[gmachinz]

Well on that split cam 290 duration on the exhaust side combined with that relatively big lift (for a street flat tappet cam) is not going to provide a lot of vacuum @ idle. The 5hg rating simply means once manifold vacuum reaches near that 5hg, the secondary enrichment circuit begins to dump fuel even if it isnt needed by the engine. If you have power brakes, TH350 kickdown line, climate control vacuum feed line-basically anything requiring vacuum is going to use a little bit of it so you really want to maintain a good manifold vacuum-say 16-18hg @ idle is pretty good for an old school SB with a decent cam.

So use a vacuum gauge and put it on a good manifold vacuum source with all your vacuum lines connected and @ idle what do you get? Cracking the throttle does drop the vacuum signal a little but it will drop more under a load (acceleration). So if you have 10hg @ idle before cracking the throttle Im willing to bet just off idle theres a pretty good shot of fuel getting dumped which can cause a momentary bog before "coming alive" so to speak.

A stiffer secondary spring rate serves to delay this secondary enrichment fuel dump so it should help a little by letting the engine speed get up there a little bit off idle so the bog doesnt happen as much or at all. I would like to point out at this time you might want to upgrade your ignition a bit to help light off the extra fuel you're using too-most people think more fuel is better when imo a stronger ignition hit is what most engines can use. If running an HEI think about a set of 40-50 ohm performance wires such as Taylor 409, Magnecor, Moroso Racing, ect. Then Id suggest a matching module/coil from somebody like DUI or Performance distributors. This gives you increased dwell time (which should help with the slightly long duration specs on your cam) and a 60K ignition coil should really light it all off after that!