[johndglynn]

Sean, I recently dyno'd my C3 and also came home with scary lean mixtures at the top end and 11bhp down on stock at 189 flywheel. I too figured it would be fuel pressure, as idle CO was spot on and was all OK up until about 4k revs, when the AFR curve dropped right off.

With no CIS gauges and only three days before a track event, I just had to get stuck into the system and see what I could find. I stripped most CIS stuff off the engine, cleaned it all out, found a few vac leaks, a duff connection on my WUR and a small fuel leak by the accumulator. I put it all back together, was definitely stronger so I redyno'd.

Now read 202bhp (flywheel extrapolated), with more in the mid range, but AFR at the top end was exactly the same. I had three hours to spare so I dropped the fuel pump and found a pinhole leak by a copper washer at the pump banjo - tiny bit of corrosion in there. Cleaned it all up and wow totally different. Not dyno'd it again yet, but it has given me lots more up top and am sure it has solved the problem. I took it easy on the day just in case, but I think we are home free - good job too as '76 C3 fuel pumps are silly money!

Good luck tracking down your problem, least you know it is an issue now rather than finding out the hard/expensive way.