When we bought our 90 with 4.2 we also had all these problems. Started going through everything ie; tune up, carb rebuild, fuel filter, numerous carb adjustments guess what I found.... nylon coating on the timing chain sprockets came off. Way too much slack! Changed the timing chain and gears and Hello issues are gone. I also actually found that someone also had bumped the distributor one tooth in order to get it to time. It never made tapping or slapping sounds. Only found it because I had an oil leak on the front cover. Only had 112,000 miles on it. How many miles do you have?
HAve not done the nutter yet and unfortunatly Can't change over to the 2100 due to Cali emissions. Runs GReat now so not worried about it for now.