Honda VFR750FJ down on power - help please, off the to TT in a few days!

I'm hoping someone cleverer than me here might have an idea where I can
look.

My VFR750 has stood for too many years and I didn't drain the carbs. After
unsuccessfully trying to clean them out with buckets of carb cleaner etc. I
managed to get a much cleaner set from ebay and fitted them. Bike seems to
run ok, but it definitely down on power.

fuel filter. New plugs.

I took the tank and airfilter off so I could see the throttle slides and ran
the engine to see if all of them were lifting. The front two carbs looked
ok, but no matter how much I revved the engine, the pistons in the two rear
carbs didn't move at all. It seems weird to me that both two carbs looked
the same. I checked both of the rubber skirts etc. and all looked ok, but
somehow it would seem that there's not enough vacuum in the two rear
cylinders? Or is it because I am running without an airfilter?

The bike seems to run ok (revs without misfiring, no popping or anything)
but is definitely low on power.

Any ideas? I'm stumped at the moment, would really appreciate some help.
Could it be air leaks in the rubber inlet manifolds? Wouldn't that cause
lots of backfiring?

All advice gratefully received, I'm finally off the to the TT (first time
ever), after thinking about it for about 30 years.....