I have drilled 2 holes in my cap but have yet to try it out. If you take you cap off and apart you can see 2 channels in the metal used for escaping pressure. I drilled a hole on the iner most side of each one, but the strange thing is if I put the air compressor to it it flows great at low psi. I put a little light oil on the valve as well to see if it would free it up a little and help the diaphram move easier. David has also mentioned fuel boiling which is what I think is wrong with the muffler placement, but not sure what to do to fix without major cost.
68 RS, Ash Gold,pwr windows,Hounds tooth