Aloha,
If the other suggestions do not prove successful, perhaps Hal has identified the problem as a sticky valve guide. I have had a recurring problem on my XPAG engine with a burnt exhaust valve on #4 cylinder (twice since head was rebuilt in 2004). Inspection by another machine shop found that the new bronze valve guide was not reamed to sufficient clearance causing the valve to stick. Hopefully a little reaming and a new valve will solve that problem.
If you are unsure of the originality or history of your XPAG engine you may not have the head that left the factory with the block. Here is some info from Neil Cairns book, Engines for MG's, Their Story After 1935:
The 'casting number' is not the 'part number' normally. It is the number of the wooden 'pattern' used to make the casting. The same 'pattern' may well be used for different applications decided upon by the way it is machined. This way a cylinder block may start as identical items but end up as a 1250cc MG TB engine or a Marine version of the Brockhouse 'Morris' boat engine; or a MG YB engine or the Wolseley 4/44 version. They will be differentiated by their 'part numbers' stamped into the machined metal or on a riveted tag. That on the cylinder block appears just behind the dynamo, the one on the head is on the top face.
Casting Numbers.
Model/Item...............................................................................Casting Number
· Early Morris/Wolseley Ten 1140cc 'X' cylinder block, oval,...................... 22500
· Later post war 1140cc cylinder block, oval water holes,.......................... 24144
· Very early 1250cc block with no timing chain tensioner,..........................24001
· Early M.G. 1250cc 'X' block, octagon cast in, oval water holes,.................24142 & 24146
· Later post war 1250cc M.G. block, octagon cast in, oval holes,.................24445
· 4/44, later TD, & TF block, no octagon, round water holes,......................168421
· Early 1140cc cylinder-head, no centre oil drain, oval water holes,.............22812
· Later post war 1140cc head, same as early 'T' type, oval holes................22952
· Later TD, & Y oval hole cylinder head, short reach plug,..........................22952
· 4/44, TD & TF head, round water holes, long reach plugs,.......................168422
· Late 4/44 head, round water holes,.....................................................168425
My engine had a later TD head that uses short reach plugs. This head has oval holes not round holes like original TF head with round holes and long reach plugs. This can lead to confusion as to which Head gasket to use. Again from Neil Cairns book:
THE ROUND HOLE HEAD GASKET CAN ONLY BE USED IF BOTH HEAD AND BLOCK HAVE ROUND HOLES.
Any other combination, use the oval holed gasket.