Holley 750 vac secondary is the carb, accelerator pump circuit is the perceived issue.

383 SBC fresh rebuild, 11:1 compression, AFR 195 heads, AFR titan intake, pump gas (93 octane around here), 236int/244exh at .05 - .603in/.612in lift - on a 111LCA, roller cam, hooker long tubes 1 3/4 primaries, dual 3in exhaust with cutout before axle and 3.5in over the axle to flow master muffler.

T56 manual transmission, 9 inch 3.50 gears

Ok, so I think this is all the needed information to provide some insight to my issues last summer. I finished this last spring and spent the summer tuning and breaking it in for the dyno. Winter came fast and I now will have to dyno in the spring. I do have a wide band AFR gauge on the pillar assisting with this tuning effort.

What I've got right so far:
- idle is strong and lopy around 850rpm
- 9.25 inches vacuum at idle. This was max achieved when adjusting idle mixture screws.
- idle set screw about 3/4 of a turn in (keeping transition circuit in play). This took cracking the secondary blades a touch.
- main jets are a little rich 12.3 at WOT but this is ok since I can fine tune at the dyno and have nitrous (rich is better than lean)
- power valves had to be changed, as I had a part throttle uphill bad lean condition. Went to an 8.5 valve, cruise vacuum is 12-13in, so this works. Made a huge difference.
- secondary circuit isn't causing bog, or blowing smoke, this is good enough until I get some dyno/ track time.

Now for the accelerator pump... my old 360, RIP, had this same carb worked best with 33 nozzle. 31 hesitated, 35 bogged, 33 lit tires. Easy selection. I started with the 35 on the new build, more displacement, better heads, etc. Undrivable, stumble, carb cough, no recovery.

I had a 37 nozzle (biggest on hand) and thru it in. A little better but still bad. So I purchased 40, 42, 45, nozzles and 50cc capacity upgrade to go with (arm, cams, diaphragm.) I'm now at 45 and it's drivable and fast but... if I go wide open from 2-2500rpm in say 3rd or 4th gear the carb still coughs and chokes before pulling. This hesitation can be felt and sometimes carb coughs even with moderately hard acceleration. My buddy was following me and said between gears under hard acceleration I puffed smoke between gears telling me I'm rich when initially punching it. Opposite of a carb cough? Monkey wench? Ha

The pump linkage is at 0 lash, but worth noting with the 50cc pump cam the throttle will move very slightly prior to squirting due to cam shape. I feel this could be part throttle issue, but not the WOT cough. No other screw hole alternative to engage the cam earlier.

I take pride in tuning and having a car be perfect and streetable. This is taking the fun ot of it for me and I'm stumped. I also feel at my expected HP level (goal - 400 RWHP not on the juice) I shouldn't even need the 50cc shooter setup. Any suggestions or things to try would be whil recieved.

Thank you!

 

Past experiences have shown me that a 50cc squirter isn't needed for builds on this level. Or rarely any level for that matter. I've found that the accelerator pump is rarely the culprit and it's tends to be timing related. This is assuming all is in good condition. Fresh builds often have a gremlin or two to work through that can get us chasing our tails. Like a vacuum leak.
I've got a similar cam (236/242 with a 110 LSA on a 104 ICL) with slightly less compression at 10.2:1 (iron heads). This cam was previously installed in an 11:1 AFR 195 headed 355 and had similar idle vacuum to yours at 9.5". That was with the vacuum advance connected to ported vacuum and about 14° initial timing.. Since swapped into my 355, with less compression, I've got the initial spark lead up to nearly 30°. This is with a combination of the initial at 14 plus the vacuum advance of 16 through the manifold supplied source. Made a dramatic difference in drivability once the timing tune was sorted out. Accelerator pump is a pink cam and a 33 nozzle.
All of that said, have you checked over the basics? Like I said, a fresh build and all that.
Have you done a compression test? Nice bit of info to have regardless.
What is your installed intake centreline?
And what's your timing curve look like?
When you're cruising and accelerating what's the AFR gauge indicating? I run a wide band full time too along with a vacuum gauge. That's how I managed to dial in my power valve as you have. I went so far as to go for the largest PV channel restriction available (.059) and that along with the more suitably spec'd valve also made a real difference in drivability.

 

99.9999% of all Holley accelerator pump problems (other than improper linkage adjustment) are caused by mis-tuning of OTHER subsystems of the carb.

The way to tune a carb is to ISOLATE each of their systems - main (jets & venturis), idle, power enrichment, transition, pump, etc. - so that you FIRST tune the one(s) that affect EVERYTHING else; then, in the order of what affects what, the others.

Start at the beginning. The primaries should be PERFECTLY capable of ALL operations, including slamming open to WOT, all by themselves, without the secondaries. Step 1 is therefore, disconnect the secondaries altogether, and work on the primaries.

In Holley carbs, the systems need to be tuned in the order of: fuel bowl level, main, idle, PE, transition (not really adjustable in most of them, ya just gotta take whatever ya get, but it's usually not a problem anyway), and last of all, the pump. Then, the same things about the secondaries; except that, depending on exactly what carb you have, there might not be very much tuning available over there at all. To tune the mains though, it helps to rough-tune the idle first; adjust the secondary throttle stop until when the primary idle setting results in the desired idle speed, there's only .030" - .050" (about 1/32" - 3/64") of the transition slot showing.

From there, follow the Holley Tuning sticky that Apeiron compiled out of abuncha my posts on the subject.

The ONLY purpose of the accel pump is to "cover up" the interval of time, occurring after opening the throttle, where the air has begun moving, but the fuel in the passages (main system) has not. Fuel is heavier than air, therefore has more inertia. Remember: that is the ONLY purpose of the pump. In a properly tuned carb, it's almost irrelevant and unnecessary. It needs VERY LITTLE tuning on its own unless something else is already seriously wrong.

Also remember always, THE ENGINE will tell you what IT wants the carb to do FOR IT. Pretty much every engine is a little different; different cam, compression, cam timing (adv/retard), head flow, intake plenum volume, CID, exhaust efficiency at clearing out the spent gases from the cylinders, etc. etc. etc. Then on top of that, every CAR imposes different requirements on the engine; weight, gears, converter, etc. etc. etc., all of which affect what THE ENGINE will want from the carb. Just because a carb "ran good" on one engine does not automatically mean that its tuning will match some other engine's wants. Disabuse yourself of whatever notions you might have about what YOU want to GIVE (force upon) the engine, and instead, let THE ENGINE tell you what it wants.

"If it RUNS good, it IS good". Too simple. The flip side of course being, if it DOESN'T RUN good, it IS NOT good". You find "good" by making a change, and seeing how THE ENGINE responds. THE ENGINE will not care what YOU think about squirter sizes, pump cc's, jet size, PV, and so forth; it will simply run better, or worse, according to how well YOU match what YOU give it, to what THE ENGINE wants.

So, get your fuel level correct, FIRST. Primaries should NOT dribble fuel out the sight plug while idling, but should take a little jogging on the fender or whatever. Start there. At the BEGINNING of the tuning procedure (fuel level, main system, ...) as described above, NOT the very end (accelerator pump). First things before last things. In order.

 

Intake centerline is 105.

Timing has not been tuned yet other than 10 deg at idle, no vac advance. I have a vac advance limited to 6 deg. Medium springs on the mechanical advance and total timing is unknown but assumed about 34 deg. Dyno is step one this spring to finalize timing for max hp. This will also help with carb main jet tuning to get it perfect.

The only thing I feel that has not been fully checked or tuned yet is vacuum leaks. Ether stil the best way to do this?

Thank you for the words and suggestions, I'll continue plugging until it's ripping!

 

Seems to me you've been at this enough to know how tuning the carb means that everything else is on spec. Lots of carb tuning trouble can be ignition related. And vice versa.
Of course as pointed out, the order that you approach the carb tune is important too. There's further support in an earlier comment about the accelerator pump just about being the least of the concerns when considering the carb as a whole.

While your dyno tune is going to be great for your maximum WOT timing, it may leave something on the table if the curve to get there is not what the engine wants. I kinda feel this is where you may be now as 10 degrees isn't much when considering the cam timing. As I mentioned, the last couple of 350's with 70° of overlap (like yours) I was involved with easily swallowed up 16°+ initial with a quick(ish) curve all in by 2800-3000. I had to limit the amount of mechanical available in the distributor because so much was used up in the initial setting. (Total in my case was 34)

As for vacuum leaks, that's always been a tough one for me as the issues I've had were either glaringly obvious, like a missing or damaged carb plug or vacuum line. Or the worst is an intake manifold leak from the lifter valley. Almost undetectable except for some weird symptoms and a reveal that comes from pulling the intake and seeing a mangled or misset gasket. Any investigations I've done used the old propane bottle sniffer trick. That said, it never uncovered anything for me as my leaks weren't such that the bottle was effective. Some here cam probably chime in with an effective vacuum leak "sniffer" .

 

Digging up my previous post. Car is driving well now. My setup just needed the 50cc pump shot and a 42 nozzle. Have a slight lean point uphill under light acceleration but as soon as the power valve kicks in she pulls just fine at about 13 afr.

Base timing is set at 10 deg and car pulls pretty hard right off the line. My HEI advance timing springs are adding advance late (about 4.5k rpm before all in) and i'm wondering how much an 11:1 on pump gas (specs above) can take early. If I go to the lightest springs will I ping? And if I ping will I know it? Never experienced 'pinging' before that I know of. If I do ping is it instant engine death?

 

What's your power valve spec and what's your "cruise" manifold vacuum?
10 degrees base timing? That's pretty weak all things considered.
You MAY find, if you can limit your mechanical advance, that base timing could be closer to 20 and that big pump shot won't be required.
This is on my experience with the same cam and equivalent compression. The difference it made all around was remarkable.
20+ MPG hiway too.

 

9in idle vacuum, 12 cruise. 10.5 power valve.

Didn't have an easy way to limit mechanical advance to less than 24 which is why I had it at ten deg initial. I am running the vacuum can (limited to 6 deg) so 16 deg total at idle.

Tonight i decided to lock out the timing and programmed my MSD box to control the timing. I didn't want to do this because I was worried about losing part throttle response without vacuum advance plus the added effort of locking the timing. Turns out the vac advance on my distributor works with the timing locked out which is nice. Eliminates the need for me to buy a GM map sensor and rig it to the box.

All in by 3400rpm is how I have it mapped at the moment. I plan to go 34 deg total and I'll start with 12deg base and work up on the base timing leaving the total at 34. Tomorrow we're getting snow, but I'll report back when I get it out.