The Mikuni is a wretched thing to work on. Lots of diaphragms, control circuits, vacuum lines and components that are hard to replace and/or source. Add to it years of service and dealing with vacuum leaks around throttle shafts, adjustments made by PO's that didn't know any better and a huge number of model variants with altitude compensation and internal/external EGR valves etc and it is no wonder some guys abandon these trucks as being in the 'too hard' basket.