yet another jetting question

This '81 CB650 has something to teach me. (the positive way of looking
at it)

I got it at the beginning of the summer with 13,000 mi. I just checked
the odometer, I put a little more miles on it over the summer than I
thought. It's at 18,000 now.

At first it would cut out if I gave it more than 1/4 throttle, under
load. Going up a hill on the freeway required that I let up on the
throttle so it would have enough power. Downshifting just made it
worse. It seemed like it didn't want to rev, but as long as I built
speed while the throttle was barely open, it would eventually get past
5,000 rpm.

I took the air filter out and it would rev to redline, going uphill
with the throttle open as far as I wanted. The filter was new. I put
it back in and it sucked again.

I cleaned and balanced the carbs a few times. Yesterday I put on some
UNI filter pods. Now it behaves like it should and sounds like a fast
bike. :)

I'm not sure if I need to rejet. How does a guy determine if it is
necessary.

Also, if anybody cares to speculate, why would a bike with stock
jetting run so much better with UNI filters? I tried setting the idle
mixture to the factory spec, but that's as far as I got, with my
pea-brained tuning.

BTW: The head gasket leaks oil, should I fix that before trying to
solve any other problems?

Thanks for your feedback.