Jim Mason - firstname.lastname@example.org - Proud owner of a '78 R80/7 with over 101K on it.
Rather than prying off the retaining nylon ring with brute force, drop the whole slide into hot water. The ring will expand enough to lift off easily. Leave the ring in the hot water while you swap your old wad of black chewing gum for a spiffy new piece of rubber (locate it properly in its tab slot!) and then pop the ring back on. Wait for all to cool before reassembling the carb and check that the ring is not going to come off.