This is a tip which may be useful to others. I have an old lawnmower with a pulsa jet carb. It has been playing me up for some time . Sometimes it would start first pull, then next time it wouldn't start. I found that if I put a bit of petrol down the carb it would usually start then stop, pointing to a fuel problem. I had taken the carb off previously and cleaned it all out but still wouldn't go. Today I got a new diaphram for the carb and fitted it. More importantly , it came with fitting instructions which tells you to hold the choke open by 3/8ths inch before tightening the screws to pretension the diaphram. Obviously last time I took it off I just bunged it back on and tightened it all up and the diaphram wasn't pumping up petrol. Since I've changed it , the engine has started first time every time, so I probably didn't need a new diaphram, but it was worth it to get the installation info.and cure the problem. Hope this is off interest to others with old B and S engines.