Twin-one pipe hot!

Think I know the answer but looking for confirmation. '78 KZ750 Twin with MAC header. It had sat for 13 years and something had packed a clay like material into the right side exhaust from the valve to 4 inches into the header. About 1/4" of the material in the exhaust port had been corroded out by the clay but the header pipe seemed solid. Once I got it started the right pipe was running 180 degrees hotter than the left. And the right cylinder fins were about 80 degrees hotter than the left fins.

So I replaced the head thinking the thinned out exhaust port was causing excessive heat transfer. Still at least 130 degrees hotter on the right pipe and 50 degrees hotter at the fins. Carbs sonic cleaned twice, float levels OK, needle jets both same position (4th slot). Ignition, advance, carb sync, cam position & timing, etc are all perfect. Both plugs look to be running slightly rich. So now I am thinking the exhaust header got thinned as well. Would a little bit of thinning cause that much difference? Thanks in advance.

Another guess could be, the valve seat and valve lip margins are corroded now and not sealing as well. So the extreme exhaust heat is not being sunk into the head for cooling.
Kinda like a blow torch heating the metal with not so much heat sinking anymore.

I did lap all the valves, mostly to make sure I wasn't losing compression. Never thought about them being potential hot zones. I just ordered a new MAC header today so we will see if that really was the problem. If not, the head comes off again! Thanks for your feedback.

Got the new header installed and it confirmed two things. One, the right pipe must have been pretty thin 'cuz the difference between left and right pipe is less than 40 degrees now. The other is that the left side is still running rich, which of course made the temperature difference between left and right even greater. Thanks to my bud for letting me borrow the temperature gun!