The pressure tap hole in a Nelson muffler is not drilled all the way through, that is if you push a small wire in the hole it won't go in more than 3/8ths of an inch. The hole is only drilled into the same hole the mounting screw goes into, if you put to long of a mounting bolt in it you can block the hole. If you put a piece of fuel
line on the pressure tap you should be able to blow through it, if you can't take the mounting bolt out and try again, if you still
can't blow through it, push a small wire through the Mounting hole, it shold go in at least 7/8ths of an inch.