Hi,
Im new to the forum and I inherited a 1988 P750 from my uncle in tough shape. The bike has been in storage since 1999 with a broken charging rotor until two years ago when it was replaced and supposedly the bike ran. I replaced the belts and changed the oil and a few other expendable items to get it back on the road. I got the bike to start for the first time yesterday and it fired right up and seemed to be running decently. After the bike had warmed up some I tried to move it under its own power and it made it half way down my short driveway before bogging down. Now the bike will fall flat on its face as soon as I start to let the clutch out. I hadn't tried reving above 5k before this but after it wouldn't move under its own power it would bog down right around 6k rpm in neutral. It was dark enough out that I noticed some sparks coming from the exhaust around 5-6k rpm. In neutral it would continue to run well up to 6k for a while before it would slowly start needing more throttle input to keep running. Also a little bit of choke seemed to help it in neutral as these problems started

I'm sort of stuck on where to go from here and would appreciate any suggestions. My knowledge of the bikes history before I got it is limited but it does have Mikuni T38 carbs. From what I've heard is that they were rebuilt and cleaned last year. My next step will probably be to take the carbs off and do a good cleaning but I'm unsure if theres any other paths I should take such as valves or ignition.

Prior to starting the bike I replaced the spark plugs and it seemed like the vertical cylinder had a weaker spark. Would this have anything affect on the abrupt bogging down under any load?

Any and all help is greatly appreciated!

New plugs & fuel filter, fresh battery?

Remove fuel inlet at carbs and run starter a few secs.
It should pulsate freely.

I agree with Gert. I would replace the fuel hoses and the filter if you haven't done that already. Also remove the fuel petcock from the tank as it has a mesh filter inside which could be clogged if there's dirt in the tank. Was the tank flushed and the fuel old replaced?
While you have the tank off also check if the fuel return and tank breather are free. Btw is there a Y piece in the fuel line with a fuel return hose to the tank and what fuel pump is installed?
Are you sure the bike is running on both cylinders? Do measure the voltage at the coils (orange wire) and let us know. This could be the "weaker spark" issue.
Other questions would be: are the carbs synchronized? Is there leak air maybe from the carb rubber boots? Was the valve clearance checked? Does the clutch fully disengage? ...