2001 Merc XR6 150hp low on power

[Desert_Howler]

This weekend I found my xr6 is low on power. The top speed is down by 25% and it is slow coming out of the hole. Compression is very close to 120 all around. Fuel pump was rebuilt just a few months ago. The only clue I have to go on is that the all new spark plugs show normal coloration on the 4 bottom cylinders. The ground electrodes are dark grey with dark oily residue on the threads. In contrast, the top cylinder on each side has plugs that look like brand new, only a very light film of fuel. It doesn't look like they have seen any combustion byproducts at all. The electrodes look pristine.
Today I started it up on the garden hose and ran it for probably 30 seconds. Afterwards, I removed the spark plugs, and exhaust smoke was curling out of the holes. All the spark plug holes had similar exhaust exiting, so it seems like maybe there is combustion happening it to top cylinders. Top plugs still look shiny new. While it was running, I blocked airflow into the carbs with my hand and the vacuum seems similar between top and mid cylinders. Doing this causes fuel to spit into the carb throats, but I could see no difference in behavior between top and mid cylinders.
I do run a jet pump on the xr6, and I started having problems when I plugged up the jet when I ran over some weeds. When the jet clogs, it can cause the motor to rev. Could this cause a carb issue, or maybe cause problems with the reeds? I'm not sure what to try. Should I rebuild the top carb?

-Cliff
There was fuel in the top-carb float bowl when I removed the bowl plug.