98 mercruiser 4.3 port side exhaust manifold runs 50 degrees hotter than stbd side

[FONT=&quot]I have the 1998 Mercruiser 4.3 Alpha carbed motor. The San Juan Engineering MC321 FWC kit installed, dry joint exhausts installed and has ran fine for the last 10 years. After changing the (weeping from the shaft) GM water pump recently, I noticed the port side exhaust manifold was 50 degrees hotter than the Stbd exhaust manifold, 210 vs 160 but the helm temperature gauge still showed the normal 170 after warm up. Compression check was good and consistent. I next removed the risers and confirmed all was clean and no water blockage issues. After reassembly there is still the 50 degree difference despite painstakingly burping the air out of the system. The 140 and 160 thermostats I used have function tested perfectly on the stove, opening widely and timely at their respective temperatures. I next removed the thermostat and ran it on the hose again. Both exhaust manifolds temperatures were dead even through 15 minutes and various rpm so we know the system has no blockages now for sure. My question is...does the SJE MC321 kit thermostat housing have a water routing difference that shows only between port and starboard when a thermostat is installed? Is a diverter needed like the OEM system to even out the flow? Or?[/FONT]
[FONT=&quot]Once again, thank you for your insights.[/FONT]

The usual reason for that is a blockage on the 'hot' side where the water and exhaust finally meet. Water is diverted to the 'cool' side which creates your problem.

Jeff

Thanks for your help, I removed, inspected, flushed the hot side manifolds/risers to the tune of $80 of dry joint gaskets and had the same problem until I removed the thermostat. Now both sides are the same normal temperature...unfortunately can't run the engine that cold though. Any other thoughts about the thermostat causing uneven flow port/stbd or if TSTAT isn't opening would that cause the one side to run cooler because the pump bypass is on that side? I added a few photos of the coolant bypass arrangement, the thermostat housing and the "hot side" manifold. Maybe that will help?

More photos upload:

Your system is what is called a "closed cooling" system. You called it FWC...
Raw water cooled = No heat exchanger
Fresh water cooled (FWC) = Closed and with heat exchanger.

That means the Manifolds and engine are cooled by the antifeeze/water and is controlled by the thermostat and heat exhanger.

That is why your engine temp is constant and correct (if it does not increase past 180-190 when running under load).

The Elbows typically are cooled by incoming COLD water as that water also mixes with the exhaust gases to keep exhaust cool and not melt rubber hoses (3-4 inch hoses).
This cold water is supposed to be equally diverted to each Elbow.

Typically a 20 or so degree difference is normal.

What I would suggest is the INCOMING water is being restricted. Either low flow (impeller) or a pinched hose or clogged power steering cooler or some other clog/blockage.

I will try to look at a water flow diagram in a bit to see what else may be of concern.