Engine shake/vibration 1400-1800 rpm

My .02 - Sounds like an issue with the a weak cylinder or bad motor mounts, or perhaps the exhaust system is touching something at that rpm and vibrating.

I'd do a cylinder balance test, some older tach dwell meters had this as a standard function, you can stop one cylinder firing at a time to see if one is particularly weak or not contributing as much power as the other cylinders, while the engine is running. If you dont have access to one of these you can do the same thing by pulling one plug at a time to see if one of the cylinders is not as strong.

Good luck with it.