as can be understood this would be highly uneconomical and another fire risk depending which way the petrol spurts out.
Experimentation and close observation is needed and it is okay to remove the top float chamber to adjust float hight while chamber is full of petrol, notice that there is a small notch on the float needle in which to slide the float up and down it a small amount. Check float spring is holding onto needle tightly and check hole is clean where top of needle fits into the top cap.
With this type of carb you need to have bike on centre stand as side stand will allow float to hit side of chamber allowing more petrol through the simple valve at bottom of valve needle, its only brass on brass and the smallest hole will allow more petrol through, and in turn petrol will then pour out of the jet block drain and down onto the magneto. In your case with a blocked off jet block drain, the petrol will either eventually run out of the bell mouth or down the cylinder bore. It's also the kind of carb where you need good working petrol taps under the petrol tank which always need switching off when bike is left unattended especially on bikes side stand.
In all... although it is a crude type of carburettor and often accused of being poor in its design. When a full understanding of its correct operation is achieved, it can be a good and economical carb.
