5.7 OMC King Cobra, RWC, 361 hrs

At the end of last season I did a complete impeller and housing replacement (found the metal sleeve had rotated and was blocking water flow). I replaced the thermostat also. This season I notice a drastic temp difference on the exhaust manifolds (checking with infrared thermometer). 160 deg. thermostat. The starboard side has consistent temps at each cylinder. The port side has one cylinder (first in line at water input) that is easily 20-30 degrees higher. The cylinders behind it (in water flow direction) have lower temps. Both riser temps are the same withing 10 deg. Thermostat housing has a clean pass through for water ( I pulled, drilled out and cleaned rust) Hose to port side exhaust manifold is clear (temps hold well here too). Exhaust flapper is solid and in place. My guess id something may have come off the old impeller (tossed it last year so I don't have it to inspect) and way be clogging up the water channel for the first cylinder in line. Thoughts or an re-assurance would be appreciated.

Thanks!

Oh, and every boater should have an infrared thermometer on hand.