So I have a Holley 750 DP square bore carb that runs pretty good. Open throttle is awesome. Idle, fantastic. Here’s my problem. When cruising, the car seems to load up and spring forward if you breathe on the gas. Part throttle is fine after it gets through the hiccup. Is the power valve too strong? It is jetted correctly and the accelerator pumps are working well. I took it apart and cleaned it to make sure the metering blocks were not, well, blocked. I can’t figure this out and don’t have any bigger power valves lying around. I know the power valve opens when a certain level of vacuum is reached in the motor. I have a #5 power valve in it as of now. What do you think? Would a larger # power valve make the difference and smooth this thing out opening the valve at a higher vaccuum (less throttle)? Come on Geniuses, I need your help. You guys have forgotten more than I know!

 

How about the build specs.

 

On the motor or the carb? The motor is a 10.5 to1 455 with Edelbrock heads, msd, headers, edelbrock intake. Cam is, lift; 520, 523. Duration @.50 230/236. Flowmaster etc. 373, gear.

 

Damn, nothing.

 

So what you are trying to say is when you give it a little gas it stumbles? and then catches up? What is your vacuum? What is your timing set to? Do you run a vacuum advance can?

 

Oneof the better articles on holley carbs. Just make sure your ignition is running correctly (initial & total centrifugal timing, timing curve, vacuum advance, dwell, etc) before you do anything with the carb.
http://www.bob2000.com/carb.htm

 

Power valve does'nt matter when cruising. Your driving around on the idle circuit and the transition slot. More tuning and adjusments are deffinately necessary. May have to start drilling the idle tubes themselves. If you can borrow an aif/fuel meter it will help.

My own car needs more tuning cause I'm dealing with the same kind of cruising issues.
I've been using holleys since 2001 and have figured out that they always run great at WOT but need some finese for everything else.
Mine started out as an 800cfm dp and is equal to an 880-890 cfm now after upgrades and drilling everything bigger.
I've been lucky to have a friend who knows carbs and nas tons of patience to get help get me this far.
Keep me posted and I'll try to help where I can.

 

Dwell, chuckle. Does anyone even know how to set points anymore? Thank you kindly for the help. None was forthcoming. The car has an MSD Billet blah, blah, blah. I think I have it narrowed down to the idle circuit like Jim said. The car is very close to being right. I think this may be above my head however. After posting this I read up on Holleys and watched some vids. I found out the power valve has nothing to do whith cruising situations. Thanks again.

 

Yes, chuckle, a lot of us still understand and can work with points! I watched most of your video and don't understand what your talking about seems to run ok to me. But possibly a better description of your hiccup and some answers to a few questions would have lead to an answer.

A lot of us have found that most perceived carb issues are not carb related and blah, blah, blah...

 

Hey.... I actually own a dwell meter.
And you are correct, the power valve shouldn't be involved in cruise unless it's blown or opening way too soon.

 

Laughed at this. The car runs fine in the vid. Notice most of it is WOT. The car loads up and then surges forward. It's enough to snap your sneck sometimes.

 

Is the power valve blown? If you've ever had a backfire through the carb it probably is blown. If it is you'll have extra fuel all the time. As far as your #5 power valve I would not go bigger (higher number). To find the right power valve hook up a vacuum gauge and go for a cruise noting the lowest vacuum reading while cruising, not getting on it, normal acceleration. Put a power valve in it that is two points lower than your lowest reading while cruising.

Unfortunately the problems you are having could be a multiple of problems and not one thing specifically.

 

The power valve is new and I have swapped it with another to make sure it is not the problem. I read about the formula to find the right number power valve and you are right on it. When I figure this one out I will post the answer. Because I WILL figure it out.

 

When yoou say "load up" I usually associate that with running rich. How do the plugs look?
And when you say it springs forward with just a little gas, that makes me think it's running lean and the accelerator pump richening it makes it run better.
If your ignition checks out ok, try larger primary jets.

 

Ah! Jetting! I thought I would try that next.

 

If the carb is loading up (check for black spark plugs) the idle circuit is too rich. Try .005 bigger idle air bleeds, primary side only. Also if the carb cold starts and idles quickly , its too rich , it should take some engine heat before it will idle smoothly(assuming you do not have a choke) , thats a start. Thanks Milan

 

Ok, found the problem. When you are running an electric choke, make sure the wire is connected! It became unconnected at some point. The motor would suck air in and open the choke but it would go back to being partially open. When I would pull the air cleaner it would be open. But not all the way. I noticed it wasn't opening correctly when I went to adjust it. It wasn't responding. I checked the wire and found it was disconnected. Hooked it up and the car runs sooo good I can barely stand not to drive it. Since I went through the carb and replaced everything it is ultra smooth. The fast idle works perfectly and now the choke does too! So, moral to the story? Check the damn wire even if you think the choke is opening. You should be able to watch it open. Can I get a DOH! on this one?