Hi,
I had a very similar issue with my 1998 Vulcan 1500 classic and it seems many others had related and/or connected issues. I'll tell you my story which I think is the weirdest of all but it might help you with the investigation of yours.
Intro: I got the motorcycle second 2015 and in 2016 the engine broke. When my mechanic opened it up, it seems some previous owner had it partially and badly rebuilt. So I ended up rebuilding it all, again (just my luck I guess). Once the engine was rebuilt, I performed by the book break-in and everything was OK. No carb flooding even if I already was using the Vance & Hines Longshots and K&N filter but no re-jetting (even if I have the kit
).
At 1600km since revuild (about 1000 miles) break-in was over and I made the final oil change. Then I drove another 2300 km (about 1500 miles) with NO issue.
Then out of the blue, the carb started flooding. Basically I rode the bike around the city, I parked it and then when I wanted to leave, I started it and it choke pretty quick while making a poodle of gas on the ground.
Note: Mind you that my carburetor is, I think, the most complicated Kawasaki installed on an 1500. Beside any normal carburetor does, this one have the electronic TPS (throttle positioning sensor), low emissions hose entry and valve and the coolant input so the carburetor is kept cold or hot together with the engine.
Then the fun begun:
1. I checked the carb float and pin assembly - it was ok but I ordered a new one anyway. My mechanic removed the carburetor, cleaned it all up thoroughly, checked jets, all diaphragms installed the float and needle. Problem was still there. Engine starts idle for a while then choke and stops. Then gas poors out of carb.
2. On forums people was telling me to check all the hoses to check if none is pinched or damaged so pressure is OK. I checked them all by removing them, then checking them individually and then putting them back. SAME issue was there.
3. Then from forums I found out I have to check the TPS sensor. In reality this sensor is just a variable resistor, nothing fancy but the price. I tested it using the procedure recommended in the Clymer after market manual, and I discovered that it was working ok for 1/2 the throttle but then it was showing a value like it was some short-circuit. I removed the TPS and then used high pressure air to clean it. I installed it back and tested it again and now it was ok. So it seems a bit of dirt, or a small piece of metal was there. Anyway, the TPS was ok. Then my mechanic installed back the carburetor. SAME problem.
4. Then, on other forums people recommended to check the fuel pump, becasue it is possible it pushes too much fuel. I measured the pump electrically like it is recommended in the Clymer manual. Apparently it might had a problem. I ordered a new fuel pump and instaled it. SAME problem. I put the old pump back, SAME problem. In the end I put the new pump again and continued with the investigation. SAME problem.
5. On other forums, people told me to check the vacuum valve, the one that is part of the gas re-circulation system (the one you disabled). I checked that valve and indeed it was malfunctioning so I ordered AND installed the brand new valve. SAME problem.
At this point, it was driving me, and my mechanic crazy. Then, since the carb was flooding even on idle I got an idea.
6. I bypassed the fuel pump: I disconnected teh fuel pump from electrical system and connected the hoses so the fuel was poring from tank, through the filter and directly to the carburetor. I've put the tank in a position a bit higher so gravity did the job. I started the engine and, at idle it was NOT flooding anymore. Everything was OK. Of course I could not do this on a test drive. So it seemed that the issue was fuel pump related, but I had the old pump and a brand new pump. It was impossible for both of them to be bad. So I reconnected the pump back and put everything together. SAME problem.
The problem was still there but, at least, I kind of isolated the root cause that is the fuel pump or the CDI system. My motorcycle has a CDI system and not an ECU. The ECU was added in later models. If you have an ECU version, there is a way to diagnose the electronic issue. The process is described in the Clymer manual.
7. Then my mechanic found some shady recommendation done by a Vulcan Main Streak owner that was telling us that, for these old models, the CDI has a system to identify an issue and enters in an 'error state'. But it does not have a system to reset an 'error state' like you have in case of the ECU systems. The solution is to disconnect the battery for 30 minutes and then reconnect it back. I did this and the problem WAS FIXED.
Basically, the CDI detected that I had an issue with the TPS (that I later fixed cleaning it with pressurized air) and it entered in an 'error state' where it was setting the fuel pump to max output so you can still reach your destination. And it stayed in that 'error state' until I reset it.
It runs like a charm now.
You can try any of the above attempt, from the most easy to implement (disconnect battery for 30 minutes, then try to bypass the fuel pump, then check float and pin, hoses, etc.).
If you manage to fix it, please reply here so other people knows about it too.
Good Luck
/3ng