[Rick Parker]

The manifold will have created the biggest impact on the carb since the signal to the mains is reduced until about 2000-2500 when it begins to work, below that it will not create enough vacuum to initiate the mains. If you check with Edlebrock these manifolds make their power from about 2500-8000 RPM. I think this is part of what you are experiencing, but I would agee that your current jetting may be a little large. These manifolds also do not have the best distribution at part throttle consequently the plug readings may not be consistant, specifically #1 & 5. They make VERY good power but in the mid to upper RPM range. I use one on my 289 and really have no regrets.