Ever since I started riding the NX, I've sensed that all was not right with respect to fuel delivery. I've wondered about corrosion in the carburetor (found and dealt with), air leaks (rectified with a carburetor rebuild kit) and timing (checks out OK). There are only two adjustments that can be made to the Keihin PD52E (PD52F in California) carburetor; idle speed and fuel/air mixture (also known as the pilot screw). Since I recently adjusted the valves, I figured it would be good to revisit the carburetor adjustments.
One of the issues with the adjustment is the limiter cap that's installed at the factory. The bikes are adjusted to be lean so that they pass emissions tests. The limiter cap ensures that the bike leaves the dealer in the same state of tune as when it left the factory. There's a stop cast into the float bowl and the tang on the limiter cap prohibits the screw from being adjusted to enrich the mixture. I've read that heat will defeat the adhesive that holds the limiter cap. I chose to simply grind the tang off when I had the carburetor apart for rebuild. You could also grind the stop off the float bowl, if you so choose. Regardless of what you do, don't allow aluminum shavings to enter the carburetor.
If you refer to the Chapter 4 - Fuel System PDF on the Resources page, you'll want to review the Pilot Screw Adjustment procedure on page 4-10. Basically, you want the engine at operating temperature and idling around 1300 (+/- 100) rpm. I use an off-set ratchet to make the adjustment.
The hard part is not burning your hand while trying to adjust the pilot screw. In the image below, the float bowl of the carburetor is identified by the yellow arrow (behind the rear brake light wiring and switch). If you use an off-set ratchet with a straight blade insert, you need to insert your right hand between the engine and exhaust pipe (pink arrow) and use your left hand to manipulate the off-set (lime green arrow). It's not easy and you can get hurt (burned).
I've found that using a welder's glove allows me to put my right hand on top of the engine case, palm up, without being injured. Yes, your hand does get warm-to-hot inside the glove but you're protected. At this point, I use the middle finger of my right hand to hold the off-set in place while I adjust the pilot screw with my left hand. You are instructed to "turn the pilot screw in or out to obtain the highest engine speed". If you're using an offset ratchet, obviously, you can only turn in one direction at a time. Chances are, you'll want to turn counter-clockwise (from the initial setting of 1-7/8 turns out from bottomed) to enrich the mixture since the initial setting is lean. I've considered a screwdriver with a flexible shaft but have yet to see one with an attachment that keeps the blade in the screw head slot. Once you've obtained the highest engine speed (rpm), re-adjust the idle screw to 1300 (+/- 100) rpm.
This "tweak" has the NX running as I think it should. It warms up quickly, uses less choke and it accelerates smartly. There are times when it still struggles for pace but much less than it used to.