GL1200 backfire

[dtwing]

My 1987 GL1200A sat for several months without riding. I went to start it up and it wouldn't. I replaced the battery. Cranked and cranked, but wouldn't start. Instead gas came out the overflow vents. So then I rebuilt the carbs with a Randakk kit and replaced the spark plugs, caps, and wires. Started it up, set the pilot screws and synched the carbs. On the test ride it ran rough and seemed to lack power. Then work got in the way and it sat a few weeks. I got back to it today. I fiddled with the pilot screws some more. Still runs rough - has a miss. Adjusting the pilot screws on #2 and #4 cylinders has little effect on the RPM. Cylinder #4 screw has some effect, but #2 has no effect at all. Also noticed that opening the synch port on #2 to atmosphere has not effect on rpm. I pulled the plug caps one at a time while running and all are cylinders are firing. It has a slight backfire/pop at low speed. It has a bad pop/backfire as the RPM slows to around 2900. Compression is #1 = 185 psi, #3 = 187 psi, #2 = 185 psi, #4 = 175 psi.
So where do I start? Do I open the carbs back up and try cleaning again? Do I pull the left side head?

[Tropicalsuns]

My 1987 GL1200A sat for several months without riding. I went to start it up and it wouldn't. I replaced the battery. Cranked and cranked, but wouldn't start. Instead gas came out the overflow vents. So then I rebuilt the carbs with a Randakk kit and replaced the spark plugs, caps, and wires. Started it up, set the pilot screws and synched the carbs. On the test ride it ran rough and seemed to lack power. Then work got in the way and it sat a few weeks. I got back to it today. I fiddled with the pilot screws some more. Still runs rough - has a miss. Adjusting the pilot screws on #2 and #4 cylinders has little effect on the RPM. Cylinder #4 screw has some effect, but #2 has no effect at all. Also noticed that opening the synch port on #2 to atmosphere has not effect on rpm. I pulled the plug caps one at a time while running and all are cylinders are firing. It has a slight backfire/pop at low speed. It has a bad pop/backfire as the RPM slows to around 2900. Compression is #1 = 185 psi, #3 = 187 psi, #2 = 185 psi, #4 = 175 psI.
So where do I start? Do I open the carbs back up and try cleaning again? Do I pull the left side head?

What type of gage are you using to sync?

If you are indeed properly synced then my guess is you may have a vacuum leak. Try spraying water on different areas of the motor while running. If the engine speed changes you have found the area of the leak.

[dtwing]

I'm using 4 vacuum gauges with small valves in the tubing to dampen the pulsation. I'll try the water. I've looked for loose or incorrect connections and everything looks ok, but there could be a leak somewhere.