I seem to have a light throttle hesitation.  If I'm cruising at say 30mph and I give it just a little gas, I get a hesitation and I have to kinda pump the throttle to get past it.  Timing is 12 bdtc with vacuum plugged.  I'm running manifold vacuum off the carb baseplate, not ported vacuum.  I can't figure out what this is.  This is just slight throttle, so I don't think the accelerator pump is involved yet.  Could it be the little set screw that keeps the secondaries slightly open?  If I really get on it, it goes, but just driving through the neighborhood and giving it about a 1/4 inch of accelerator pedal causes a hesitation.

Any ideas?  Power valve?  stock jets too small for E10 gas?  I'm pretty sure I don't have a vacuum leak.

Assuming the advance is working properly, the power valve would be my first suspect. I wouldn't rule out the accelerator pump, either...it is still in play at that speed/throttle opening. I would not be looking at anything on the secondary side of the carb. If the secondary plates were open too much, it would effect the idle quality.
Is this a basically stock LT-1?

MSD tach drive distributor and spark box.  TCS  and air pump are gone.  The rest is stock.  I think it has an 8.25 powervalve.  It has backfired through the carb a few times while driving.  I started the timing out at 18 degrees and have backed off to 12.

A backfire thru the carb will almost always pop the power valve...depends on the maker of the valve, and the material the diaphragm is made of. Some of the later PVs had a better diaphragm that would handle a backfire better.
I duno...I think I would start with that...

Yea, that's what I was thinking.  I also thought about getting one of those power valve blowout protection kits.  Problem is, I don't get the typical blown power valve indicators.  It doesn't idle rich, if I turn the idle mixture screws in it dies, no black smoke, etc.

It may not be ruptured, but....with the fuel we get these days(10% or more ethanol), it's possible the diaphragm has turned into a solid chunk of rubber, and not allowing the valve to actually open....I've seen that with some racing fuel in the past with "stock" type power valves. Just another idea.

Something else I just thunk of...does this thing still have the EGR system intact? That might be something else you would want to check....

|UPDATED|4/18/2012 7:23:19 PM (AZT)|/UPDATED|

Never had an EGR valve.  Plus the TCS and air pump are gone.

Okay...took the front bowl off and checked the power valve.  I holds plenty of vacuum on my MityVac.  It's not blown.  I noticed that my idle mixture screws are about 2 turns out from seated.  Is that excessive?  The accelerator pump diaphragm rubber seems to be fine and there is no play between the arm and the actuator at idle. 

What kind of changes need to be done to a Holley with E10 gas?  Bigger jets?  Bigger pump shot?

But...does the valve actually move?

2 turns out sounds normal...maybe a 1/2 round farther out than "spec", but...every engine combo is different.

You might try one step up on the jets, just to see if it could handle it, and maybe even take care of the flat spot too.

before you start changing anything try going to ported or timed vac. port on the carb instead of direct  vac.and see if fix's the problem, you will probably have to raise the curb idle and take it for a ride let me know.  I'am not going to x-plane it right now cause it takes to long, if it fixes it then I'll will. 

|UPDATED|5/4/2012 5:48:14 PM (AZT)|/UPDATED|