As some of you know I was having issues with my bike passing emission test here in Phoenix. I had modified the intake jets and done the white spacer mod. Well long story short, I had to put it back to stock including the stock spring in the enrichment circuit. It passed the emissions test gasping for fuel the whole time. However, I started having the old problem with the gas flow stopping at about 60mph out on the freeway. Moving the petcock to reserve would correct the problem. I also had the ''afterfire" problem again. So, I now have a virtually stock carb set up (I do have a 150 main) and I replaced the enrichment circuit spring with a softer spring. I bought a couple of Team Associates # 21202 springs from my local hobby shop for about 5.00. The afterfire is all but gone (with a stock white spacer) and in the last two months I have not had a petcock problem.
I now wonder if the petcock problems are tied to the vacuum in the enrichment circuit. Is the spring tension in the in the enrichment circuit preventing the diaphragm from working as it should?
Swapping the spring in the enrichment is a five minute job and is totally reversible. It any of you want to try this, the springs are on line.  My local Hobby Bench sells them for use as coil spring shocks on RC cars. You can even get them in red or blue. If any of you in the Phoenix area want to give it a try let me know, I got a spare...and a screwdriver.