All OE 1996 7.4 LX MPI w/VST.

Raw water cooled (85* water on test day), new merc. impeller 4/'23 with 6 hrs. on it, stainless impeller housing, oil cooler picked clean 4/'23, no PS cooler, entire transom assembly reconditioned 4/'23 i.e. water plumbing, hose etc. so no bravoitis, new merc. 160* t-stat with 2 1/8" holes drilled for flushing 4'23, engine running 160*, OE type manifolds 2017 but sat for 2 years and risers are OE type stainless. For below comparison both engines state of maintenance and reconditioning above are identical.

Problem starboard riser was 206* after a 20 minute 2,800 rpm cruise.

Sister engine starboard riser was only 145*, both engines port riser in the 115*'ish range. Exact same water flow from all 4 tail pipes including the hot one.

Any other reason why said riser would be hot other than the manifold may be plugged up with rust from sitting 2 years? Seems to be disproven by the volume of water flow from the tailpipe being the same to me but I'm definitely not an expert.

Thanks for any insight you can lend!

Thermostat housing just makes the starboard side hotter. Pretty common from what I've seen.
Sometimes it just be like that.

Very common as stated above - you did mention new impellers. Were the old ones still in tact when replaced? Another common thing is the remains of those getting lodged in various places.

Correct but as noted above the other engine starboard side is 145* in comparison; therefore, 206* seems out of bounds. Looking for data on what the max temperature of the riser should be.

They were intact and I addressed impeller remains in my OP. Last time an impeller blew was 2016 from the owner previous to me, been changed 3-4 times since then.

Replaced the impeller, all good now.