Yamaha s are notoriously lean. You have the opposite problem. Anything could be wrong with the history behind your carb. Verify that the low speed jet is even in there and is the right size. Check or replace the needle/seat assembly and verify the proper float level. (If the needle seat and float are bad it will show up as a leak out of the vent tubes.) Your choke could be causing the problem. That carb uses a choke plunger that seals against the bottom of it's hole when in the off position. If the cable is pulling on it the choke is partially open and the engine will be too rich. Verify that you have slack in the cable by moving the cable housing with your hand. It should have about a quarter inch of slack. Verify that the handle bars are not pulling the choke cable from incorrect routing.

I would do a compression check, maybe it's just wore out. How do you know it got weak spark?? That could the plug wire or a weak coil????

The first thing I would do is put in the correct heat range plug and gap it correctly. Alot of carb problems are not carb problems at all, but iginition related. Every single carb manufacturer from edelbrock to Barry Grant to Holley etc will tell you to check the ignition first. Make sure you have good plugs gapped correctly, good wires, good coil. I don't think the timing can be adjusted so that should be fine. If problems still exist then dig into the carb.

C'mon guys. Black smoke at idle isn't ignition. Running all day after it starts isn't ignition. He only has an idle problem.

I put the correct heat range plug in, took the carb apart as much as I could and blew it out with carb cleaner. Moved an e clip on a rod that was set at the richest setting, put everything back and it's a little better. Still a little smokey and seems to load up periodically. Could it be that the carb is just shot? I can't figure out how the choke works (doesn't have a butterfly like I'm used to seeing)but there is a difference when I move the lever.
Is there such thing as a universal carb for these things? The replacement is hard to find and is real expensive, repair parts are not available, and a new one would just get me a new carb that I don't understand.

Thanks, Dave

You need to check the low speed jet to see if it has been replaced with an oversize, drilled out, or removed. It's in the main body of the carb and is recessed. Remove with a screwdriver. The choke is simply another jet to allow the carb to suck more fuel in. If the engine changes when you engage it it's probably working ok. The clip you moved affects the mixture at mid throttle ranges and has little effect on idle mixture. If you carb has an air screw toward the back side of the carb (not the idle adj screw under the slide) (air screw is farther back) you should try opening it up about 2 turns.