Do headers weaken the vac on an engine?

My vac secondaries wont open after installing them, and the only things that's changed (not counting coincidence) is that I put the headers on and adjusted the carb (holley 650) a bit rich to increase the lope (for the new cam).

The secondaries open fine with the new cam before I put the headers on.    

I've seen ported vacuum drop slightly with long-tube headers, but manifold vacuum shouldn't be affected.  At least not to that degree.  I'd take a closer look at the carb.  Also, put a vacuum gauge on the manifold to see what you've got.  If you've got less than about 15" at idle, you may need to step up to a mechanical secondary carb.  

Assuming, that is, that the bottom end is in good condition with good compression, etc....  

Thanks for the info.

Bottom end is in good shap (rebuilt to stock).  

Headers can really lean out the mixture, and if you didn't conmpenstate for this this can have a pretty large effect on vacuum.

I don't know what you're talking about with making a rich mixture to increase the lope? Lope is a low-speed misfire, you don't want it! And you would really have to screw up the mixture to make it sound like the engine has more 'lope.'  

GigTG is right with less exhaust restriction manifold vacuum does drop.Hook up vac. gauge to intake vacuum and adjust carb idle mixture screws to get highest vacuum reading,if auto trans adjust in gear.Your plugs will thank you      WIEDER  

You can also adjust ignition timing to peak manifold vacuum.  Just be careful of detonation.  

The richen (for lope) was the idle mixture.

Mine seems to lope more with a rich mixture.

Thanks for all the info. I'll adjust mixture and timing with vac gage and see how that works.

Thanks again.