fuel tap question

[gray am I]

I am wanting to check the fuel taps on my 78 special and I was told to take the hoses of the fuel taps, plug the hoses , put them on prime, he also mentioned I should get a drop in rpm, presumably when I unplug the hoses one at a time. Can someone please explain in I understood him right?

 

[gggGary]

The 78 on XS650's use a vacuum petcock

Yes you can do as he says, remove the vacuum line at the manifold and plug it and the manifold port. OR you can pull a vacuum on that line and that should cause fuel to flow in the fuel line, the flow should stop when vacuum is removed. Think twice about sucking on it, a fuel leak through the petcock into that line can happen.
Hint; a simple way to cap the port a is a bit of proper size fuel or vacuum line, push a ball bearing in it to plug it. Use a clamp on the vacuum port, they can get blown off if the motor backfires.