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.


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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?


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Joel Adams
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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.

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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...

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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.

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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|


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

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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?


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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.


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Joel Adams
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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|

Have you made sure there is no vac leaks ? When i put my holley on I had a big hesitation and remembered I left a vac line off , once i put that on I had a small hesitation usually over come with a full throttle pull but always present at light throttle , i found a cracked vac line and all is well now 

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1980 , ZZ430 , 3.92 rear , 700R4 2400 stall NEW EZ EFI 

I think I fixed most of it!  These Holleys have a small set screw to barely crack open the back barrels at idle.  It's upside down, tiny, and you have to use a right angle screw driver with a very small bit to get to it.  I monkeyed with that a couple of years ago, so I backed it out a quarter turn and most of the hesitation is gone.  I guess I was getting too much air through the back barrels.  I still might try going back to ported vacuum to see what that does.

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Just for the edumacation of us carb-challenged folks, what is the power valve?

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They are in carburetors.  Mostly Holley.

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the power valve  works in combination with vacuum, to richen the fuel supply to the carb under heavy  load, the number stamped on the power valve tells you when it opens, my 750 has a 65 which opens at 6.5 Hg of vaccuum. 5 months ago I suggested that you go back to portd vacuum rather than direct vac, I assume your MSD dist. has a vacuum adv. canister on it, did you try it, and if so what happen, I'am friggen pissed because this afternoon I spent about 45minutes explaining why you don't use direct vac to the canister and when i Clicked on the post button it went to never never land, think about it, if you put 12 degrees of initial into the motor and plug you canister into direct vac. and the canister has lets say 12 crank degs. of total advance built in it you now have 24 degrees of initial at idle, right? wrong! static means it doesn't move, because  the advance canister  varies advance with throttle position and vaccum  at low to mid range your initial is moving around causing either a surge or stumble, I'll say it again, plug it into ported vac. just like factory, holley, edelbrock suggest and forget the BS on how cool, and good they run with direct vac. and let me know.

|UPDATED|9/29/2012 6:07:44 PM (AZT)|/UPDATED|

Serenity now, serenity now.    Thanks for the reply.  For the first time in a month, it's been raining all day here in N. Tejas, plus, it's College Football Saturday.  So I haven't been able to play with the vacuum source yet.  I'll get to it sometime this week...err... month.  Gotta chase down this blower high speed thingy too.


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