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Discussion Starter #1
Hey y’all so I’ve been trying to get this issue figured out since I bought the car a few months back. Either from a dead stop or cruising around if I put the pedal to the floor the car stumbles and sounds like it wants to die but doesn’t, even had a small back fire through the carb. If I ease into it
it’ll go but it feels like it’s not giving all it’s got.

It has a Holley 750 double pumper which I just replaced both accelerator pump diaphragms in. Before this, if I put it to the floor it would completely die. I have also put a vacuum gauge on it and adjusted all four idle mixture screws to achieve best vacuum which I believe was around 25 at the manifold. I also found that if I work the throttle linkage at the carb and snap it to wot it doesn’t seem to have any stumble or hesitation.

The car came with the 400 with the Holley 750, Edelbrock aluminum heads, beefy looking rockers, a good sounding cam, and long tube headers. I’m not sure of any of the specs since I was unable to get the build sheet for the upgrades. I just feel like this setup really should have some get up and go but it can’t get out it’s own way!

I can also try and upload a video if that would help

Any input or advice would be great!! Thanks!
 

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"Working the throttle" at the carb won't tell you anything or mean anything because in that situation there's no load on the engine.

There are references all over the internet about how to set up Holley carburetors, and a handful of those even have useful information (lol) - so you might do some searches on your symptoms. Because changing out your pump diaphragms "changed something", that suggests that you're in the right ballpark. Make sure your pumps are adjusted properly (internet references), your power valve (valves, if it has two) don't have ruptured diaphragms, and also that your ignition timing is correct. Getting the correct timing is more than just setting it to the factory 6-degrees initial and forgetting about it. There are other posts on here detailing the process.

You may need different pump cams to either speed up or slow down the pump shot, larger (or smaller) pump nozzles. The trick is to make small changes, test, understand what effect the change had, and act accordingly.

Getting a Holley set up correctly isn't as simple as everyone makes it out to be, I know from direct personal experience. It really helps to understand all of the carb's internal systems and how they're supposed to work, how they can be expected to respond to changes, how to interpret what your engine is telling you, etc. There are books available that have that information.
A vacuum gauge like it sounds like you already have is invaluable. Also very helpful is a wide band air/fuel meter, either a hand-held one or a permanently installed one.

Go ahead and buy yourself a tall stack of bowl and metering plate gaskets and get familiar with removing, disassembling, reassembling, and reinstalling it because there's a good chance you'll be "into it" multiple times by the time you get it right.

Start by make a record of everything you can: primary and secondary jet sizes, pump cam, pump nozzle sizes, power valve specs (stamped on the valve), idle air bleed sizes (if removable), high speed air bleed sizes (if removable), idle feed restrictor sizes and location, how much of the transition slots are exposed when the throttles are "closed" (idle setting), power valve restrictor channel sizes (if tunable)... everything you can measure or examine.

Keep a record of every change you make and what happened. (I used a spreadsheet for all this)

Back to your original problem...

If we ASSUME that your ignition timing is correct (again, verify it first), and your power valve(s) aren't leaking, and everything else is in good order (no vacuum leaks, etc.) the behavior you're describing is probably because the accelerator pump shot isn't "right".

It can either be too rich / too much / too "quick" or too lean / not enough / too slow. Both conditions can cause what's happening. Watch your mirror or have a helper watch when you 'stomp' it from cruise. Does it 'go' for a fraction of a second and then fall on its face or does it stumble immediately? Is there a puff of black smoke out the exhaust? What happens if you roll into the throttle as opposed to hitting it all at once. Can you find a rate where it doesn't stumble as long as you don't hit it any 'quicker' than that? These answers all help you determine if your engine is wanting more or less pump shot. Different pump cams control the timing and volume of fuel (to a degree). Pump nuzzles control the duration of the shot. Larger nozzles put out more volume quicker, but the duration of the shot won't be as long. Smaller nozzles "slow down" the shot over a longer period of time. If you find out that your engine is wanting a LOT more fuel from the pump shot, you may have to move up to the larger 50cc pumps on the secondaries, primaries, or both.

Every combination is different and what works for one Pontiac 400 may be horribly wrong for Pontiac 400. It's a process. That's why it's called "tuning".

There's a guy over on PY, Tom Vaught, who's an expert and has helped a lot of people. He helped me a lot when I was doing battle with mine.

Bear
 

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My first suspicion would be the power valves. If it has a cam that lowers the idle vacuum, then you need lower numbered PV's.
If it stumbling at w-o-t where theres no vacuum then the power valves are already open no matter what size they are.

I would do as Bear suggests and work on your accelerator pump sizes. Sounds like its not getting enough fuel and you have a lean backfire condition. If it was too rich you would have more of a bog than a stumble with backfire thru the carb. What size accelerator pump nozzles do you have in it currently? If you’re already at number 35’s then you need to up the pumps to the 50cc version Bear referred to in order to go to a bigger nozzle size.

Changing the accelerator cam size will help when you’re getting on it but not at full throttle or rolling into the throttle until w-o-t. When you stab the gas all way to floor instantly, it doesnt matter what cam type is in it the pump shot is at 100% immediately.

Assuming cruising conditions feel fine i would leave the primaries alone And possibly give it a little bit bigger jet size in the secondaries in addition to more pump shot.

Again as Bear stated you’ll have to play with it allto really dial it in. Make one change at a time until you get the results you’re looking for.

Also whats your timing curve look like?
 
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Discussion Starter #5
Thank y’all for all the info!! It’s definitely a lot to consider, it’s been a long time since I’ve done anything with a carb. I believe that the pump nozzles are 31 but I will check this afternoon. When I put my timing light on it at idle it is past the factory marks on the balancer by a little maybe 15 or so since the last mark is 12. When I get the rpm up high enough to where it stops advancing, with the vacuum advance off the distributor and plugged, the timing light dial is set to about 32 when it goes back to 0 on the balancer.

Also, when I roll off idle just a bit and give it gas it seems to kind of sputter but not bad. As for putting it to the floor it instantly stumbles no delay.
I had my wife work the throttle last night while I looked at the linked and when she held it to the floor I noticed I had about an 1/8” more throttle available that the pedal wasn’t using.

When I get it out next I’ll take a video of it at wot

Hope I am explaining this all right haha!
Thanks again for all the info
 

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About a zillion variables there. Agree with most all posted. Assuming timing is somewhat close, it sure sounds like you have a lean condition. Since changing some items in the carb seemed to have some effect, keep working there. I'd get the stack of bowl gaskets and a selection of accelerator pump cams and squirters. Since lean is suspected, I'd go real big right off the bat and see what direction your problem goes in.
Timing though, got to be sure of that. The "off the cuff" diagnosis is either timing or carb. You might be masking timing issues with the carb adjustments.
 

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Discussion Starter #7
Could y’all point me in the direction of the bowl and metering things I would need? I googled it but only see gaskets and not sure if that’s what I’m looking for.
 

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For example:
Gasket sets
The 'open' gaskets go between the fuel bowl and the metering block/plate. The others go between the metering block/plate and the carb body. These are for '4150' family carbs which I'm assuming you have, not Dominators.

If you're very careful you can often reuse the same gaskets multiple times. There's a torque spec for the bolts - around 15 inch pounds as I recall, but don't take my word for that without looking it up yourself.

When you remove the lower bolts, the fuel will drain out of the bowl(s) so the trick is to do that over some container to catch it, remove one of them, and drain the fuel before you continue.

There are also nylon gasket seals under each bolt head that are easy to miss/lose if you don't know to look for and be careful with them. Sometimes they'll stay stuck to the fuel bowl when you remove the bowl screws then fall off later as work on the carb, then when you put it all back together and reinstall it, you'll have leaks.
Screw gaskets

Power valves will usually have a number stamped on them. That number is "inches of vacuum" below which the power valve will open and feed fuel into the power valve passages. If you have a power valve with too "high" of a rating in a carb on an engine with a rowdy cam and doesn't make much vacuum, then it may not make enough vacuum to keep the valve closed when it should be, and the car will run too rich at other than wide open throttle.
Many times only the primary side will have a power valve. There'll be a plug in the secondary side. One of the ways to test for a power valve problem is to plug to the primary side also, disable the secondaries completely (disconnect the linkage or disable the vacuum actuator if it has vacuum secondariesand block them closed) so that you can drive the car on the primary side only - see if that makes it act differently. It'll be down on power at full throttle because you won't have the secondaries to provide more air and fuel, but that can help you figure out the cause for problems that are happening at the various transition points (idle to cruise, low cruise to high cruise, cruise to wot, etc.)

Holley's have a reputation for being 'easy to tune' --- they aren't. What they are is easy to customize because so many of the parts can be changed (air bleeds, pump nozzles, pump cams, Idle feed restrictors (on some), emulsion well restrictors (on some), Idle bypass air (on some), power valve restrictors (on some) etc. etc. etc.) To take advantage of that you have to understand how they work, what your engine is telling you it needs, and what to fiddle with to deliver that. It's also easy to chase down the rabbit hole, making all kinds of changes without understanding completely what's happening, and just hoping to trip over something that works.

Before I changed my car to a Holley format (actually an AED brand - 850 HO with annular boosters, mechanical secondaries, no choke horn, double pumps), I ran a real 455SD Qjet on it. My personal experience is that I found the QJet much easier to set up and get right, after I got Cliff's book and understood their systems. It was a big struggle for me to get that Holley 'right' everywhere: idle, low cruise, high cruise, launch, and WOT, even with the help of a quality wide-band digital air/fuel meter, Dave Vizard's book, and Tom Vaught's generous advise. The only problem I ever needed help on with the QJet was getting rid of a bog at launch, which turned out to have a cause not mentioned in any of the books or materials I had - fuel sloshing up through the secondary rod holes over into the secondaries and causing a momentary rich condition. I fought that one a good while until Cliff told me to try making a seal around the rods out of electrical tape.

I went to the Holley because I talked myself into believing that my engine needed "more" than what my QJet could deliver, otherwise I would have stayed with it. If I was wrong about that, I'm not sure I want to know... o_O:censored:

Bear
 

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When I'm in Holley tune mode, I smear a super thin coating of Vaseline on my gaskets. Frustrating to repeatedly wreck them.
Don't over torque the screws! You'll mess up your bowls/power plates.
 

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Bear and Mr. Taylor are both on it. 9 out of 10 carb problems are timing problems. You should always ensure timing is correct before carb tuning.

but since you are asking, Inthink you are lean going into WOT, that would be too small a stirrer and pump cam combo...

a stumble off idle is not usually the squirter from the acc pump, but the transfer slot in the side of the carb. If you have too much transfer slot exposed at idle, a common problem. Then you don’t get a smooth “transfer” from idle to off idle.

to fix that take carb off turn over and square the transfer slot hole try to set your idle mixture screws while either not disturbing that slot or only very minor changes...

WOT should also bring on secondary jets, as that circuit must be right as well...
 

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Discussion Starter #11
update

So I confirmed that the acc nozzle is a 31 size in the secondary but with the choke flap in the way I couldn’t manage to get the primary out to check it without fear of dropping it. I also reconfigured the throttle linkage and that greatly improved the off idle issue as well as transitioning to wot. Still seems sluggish but not much bog. I’m assuming this improved because of the slack in the cable was causing it to not get a clean crisp pull on the throttle. I did order a new throttle return spring since the one on it looks like an old screen door spring haha. I order it because when I would give it gas and let off it would raise the idle slightly even when I was off the glass but if I taped the pedal it returned to its normal ideal speed so I assumed the flimsy spring didn’t have enough to pull the linkage back fully. I got the one with the double spring setup, like one inside of the other.

I’m really happy with this improvement, what would be a good direction to go next to hopefully improve the power I feel as if this motor should make? Any suggestions on timing changes? Initial is around 15 and with vacuum advance off and plugged looking like 30-32 at increased rpm. Should I maybe raise it a few degrees?
 

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You should add the correct vacumn advance,...because you are giving up 10 more degrees of timing advance that you cannot get from the total mechanical advance.

racers take off the vac advance because they run at wide open throttle or on a drag strip,...light goes green pedal to floor..no need for vac advance if that is what you are doing.

But if it is a street car you are making it run hotter at idle and making it run poorly at most light throttle conditions because the timing as you are running it is tooretarded to burn those leaner mixtures.

if you are not just racing it put on a proper vac can... 10 more degrees at idle and light throttle will really help your engine run cool, smooth and powerful.

also it has no detrimental effect on top end as there is no vacumn at WOT so no Vcaumn timing...just your total mechanical which I recommend you start at 36 degrees....total and work from there.
 

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Hopefully this isn't too simplistic but I just added an M/E Wagner Performance adjustable PCV valve to my set up and it made a huge improvement. I've got an Olds 455 with a mild cam upgrade and Edelbrock Performer intake and carb. The power was pretty decent but it felt like it was restricted somehow. It would stumble at acceleration, had two small oil leaks I couldn't remedy, and had very strong emissions. I happened upon this product, read about others who had the same frustrations, and figured I'd give it a try. Really glad I did. Maybe it can help you, too. (I'm in no way affiliated with or compensated by them.) Good luck.
 

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Discussion Starter #14
You should add the correct vacumn advance,...because you are giving up 10 more degrees of timing advance that you cannot get from the total mechanical advance.

racers take off the vac advance because they run at wide open throttle or on a drag strip,...light goes green pedal to floor..no need for vac advance if that is what you are doing.

But if it is a street car you are making it run hotter at idle and making it run poorly at most light throttle conditions because the timing as you are running it is tooretarded to burn those leaner mixtures.

if you are not just racing it put on a proper vac can... 10 more degrees at idle and light throttle will really help your engine run cool, smooth and powerful.

also it has no detrimental effect on top end as there is no vacumn at WOT so no Vcaumn timing...just your total mechanical which I recommend you start at 36 degrees....total and work from there.
Just to make sure I understand correctly, I leave the vacuum hooked up to the distributor while I set total timing?
 

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Hopefully this isn't too simplistic but I just added an M/E Wagner Performance adjustable PCV valve to my set up and it made a huge improvement. I've got an Olds 455 with a mild cam upgrade and Edelbrock Performer intake and carb. The power was pretty decent but it felt like it was restricted somehow. It would stumble at acceleration, had two small oil leaks I couldn't remedy, and had very strong emissions. I happened upon this product, read about others who had the same frustrations, and figured I'd give it a try. Really glad I did. Maybe it can help you, too. (I'm in no way affiliated with or compensated by them.) Good luck.
I’ll looks into this, thanks! Was it hard to setup or install?
 

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No, sorry Imwas not clear,...disconnect vac and plug to set base timing. Just make sure that the vac can you use pulls in the correct amount of timing, below your vacumn numbers.

some guys pull their cans out, you have not done that, but also some cans pull too much timing, and on today’s gas that does not help you run good.
 

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Just to make sure I understand correctly, I leave the vacuum hooked up to the distributor while I set total timing?
No. Never set timing with the can connected. So many people get confused by this, probably because the word "total" implies "the sum of everything".

However, when talking about setting ignition timing the word "total" means the sum of initial timing plus the maximum amount of mechanical timing in the system and does NOT include vacuum.

Here's the reason: Whenever you're adjusting timing, what you're doing is trying to optimize the amount of ignition timing for best performance under heavy load, like wide open throttle or pulling up a steep hill. Under those conditions, engines don't make any manifold vacuum to speak of, at least not enough to register on a gauge or to activate a vacuum advance canister. Whenever you're setting timing, you're adjusting for best power/heavy loads - where there isn't going to be any vacuum. You can't duplicate those conditions with the car sitting in neutral/park, so you set timing with the can disconnected. The only way you might be able to do it would be to do it while the car was strapped to a chassis dyno, pulling a load at WOT. Certainly not something I'd want to attempt, let me tell you.

The only time you would get the light out and use it with the can connected is if you want to MEASURE how much advance it's adding under various amounts of manifold vacuum, but you never SET timing with it connected.

Bear
 

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Discussion Starter #19
So how do you go about adjusting the vac can? Leave it plug up to vac and then use the timing light to see how much of a difference it makes at the higher rpm compared to the same rpm with the vac line off and plugged?
 

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The easiest way is to read the number on the vac can, if it has one...some do some don’t it will have a number,....like 15 or 20. Some are marked for crankshaft degrees and some for distributors degrees,..which are Hal’s of crank. It does get confusing there, but a place to start.

If you are hooking to full manifold vacumn, which I recommend for the distributors that I curve,...some disconnect and plug dist vac and set base timing with your timing light and turning the dist by hand. Let’s say you want 10 BTDC, set it and lock it down at the dist.

Now just reconnect your vac advance and read the timing,...I set all vac cans for 10 degrees advance....so in this example you would have 20 BTDC showing on your timing light at idle.

the vac can is locked out at 10 it never goes higher, only lower as the throttle drops in this method.

Now if you hook to ported vac you will have to rev the engine and watch the timing light as the timing will start to advanceas the throttle plate barely opens...which allows vacumn to reach the port for the vac can....it will then add timing and then drop off as the throttle opens more.

ported vacumn is an emissions control method,nit runs the exhaust manifold real hot at idle and then burns off the noxious gas. It heats up your engine and makes your idle temps hot as a result.

there is no reason for it except emissions. If you need emissions put it there, if you don’t use full manifold vac.
 
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