Low RPM stumble

[540Valiant]

Using my rule of thumb, the target idle-timing needs to be about 22* to 27*;starter kick-back being the limiting factor.
On Holley type carbs,the accelerator pump freeplay needs to be readjusted after nearly every idle speed adjustment.
With the new idle-timing, it may be possible to close the primaries to get a better transfer port sync-up. Put a vacuum gauge on the spark port. Crank the primaries closed until the sparkport reads less than 2 to 4 inches vacuum. Then set the idle speed with the secondary cracking screw. Reset your idle mixture screws for best idle quality. Reset the accelerator pump freeplay(s). That cam should be ok with the standard A-pump, but all the checks need to be in there, and with 2.76s will need a pretty good shot to get going at low revs.

2.76s with a 245 cam! Are you serious? You cant really expect to tune that very well.Wait til the 4.10s are in there and a hi-stall. Everything will magically come together!

Nicely diagnosed! For what it's worth! Finish the combo before troubleshooting!

 

[Breadburner]

Yup! How would I diagnose a bad power valve?

http://forums.holley.com/entry.php?430-Holley-Power-Valve-Tuning

 

[Treblig]

http://forums.holley.com/entry.php?430-Holley-Power-Valve-Tuning

Yes a bad or incorrect size power valve will definitely give you a hesitation at ay speed when you depress the accelerator. Sometimes it's off the line and sometimes it's at cruising speed but it almost always happens when you step on the gas.


Oh, I forgot...on a Holley there are also different sizes of accelerator pump "cams". I have a box of all the different accelerator pump cams for trouble shooting. The cams determine how much and how fast the accelerator pump shoots the gas when you accelerate.
Treblig

 

[Snake]

IF you mixtures do nothing in or out at idle.Bad power valve.................

 

[mguner]

IF you mixtures do nothing in or out at idle.Bad power valve.................

It is probably not so much a "BAD" power valve as one with too low a vacuum rating. A bad power valve would not hold vacuum and would also leak fuel into the intake. Too mild a pump cam profile as mentioned or the check ball or needle under the squirter nozzle is missing. You could also upgrade to a 50cc pump if not so equipped and have a longer range of shot available from the pump.

 

[AJ/FormS]

The way I diagnose a power valve on a Holley, is to pull it out and stick it in a power-valve tester. Shazzam!.
Okay, so you dont have a tester?
Option A) Pull the carb off, dump it upside down and drain the gas out. After that stand it up on the bench,primary bowl up. Remove the bowl and the Mblock. Check for liquid fuel in the vacuum cavity where the PV diaphragm resides.If its dry, the diaphragm is good.
Option B) Remove the PV and sub in a PV plug.
Option C) Replace it with a known good valve.
Option D) Check for fuel dribbling out of the main nozzles at idle.This is D), because on low vacuum engines this doesnt always happen. On older carbs without the anti backfire checkvalve, fuel can pass through the diaphragm and flow down the signal passage and dump into the manifold under the throttle blades and you cant even see it.

If you have a Holley, you should have spare parts. Actually, swap out the word Holley, and sub in carb. Sooner or later you are gonna need tuning parts, on any carb.
I like Option B), cuz I can take the car for a drive (with a vacuum gauge installed)and see just where the PV needs to be calibrated at. When the MJ circuit quits(this will be obvious), check the vacuum reading. Then from your PV collection, select one with a close rating and verify it on your PV tester. Install it and roadtest. I like to delay the PV circuit as long as possible without getting into driveability issues. I also like to run the MJs as lean as possible. To that end I always disable the secondaries, and fine tune the primary side first. And every Holley I ever worked on has a fat low-speed circuit.Which is OK, mostly. But if you are gonna hiway cruise it for lotsa miles, it sucks a lot of gas. So I usually lean it out quite a bit, and then fatten up the MJ a hair.The plugs will last for, lets see now; I think my S has well over 100,000 miles on its original Champions. Thats 15 years now
If its not clear, this is how I do it;Get timing right, Dis-able secondaries,Install PV plug,Install too-small MJs,roadtest. Sneak up on correct MJ size. Install the ballparked PV and roadtest.Install a later opening PV and roadtest. When you encounter issues, back up one PV size and begin reducing MJ, until issues reappear. Increase MJ to last good roadtest. Move to pumpshot. Get that dialed in. Enable secondaries and dial that in. Revisit timings. Tune Vacuum advance. Revisit primary MJs.
Lots of times with a well-tuned Vcan, the primaries can be run quite a bit leaner, than with no can. This really sharpens up the part-throttle response.
Expect to spend a lot of time on this. A lot.
Unless you have an A/F gauge or two. I dont recommend the Edlebrock dancing light one.

 

[AJ/FormS]

With that big cam the idle A/F will be jumping around some. Thats normal. You must have a pretty fast gauge to catch it. You just need to understand whats going on in the manifold. At idle, its chaos in there.To get a sense of that,put a vacuum gauge on it. Then slowly bring the revs up to about 2500. Watch the needle climb.It will start off pretty low,climb rapidly, level off and then,perhaps, begin to fall again;a bit. This is all because of that cam and whats going on with airflow between the piston-top and the bottom of the throttle blades.
BTW, the lowest rpm point where the vacuum levels off is the lowest cruise rpm to entertain. Thats the point at which the engine first becomes efficient. If you gear the car to cruise there and lean the carb out there, it will return the best fuel economy there. You can cruise it slower or faster,but it wont likely get any better mileage.But I suspect you aren't concerned with mileage now are you?