Effects of old fuel?

So, finally got my '98 M3 out on the road today.  First off was an alignment/corner balance, then it was off to the dyno.  

Car definitely underperformed on the dyno, made less horsepower than it did last time, despite having cams and a ported throttle body now.

A/F ratios were super rich, like below 10:1.  Car otherwise drives fine, no CEL, runs smoothly.  Is guzzling fuel like it has a big block tho.

I'm wondering if its because the fuel is old?  I don't remember when I last filled the car, but its possible it was as far back as last July and the soonest would've been November.  I really think I didn't fill it in November though.  Regardless, fuel is at least 5 months old.

Could that cause the ECU to run really rich and have the motor drop ~30rwhp on the dyno?

I don't think 5 mo old fuel would do that.  5 year old fuel, yeah, OK.  The super rich A/F is why the power was down.  Now why was it so rich?  Oxygen sensor problem? Something clogged up?  Put a tank of new gas in and a bottle of Sea Foam, run that out of it, then fill up again and see how it does. 

Another M3 owner had the same thing happen, but his fuel was quite a bit older than mine. 

No codes for O2 sensors and I replaced them not that long ago.  I don't run cats in this car (track car) so nothing there to get clogged up.

I'll run through this tank of gas as dry as I can get it, then put in 3-4 gallons of fresh and see how it feels...

Have you tuned it since the cam installation?

So, this new pig rich thing is after you did the cam and the throttle body?  Yeah, that's gonna need a tune of some kind.

I doubt it. When I did my S52 swap in my E30, the M20 headgasket blew in early January. The same fuel was in the tank when the S52 started in September. I think we just topped it off when it put down 235whp on the dyno. 

I wouldn't blame the fuel.

Assuming that richness is intentional- for that era, it could be the fuel learning is not updated since it's sat for so long.  And when it's not learned, it will tend to be conservative for exhaust temp protection.  Same thing applies for knock- if it assumes the wrong fuel, and has no time to really learn, that will be a double effect of retarded spark and higher exhaust temps (which increases the fuel to keep it cool).

I would not rule it out, but it would be a stretch.