Use spray starting fluid. When you spray near the leak the engine will surge.

The WD40 doesn't work for me either.

 

one thing you can do is take a small propane torch (plumbers soddering torch) have a buddie listen to the motor while its running then open the torch (dont lite) then run it by the vac lines and then if your buddie hears the motor speed up you found your leak (propane got sucked into the engine)