Rough Idle & Loss of Top End Power

OK, 1st off, if there is already a thread with the answers to my questions can someone PLEASE point me to them? And if so, Sorry for the redundant post.

Ok, here is my situation. We bought the truck, a 1989 Dodge Ram 50, with a 2.0, G63B from a local dealer; very clean, 1 owner, for my 15-year-old son, he is helping me work on it and it has been 20+ years since I have had to deal with ANY carburetors and NEVER with a Mikuni, AND as of right now… I don’t plan on having to change out to a Weber, maybe in the future, but not now, but I digress. It was idling a little rough so I checked the timing, it was out a little so we trued it up. The truck was also initially running EXTREMELY rich, we did our diligent research and found this document on the bowl vent valve diaphragm. Sure enough it was torn. After much cussing, fussing, praying and searching I was able to find one from Carbs Unlimited, we also purchased a Walker rebuild kit with all the O-rings and other diaphragms from our local O’Reilly’s. With my supervision and help, he went through and replace all the diaphragms, gaskets, and O-rings, and we made sure that all the passages were clean and clear. We have replaced all 25+ feet of vacuum lines, including the ones inside the vacuum control box on the fender. I have tested and verified that all solenoids, with the exception of the AC throttle control solenoid are working, he’ll just have to do with A/C for the time being. I know I’m being mean… We have adjusted the curb Idle speed down to the specified 850±50 RPMS and now I get to the heart of the issue. In 1^st his acceleration and power are fine. When he shifts into 2^nd, it all goes to !@#$. The truck will barely get above 2000 RPMs and it seems as if it is struggling to do that. It seems to me that it is almost flooding out the carb OR it’s way too lean. No emissions test here in GA thank God, but in my due diligence and OCD nature we did check and make sure that the idle emissions are with in specs. Also after a little while it seems to be searching, not holding a steady RPM, it is within 100 of the setpoint but it will never level off. I have also verified that the fuel filter is not clogged and we are not running ANY ethanol fuel through it. Any ideas on my next step???

Thanks

Is the secondary vacuum solenoid/servo working? The secondaries on Mikuni carbs are vac actuated - if the actuator fails, you lose everything from midrange throttle upwards. Easy to test. Take the small vacuum elbow off the actuator, add a longer hose and suck on it. If it doesn't hold vacuum, it's split (the secondaries won't open while the primary throttle is closed - just be aware of that). This unit can be be replaced without taking the carb off (some carbs have a rivetted shroud on them preventing access to assembly screws and auto choke fine adjustments) It takes a little prying and removal of a small 'C' clip on the secondary throttle shaft to get it out and reinstall.

*foot note - also check the vac advance on the distributor. They can bind up from gunk and the vac advance unit can also suffer the same fate as the secondary on the carb. This will nerf power/acceleration as well if it's inoperable

follow what gezer said, and for the love of jesus help that 2.0 out and put a 32/36 weber on it. so much simpler to work on and it really allows the engine to breathe. plus the secondary on a weber is activated by a slip joint on the linkage.

im in the state north of you (south carolina) and my uncle gave me my 1987 PR50 for my birthday, he got it free from the original owner. ive been in the process of an engine swap (2.6 -> 2.6) and i will say that a carb and a header are some of the best things you can do to that motor. if you keep that raspy open header sound down with 1 3/4 exhaust and a muffler, youre good to go. just gut out the underbody car, or remove it and put a peice of pipe there, and keep the factory muffler. itll sound good and perform great.

If you can rebuild the Mikuni and all of the control mechanisms are functioning properly, it'll run once you have it adjusted. The Weber is an easy carb to work on but it does take some assing to set it up. At least with the Mikuni you have tuning guidelines that don't have any guesswork.