Slanty, 904 issue

Slant's are real long stroke lower rpm torque engines but unless someone has changed the gears to something real low something is wrong and I highly doubt it's a transmission issue. Most likely running out of fuel at high speed (weak fuel pump or plugged fuel filter, dirty carb, etc..). Or it's possibly misfiring at high speeds. The engine may sound like it's running hard but that doesn't mean it's actually spinning a lot of rpm. That's where you need a tach to see what's actually happening