Lookin' for some wisdom.

I just bought my first motorcycle, Suzuki GS400 1977. It has a problem I'm trying to overcome. The rev's will stick sometimes at 3000-4000 rpms and if they don't it descends to idle very slowly from that point. I bought new diaphrams off ebay, and though its helped it hasn't solved the problem 100%. Recently I took off the fuel cock and cleaned it out and found that the vacuum hose was clogged, once more air got in from the tank it seemed to work even better. I rarely get the RPM's stuck anymore but it still descends very slowly. Could air be getting in through the fuel line or whats going on here?

Thanks!