S-10 carb runs too lean mid-throttle?

I have a performance issue with my engine. There is a point in about the middle of the throttle range, just about the point where the second barrel starts to open, where the engine acts as if the mixture is too
lean (hesitation, loss of power, difficulty climbing hills, sometimes backfiring through the carb). When the engine is cold, it doesnąt exhibit this problem if I press the accelerator all the way to the floor. But after the engine warms up, flooring the accelerator doesnąt cause the engine to go any faster.
It's a 1982 Chevrolet S-10 pickup with the original engine (over 200,000 miles, but with oil pressure still around 30 PSI at warm idle) and a łnew˛ stock Rochester 2SE carburetor (a VaraJet II without the electronic feedback components).
Modifications: - Instead of the stock mechanical fuel pump, it is running a universal inline electric fuel pump running 6-8 PSI. The fuel pump is brand new. - Aftermarket true dual exhaust which was added before I bought the truck. A pipe going from each exhaust manifold to a muffler and tailpipe. No catalytic converters. - The charcoal canister for venting and emissions is bypassed.
Maintenance and Troubleshooting: ­ The fuel pump is brand new - The carburetor was purchased rebuilt by Champion brand and replaced less than a year ago. ­ I replaced the gasket/adapter plate between the carb and intake when the carburetor was replaced. ­ I've checked for air leaks, blocking all the vacuum stems at the carburetor, and checking around the intake with a stethoscope. ­ I replaced the vacuum distributor advance a couple of years ago. It is connected, and checking with a timing light reveals that it is working.
Any ideas where to look next?
Many thanks in advance.
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I can't help you but maybe these folks can:
http://www.chevytalk.com
click on forums and scroll down to the s-10, gmc sonoma forums
two websites dedicated to s-10's:
http://www.mys10.com
http://forums.s-series.org
-W
-Woogeroo
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