CB700 Stalling, Carb Problem?

[alk]

Hi everyone, I'm not an experienced mechanic but I've got a service manual and I enjoy trying to figure things out for myself. I think I've come to a point now though where I need some guidance. The motorcycle in question is a 1984 CB700SC.

A little while ago I noticed it was getting harder and harder to start my motorcycle. Sometimes I'd have to sit there for ~10 minutes fiddling with the choke until the engine would finally catch and run. I checked the battery and it was ok so I figured it must be a carburetor problem. I took the carbs off, opened the top part and bottom float bowls, and sprayed everything with carb cleaner. I didn't mess with the jet screws at all because I don't know anything about tuning them and didn't want to mess up the fuel air mixture, I just sprayed carb cleaner into each passage. I'll note that I didn't see much varnish or other crap in the carbs, they looked pretty clean when I first opened them.

I put everything back together, put some fuel stabilizer in the tank, and let it sit for a few months. So after that when I tried starting it the bike didn't run as well as it used to. It sounded like soft muted microwave popcorn popping in the tailpipe. I replaced the sparkplugs and poured some seafoam into the tank and directly into the carburetor fuel intake hose. These helped some, but didn't completely fix it. As it stands now, the bike will start right up and idles ok. It's a little sluggish at first, but after a minute or so I can rev hard and it will settle back to idle with no or just a little of that popcorn noise.

The problem is that when I ride it, after about 20 minutes or so when I decelerate, especially coming to a stop at the top of a hill I can hear popping from the exhaust, which eventually becomes full blown backfiring and soon after leads to a stall. After it stalls I can't start it again until it cools down after 20-30 minutes.

Is this a carburetor problem? Could I have accidentally adjusted the fuel/air mixture even though never used a screwdriver in there? I read that backfiring on deceleration means I could be running lean, but I also read that not being able to start after a stall means the fuel air mixture is too rich. What should I do now? Take the carbs off and clean them again, this time unscrewing the jets and cleaning them more thoroughly? I read that you can test if your fuel air mixture is rich/lean by turning off the engine while you’re riding and inspecting the sparkplugs. Is there a better/easier way to do this?

Any advice on what I should do next would be greatly appreciated.

[Munkey]

Backfiring on deceleration can also be caused by an exhaust leak or timing. Ignition timing on your bike is not really adjustable. If your cam chain has stretched enough, the sudden reversal of slack when you let off the throttle could cause the cams to drop slightly off time and cause a backfire. But, seeing as how this backfire issue wasn't happening before you cleaned your carbs, I'd suspect something there first. Maybe something is not assembled correctly?

[Hondo]

Drain the fuel bowls, get rid of the old gas, and add new gas + 1/2 of a can of Seafoam to a tank of fresh fuel.

And change the spark plugs too.

[alk]

Ok, I'll try fresh gas before I open the carbs back up. Do I really need to replace the spark plugs, they are almost brand new? Would running them for the ~30 minutes under the current broken conditions have ruined them?