Someone here showed a method of lowering the slit for SPI. The method showed using tie wraps top and bottom, the sawing with a hacksaw blade. I see no reason that won't work here and since the tie wraps aren't in contact with an abrasive surface, they should last the whole job.
Chris, sorry to hear it wasn't the tach. That would have been the best news. But, that just means you get to play a little more. I doubt removing the slit did anything but make you feel better. If you have ever tried to throttle one of these things, you have to close it a good bit before the throttle responds. The venturi
may be the same way. Looking forward to your results! Hey, if nothing else, you've made it a steady running engine.
PS, you may try more gaskets under the galbreath head. I use three with 35% to get the best results. One gasket was overcompressing and cost me a couple Nelson's.