Cleaning the carbs 2
Read Cleaning the carbs 1 first for necessary background information and tips.
Does the inside look like this?
When you drained the float bowls, did something like this come out?
The usual culprit for this affliction is time. These carbs are from a neglected bike that had been sitting for over a year. It doesn't take that long for buildup to occur, though. The passages and jets in the 250's carbs aren't very big, and un-Stabil-ized gas will gum up in them after only a few weeks (YMMV).
The best way to ensure that the pilot jet circuits are completely clear is to do this:
When you're holding the carbs, look at the side that plugs into the airbox. If you look into the mouth of the carb, you will see the slide and the needle right down in the center of the carb. You will also see two small brass ports, one of which is directly in line with the needle, and another one just off to the side.
Those 2 ports pressurize the float bowl and allow the fuel to enter the airstream. The one directly in line with the needle feeds the main jet circuit, while the other one feeds the pilot jet circuit.
Remove the following parts from the carbs. See Carburetor photos if you're not sure where these are.
You also need to remove both main jets. The main has the emulsion tube (called 'holder' in the left picture) behind it, with the jet needle (also called 'collar') behind that. Be sure you get these little parts out (part #16017 on the diagram), as they could fall out on their own if you don't. When you replace them, check in the carb intake to make sure they're poking through. They only go in one way. The longer, narrower end goes in first.
Some notes on the main jet:
The first thing you should do is look through those pilot jets that you removed. Make sure you can see that the very small hole isn't plugged up. Spray it out with carb cleaner, then pull a single strand of COPPER wire from a piece of 14-18 gauge that you probably have laying around someplace, feed it through the pilot jet hole, and roll the jet around the wire.
Don't go overboard; you just want to remove the deposits from the brass, not remove the brass from the jet. Letting the jets soak in a powerful fuel system cleaner, like Chevron, will help if they are really bad. The worse the deposits, the longer they need to soak.
Now, take the straw from the can of carb cleaner, press it into the port on the carb for the pilot circuit, and spray carb cleaner into the circuit. Carb cleaner should spray out of the port in the float bowl that you removed the pilot jet from.
When you have a good steady stream of carb cleaner coming out of there, press your finger against the pilot jet port and spray carb cleaner again into the circuit. This time you should have carb cleaner spraying out of the mixture screw port, where you removed the mixture screw, the spring, the o-ring and the washer from.
Once you have cleaner coming out of the mixture screw, install the mixture screw and parts back into the port and screw it all the way in until it just seats (don't crank it down). Now back it out 2 1/2 turns, make sure your finger is still covering the pilot jet port, and spray again. This time you should see cleaner coming out of the mixture screw port on the engine side of the carb at a tiny little hole.
If you screw the needle all the way in until it seats, you will see the tip of the needle sticking out of the hole the cleaner will spray through.
Once the carb cleaner comes out of the mixture screw port, you need to screw the mixture screw back in until seated, open up the throttle plates, and spray again. Right where the throttle plates lifted up away from are 4 small little holes. This time the carb cleaner will spray out of these holes.
Once you have carb cleaner coming out of all the proper places, set the mixture screws to 2 1/2 turns and reinstall the pilot and main jets, which should also be clear.
Now reinstall the carbs onto the engine, and make sure the carbs are FULLY seated into the intake boots on the engine. If they aren't properly seated, the bike won't run properly.
If you have trouble figuring out where everything goes when it's time to put it back together, see Carburetor photos.