The only drain line that has any sort of plug or cap on it attaches to a nipple on the clean air side of the air box on the bottom near the right side forward of the filter and screen. It is up behind the exhaust pipe making it almost impossible to see or reach the nipple on the bottom of the air box .
You can probably get to it by removing the muffler, the exhaust pipe and the rear brake fluid reservoir. As I recall I got to mine when I had the rear wheel off and the inner rear fender fender that keeps mud off the shock spring off.
The nipple on the bottom of the air box has a filter in it to keep dust from being sucked straight into the engine if the tube and cap are lost. If you find the nipple on the bottom of the air box is broken off as was mine, I can tell you how to fix it using a valve stem off an old inner tube.
The purpose of that line is to drain any oil that gets into the box from the crankcase vent and any water that gets through the filter. The cap/plug is so you can catch and see what and how much is draining out.
I just made a new nipple that won't melt in the original location.
I measured the nipple location forward of the screened divider in the air box and inward from the right side.
Using a 1" hole saw I drilled a hole in the same location on top of the air box.
I used an 8" long 5/16" drill bit to drill down through the top hole and clean out the bottom nipple hole.
Cut the valve stem from an old inner tube and removed the valve core and nut from it.
Using a long piece of coat hanger wire through the top and bottom holes slide the stem down the wire and through the bottom hole.
Run the stem nut up the stem for a snug fit. Use some RTV if needed.
Put some RTV on the threads to seal it and attach a drain tube and rig a tight plug on it so it can't suck dirt.
You can even put a small piece of oiled foam in the to be certain no dirt gets in.
Get a 1" round snap-in cap/plug from an auto parts store and some RTV to seal the hole on top.
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