I have been having a problem with port motor and was wondering where did sonic put the Anti siphon valves on the fuel tanks. I tried switching tanks but still have same problem could they have put it on the fuel tank selector switch?

They did install them on my 38SS back in 1989. They where located just after the 90deg fitting on the tank pickup on the top of the tank if that makes sense. So it goes tank pick up to 90deg fitting then anti siphon ball then fuel line hose barb nipple then short piece of hose then to tank sellector valve. Hope that helps. There is also a corse sreen under the 90 deg fitting going down into the tank pick up taht can get clogged up with stuff.
Andy