300/440/550 Swamped 550 last summer, not running right since.

A friend of mine has a 94 550sx, westcoast head, half pipe and mariner water box. Ran pretty good last year. He swamped it last summer and it hasnt run right ever since. We took it out today, gave it a good carb cleaning, played with the highs and lows since we have no clue where they were before he tore it down to clean it. Its running with half the power I remember.

Here is the info I got after we got back to my place after hours of messing with it... We were in my driveway and he told me he pulled the head and re used the old copper head gasket. it was also covered in sticky copper spray gasket. We pulled the head and one dome was black and the other was light tan, almost wet. The bores looked good and he did a compression test before bringing it over and has 145 in each hole. Im leaning towards a new head gasket. Any suggestions?
 

N2 H2O

Previously Tbone550
Location
Charlotte, NC
The head gasket might be leaking water into one of the cylinders, which is why it could look tan. I would definitely replace that before I did anything else.
 
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