About a year and half ago I replaced the manifold on my 350 with an edelbrock airgap. At the same time I had my Holley D.P.'r rebuilt. The car would hesitate, chug, sound like it was flooding and be ready to die, and then would take off. I initially thought it was a bad rebuild on the carb. Last Oct, I dropped a crate ZZ383 into my 64, with new Holley 750, new fuel pump, etc. Put on the Edelbrock airgap on the engine. It still runs very rough at low rpm's. It lags, thinks about it, and then takes off like a raped ape. Dropped the clutch last week and put the pedal to the floor, and the car almost died. The airgap is the only thing that came with the new engine. Any thoughts?
Bill

 

Bill, 2 thoughts come to mind, timing and maybe too much carb? Bill.

 

i agree.. i think it "should" have around 650 cfm.. also, electric fuel pump of manual? ive been told electric doesnt put out as well (at least the lower end ones)
maybe to DP wasnt rebuilt correctly?
do you have a second intake you can swap in and check?

 

The carb from the factory on a Z/28 was a 780 as was the carb on the LT-1, so I submit it's not to much carb.

It could be as simple is not enough pump shot. There are squiters on the primary and secondary of this carb, they might need to be a little bigger. It can also be the air cleaner base and how it funnels air to the carb.

The only thing I heard so far that makes sense is checking the timing, then start looking at the carb setup.

 

Thanks for the help. I'll start on the simple things, and move from there.
Bill

 

I had a similar problem and talked with Summit tech in detail. He advised on raising the float level as he had the EXACT same problem. His explanation was poor gas quality and you'll notice Holley sells jet extensions for this exact reason is my understanding. I have a built 355 400/400 HP torgue and run a 750 Holley DP with a Torker 2 and I can't imagine you having too much carb with a ZZ383. I'm not a mechanic but that is a very simple change to see if it does solve it. Just advice I was given. Good luck. My problem turned out to be different anyway.

Also you need at least a 1/4 inch spacing from the top of your carb to the top of your air filter cover for the vents, put a nut under the base to tighten down the top. I bet it'll turn out to be one of those simple things that are so obvious you just overlooked it, mine was. Another thought is how much fuel pressure your pump is putting out. Call Holley tech support as I believe they only need max 8lbs pressure, I may be wrong though.

Hope it helps.

 

A 750 should not be to much carb for what you are driving

However it does sound a lot like your accelerator pump is not adjusted properly. It would be best for to get a Holley how to book but if the following makes sense then you can at least have them adjusted correctly....... first forget the gap measurement methods for setting the accelerator pump spring

Then with your motor warmed up and at idle with just VERY light downward pressure touch the acorn nut on top of the primary accelerator pump spring...... there should be an immediate response of fuel coming out of the two primary pump nozzles....... if not put a wrench on the bottom nut and turn the top with a socket until you are unable to get any movement from the pump lever bar (that moves with the throttle linkage but not the lever under the fuel bowl) when you tap on it....... when the smallest adjustment that will keep the lever from moving when you tap on it and still allow fuel to squirt out the nozzles immediately upon putting slight pressure on the acorn nut on top of the accelerator pump spring you will have it adjusted correctly.

There are some other reasons that cause fall on your face lag and then take of situations but IMO the accelerator spring adjustment is the most common cause of that....... but you really need to get a good carb tuning book !!!

Try the one written by Dave Emanuel "Super Tuning and Modifying Holley Carburetors"

Doug

 

Check to make sure that the tab on the accel. pump link is actually riding on the pump cam. Sometimes this tab is bent wrong and rests on the throttle shaft spring instead on the cam and this causes a lag before the accel pump shoots. Just bend it a bit more.
I had this on 2 Holleys so far. This tip was posted by Lars awhile back.

Daffy2

 

If I read you correctly this was a problem with an old and now a new Holley? What distributer does the new engine have? The old one or a new one? Do you have Vacuum advance?

 

I have a 790 Holley on a 327 so it is certainly not too much carb. It sounds like not enough air as was stated above. Try a K&N racing air cleaner - the one that breathes through the top as well (no cover) and see if that helps if everything else fails..

 

Actually, the gap measurement method works great, but most folks don't bother to read the instructions first (which require that the throttle lever be at the wide-open throttle position when making the adjustment), and set it with the throttle lever at the idle position - that results in a delayed pump shot.

If you do the adjustment as specified, you'll have an instantaneous pump shot with the slightest movement of the throttle lever.

 

Carb sizing:

Cu In/1728 X RPM /2= CFM

Thus 350 CI/1728 X6000 RPM/2= 608 CFM at 6000 RPM ASSUMING your engine is 100% volumetric efficient, which it isn't.

600-650 CFM would be proper for this motor, but your accel pump sounds like the culprit as others have mentioned.

Too large of carb just wastes gas due to extra accel pump fuel needed to over come too large of primary bores,and resultant poor air/fuel atomization and gives less than optimum part throttle/low speed response.

780 was way too large on a Z28 unless you needed a carb that large that came from the factory to meet some sort of racing rules, and your motor was built way beyond what was streetable.

Doug

 

He has a ZZ383, not a 350 so a larger carb would necessary.

Let's talk about the Z/28, I had a 68 and it was just right for the motor. It was needed when twisting that baby above 6500. That motor was pure performance, even in stock form and bulit to rpm.

You did't mention the LT-1, by your formula it has to large a car, the 780.

I think they worked because the were not double pumpers.

 

Exactly. The Z/28 only had a 780 on it to make it legal in the SCCA Trans-Am series, where it only needed to make big power from 4000-8000 rpm. For "normal" driving, my Z/28 runs a lot sweeter with a 600 Holley on it - used the 600 for a couple of months when I was having the 780 (4053) restored, and it was a delight to drive.

 

BINGO!

 

Well that is very interesting John and I must confess that I never heard that to correctly set the gap with a feeler gauge that the linkage must first be set at WOT........ so I learned something new

However and not disputing you whatsoever (I listen when you speak and you have been very helpful to me) nor what I shared because I know from experience that that method works very well but because of the inaccuracies of using a feeler gauge, when both perimeters are not fixed it may likely cause more problems than are solved.

Recently I mistakenly set my valve adjustment at .014 but the next day while looking at my cam card I realized that I should have set them at .016. So I rechecked them at .016 with a feeler gauge but did not find it necessary to change a single one.

So I looked into it and found that a human hair averages about .003 in thickness. Consequently and IMO the bottom line is if you are not dealing with fixed perimeters such as spark plug gap feeler gauge measurement is useless when it comes to the thickness of only a human hair.

Doug

 

The hesitation problem you discribe is a common complaint we often hear from customers that have a engine with a air-gap intake manifold, the factory intake manifold used heat from the exhaust crossover on the bottom of the intake manifold in an effort to help further vaporize the air/fuel mixture as it passed thru the intake on its way to each cylinder of the engine. The lack of heat in these air-gap style intake manifolds can cause some of the fuel in the engines incoming air/fuel condense back into a liquid form instead of the vaporized form the engine needs for proper combustion. In my opinion the added high rpm performance that an air-gap intake manifold offers is often offset by a loss of driveability and power at lower engine speeds.

When we are tuning a engine with a air-gap intake we use a slightly richer than “normal” air/fuel cruise mixture, increase the accelerator pump squirter size plus we also use a stronger accelerator pump duration spring (the spring on a holley pump arm). The pump cam we most often use is the "pink" cam. A engine with a air gap intake will also like more initial timing than a engine with a non air-gap intake.

I hope this helps Henry @ olescarb

 

Gents,
Thanks for all of the help. My initial read from two years ago, was that the air gap manifold was the problem. It started once I put it on my LT1 along with an MSD ignition/dist, and a rebuild of my Holley 650. It ran very rough and would bog all of the time. I first thought it was a bad rebuild on the carb. I swapped carbs with no change. The guy that did my engine swap last year is very good. He felt confident that the timing and carb was set correctly and that the fuel air mixture was solid. The only thing that I have not swapped out, is the intake manifold. I'll get another manifold and see if it fixes the problem. Last night I searched the internet and found several sites with threads that discussed the same problem that I am experiencing and all had an air gap manifold. One of the techs at Summit told me that he had the same problem, swapped out his air gap, and the problem was fixed. Too bad their the guys that sold me the air gap anyway. I don't race my car, so I'm not too interested in max power above the 6000 rpm range. What I do want is a smooth ride with plenty of instantaneous low end power and torque. Any thoughts on a manifold?
Bill

 

I have had very good luck with the Performer RPM or the GM LT1 manifold pn 3972116 and a 750DP, 780VS or 800 DP. If performance is not an issue, try a smaller carb such as a 650 or 600 and the Performer manifold or the GM 461 hi rise manifold used on the 327/365hp L76.

 

Well you sure helped me Henry. I think you have just solved my hesitation problem I have had ever since I blocked the exhaust crossover on my L79. I managed to get rid of some of it by riching out my idle, and adjusting the accelerator pump,but a little hesitation still remains. Thanks