2001 Suzuki GS500, can't get it to run consistently

I've posted this on gstwins.com, but figured it couldn't hurt to ask here, either, in case someone has any good ideas:

I'll try to do this somewhat chronologically:

-Picked up the bike, it had been out of commission for about 4 years with the carbs removed to be rebuilt, but it never happened. Before that, the bike would have a hard time running without the choke, and idle would go up and down, usually running around 3K RPM. While it was having this problem, the prior owner also checked and adjusted the valves.

-Drained the gas tank (no rust) and refilled it, and did round one of the carb cleaning. Jetting is stock.

-Couldn't get it to start.

-Checked the plugs(clean and in good shape), and used my timing light to make sure they were actually getting a signal. Also, pulled one and checked for spark visibly.

-Another round of cleaning. Had to get two different style carb kits as the original ones I bought advertised as for 2001-2002 carbs contained the wrong float bowl gaskets. Replaced some O-rings on the tubes the main jet screws into. Also replaced the float valve (and the cylinder it sits in) for one of the carbs, as it was pretty corroded, and the o ring on it was bad.

-Couldn't get it to start.

-Pulled the air cleaner box, and sprayed the carbs with starting fluid.

-Engine would fire up for a few seconds then die.

-Noticed my fresh gas was pretty dark. Figured out that the new gas was dissolving the old, varnished gas that had dried out some and coated the inside of the tank. From here on out, any testing I did was with pouring good fuel straight into a fuel line directly to the carbs. I'll clean the tank once I figure all this idle issue out.

-Cleaned the carbs again in case the bad gas gummed anything up.

-Friend (and former owner) helped me get it started on starting fluid, and was able to keep it running by manipulating the choke and the throttle. Was back to having the 3K roaming idle, and only running on choke. Closing the choke killed the engine, and it seemed like it was running lean.

-A few days later, went to see if I could figure anything out, and went to fire up the bike again. Could only get it to start on starting fluid, and then it would die fairly quickly.

-Pulled the carbs, and checked the idle mixture screws (didn't think of this sooner, since I figured they'd have been properly adjusted before). Turns out one of them was only 1/2 turn out, and the other was a smidge under 1 turn out. Set them both to just a bit over 2 turns out.

-At the same time, pulled the cam cover, and checked valve clearance. All seems in spec, except the left exhaust valve may be a bit loose, but not by much.

-Was able to start the bike on starting fluid, and it ran a bit longer, but still died. Revved up pretty high before dying, could keep it running for a few more seconds by adjusting throttle position. I had been planning on using my propane torch (not lit, obviously) to check for leaks around the intake boots, but couldn't keep the bike running long or steady enough to attempt it.

Sorry for the novel, but wanted to make sure I'd covered everything I'd done to the engine before asking advice on what to do next. It seems like the bike is just running way too lean as soon as the choke is shut, so I'm assuming its a air/fuel mixture problem of some sort. I may not have time to look at it in the next week or so, so now seems to be a good time to see if the next course of action seems right:

1. Pull the intake boots, and visually inspect them for cracks or bad O-rings, although they look fine to me at a glance.

2. Remove the idle mixture screws, and check for bad O-rings (although I'd think this would lead to a rich condition)

3. ??? I have no idea what else to do at this point. Also, beginning to wonder if the phrase "90% of carb problems are electrical problems" is the issue at hand. But, since I'm getting spark, I just don't know.

Its also not helping that it seems like the carbs on a 2001 are a hybrid of the earlier and later carbs, and I'm having trouble getting accurate information and parts (see the rebuild kit issue above).

Any advice would be highly appreciated.

Do you have the tools to do a leak down test? If it was run with valve lash tight before the P.O. pulled the carb it might need a valve job.
Your on the right track with propane but stick it in the air box flowing in to the carbs instead of starter fluid. You should be able to adjust the gas flow o make it idle if you can run it a couple minute then try to run on just pump fuel. Engines that sit tend to build up all sorts of crap on the valve faces from old bits of carbon and old oils scraping off cyl walls ect.

44dwarf is giving sound advice. Also, have you tried physically inspecting all of the idle passages? Run a piece of wire through every possible port and orriface to make sure its clean and clear.

Have you confirmed the idle jets are clean?

Yes agree on all above. What does your carb cleaning regimen look like? Just spray and chemical cleaning? You very well may need some thin wire and compressed air in all the tiny jets/passages such as the idle jets.

The local pharmacy dental section is a good source for small disposable brushes to clean the tiny openings in pilot jets. Sewing needles also work for those small jets ( borrow from SWMBO at your own risk).

Pipe cleaners are getting tougher to find, but work well for cleaning the larger jets and passages.

If you didn't pull the idle screws out of the carburetors when you cleaned them, then you didn't clean them. The job hasn't been done right.

You've got contamination from the new gas dissolving the old gas. That makes a slightly gooey comparatively incombustable liquid.

You need to thoroughly clean the fuel system. Not sorta clean only some of it.

Is the air box still there or did the PO use cheap pod filters for a cool look? The cheaper pods can block some of the vent holes when installer on CV carbs.

Got back from travel, and borrowed a compression tester. 150 in the left cylinder, and 155 in the right.

My carb cleaning consisted of carb cleaner, canned air, and pushing a wire strand through wherever I could. Did not get the idle mixture screws, so when I pull the carbs again, I will get those, too.

The idle screws will be capped off if they haven't been removed already. Canned air seems like a weak source of pressurized air. Do you have access to a compressor?

These bikes are tuned pretty lean from the factory. GSTwins.com has a lot of write-up on preferred jetting. For now I would just back the idle screws out a turn or so after you get the carbs clean. There's also the vacuum petcock that needs to be cleaned along with the tank, etc. My first bike was a GS500 that hadn't been treated very well by the previous owner and getting the entire fuel system clean and reliable was a chore.

Idle screws are not capped off. I ended up messing with them a bit the last time I tried to get the bike going, but it didn't help much. We'll find out how badly gunked up they are when I pull the carbs.

Tore down the carbs again. Idle mixture screws looked clean, and the O-rings on them looked good. Ran a smaller wire than I had before through as many holes as possible. Blew air through them to get carb cleaner out.

I also adjusted the idle speed down, as the throttle plates seemed more open than they should be to me.

Putting it all back together and testing is on hold, though. I somehow lost a carb drain screw after I'd removed it to clean up after having to dremel a slot in its head, since it was too stripped to open. Before, had just drained the carb by loosening the screws on the bowl and tipping it over.

Ordered some new parts, but found the drain screw in the meantime. After reassembling everything, the bike fired up with no need for starting fluid. RPMs seemed to want to run away with the choke on, but after I eased it off, it idled around 1500 RPM (1200 is the recommended idle speed, I think). Fiddled a bit with the idle speed adjuster, but it mostly settled back around 1500. There were a few occasional very small backfires, too.

So, I think the main problem all along was the idle speed adjuster. It was set so the throttle plates were probably letting a lot of air by, and probably making the bike go lean while it was cold. I'm sure that's not the only issue, but at least now I can get he bike to run long enough that I might be able to sort everything else out. The spark plugs are light gray with no build up, so I'm assuming the idle fuel mixture is close to correct, I just need to figure out which carb is backfiring.

Also, drained the fresher gas out of the tank, after I've been swishing it around occasionally for the last few weeks. It came out a lot clearer than before, but still not as good as completely fresh fuel, so I think its dissolved some of the varnish. I may actually try to run it from the fuel tank as is.

This is making me appreciate fuel injection more and more.