Weird issue truck accelerates faster at half throttle??

[yellow400ex05]

I've got a 1984 chevy c10 with a 427 bbc 780cfm quickfuel carb it runs on the rich side but I noticed while on the highway tonight that it'll accelerate much slower when you mash the pedal to the floor and when you gradually let off Til half throttle you can hear a whistle or a hiss sound sometimes and it will accelerate much faster at 3/4 throttle than full throttle. I have no cats and a fresh fuel filter as well.

[roger97338]

I've a 92 Nissan pickup with a 4-cylinder, and a 75 C-10 with a 350, both of them do that. The vehicles that I've noticed not doing that have been much more "modern" engines. (Multi-cams, distributor-less ignitions, fuel injection and such.)

I've always assumed that on pre-computer controlled motors, when you mash on the throttle, they aren't able to deal with the sudden surge in fuel suddenly pouring in. Where a newer motor with computer control will see that you want to go really fast right away, and adjust the timing as it gives the motor as much fuel as it will handle...which may not equal the amount of fuel you'd be dumping in if you had manual control of the fuel flow.

[geezer#99]

THat's about normal for a mechanical secondary carb. What stall, timing curve, rear gears and tranny you got?

[yellow400ex05]

I have vacuum secondaries not mechanical. i have a stock stall i believe which is like 1900 on a TH400, timing curve, somewhere around 40-45 degrees timing. I've got a magnum 282S comp camp which is .561 intake and .561 exhaust. the timing was backed off by a mechanic to 17 degrees of timing but it ran a lot rougher and choppy than it does with more timing, i didn't like it at all. So I advanced it from the vacuum advance a good bit and it feels much better, but maybe i have too much timing? i can back it off a little bit. but i dont see how it can affect my acceleration being slower at full throttle than half throttle.

and i have stock 3.08 gears

[geezer#99]

Have you played with the vac sec springs?
Does it ping when you floor it?
Does it kick down to second when you floor it?
What's your initial timing?
Where's your vac advance hooked to?

[yellow400ex05]

no i haven't touched the secondary springs. and i dont hear any pinging when i floor it. and my kick down isn't hooked up. my vacuum advance is hooked to the line going to the top of the carb on the side not to either of the bottom two

[geezer#99]

Have you tried pulling it back into second and then putting pedal to the metal.

[imjeff]

Sounds like you're dumping velocity under WOT because of low demand. You've got 3.08 gears and no kick-down so I'm assuming the rpm's were well below 2500 when you floored it. You don't have enough air velocity to make your primaries efficient. A vacuum gauge would show you where this happens. I would get the kick-down hooked up so you can use that carb.

Jeff

[yellow400ex05]

well it'll only kickdown at 45mph or less. and it wont do it everytime. it seems to be okay in 1st and 2nd gear but when it hits 3rd it'll bog way down, and when i let up from WOT it'll make a hissing or sucking sound and start to accelerate faster, then if i lay back into it sometimes it'll keep accelerating better, almost like my secondaries aren't opening. plus i cant figure out why its running so damn rich. you can see it idling black smoke. i love this truck but it runs like crap! everybody i talk to wants to set it up differently.
when i put it into 2nd and floor it it'll pull fairly decent, but still when i let back on the throttle it starts to pull harder. this truck is supposed to run like hell broke loose, but instead it runs like garbage.

[yellow400ex05]

here's where my vacuum advance is hooked up now, i moved it from where the red circle was to the left vacuum port on the bottom. if i move it to the right bottom port the idle will drop but the one its hooked on now and the top one the idle doesn't change.

where do Chevy's like to pull their vacuum from on these?

[geezer#99]

If you're describing that right and there's no change on idle then it's possible your vac can on the dizzy is blown.
Top one is ported, bottom one is manifold vacuum.

[yellow400ex05]

so where is better to pull vacuum from a local engine builder told me to pull it from the manifold and other guys tell me to pull it from the top port. and ill check into a new canister.

[yellow400ex05]

I'll check into a new vacuum canister, but i regardless it shouldn't affect my acceleration at full throttle or under any throttle as an engine doesn't make vacuum when under a load correct?

also since its running really rich, should i try just advancing the timing a good bit more? i just backed it off a little bit ago and its still dumping smoke, the engine seems to like more timing

[68GMCCustom]

if its that rich...are you sure your choke is working properly?

[yellow400ex05]

yes the choke is fully open when the engine is warm

[imjeff]

Originally Posted by yellow400ex05

well it'll only kickdown at 45mph or less. and it wont do it everytime. it seems to be okay in 1st and 2nd gear but when it hits 3rd it'll bog way down, and when i let up from WOT it'll make a hissing or sucking sound and start to accelerate faster, then if i lay back into it sometimes it'll keep accelerating better, almost like my secondaries aren't opening. plus i cant figure out why its running so damn rich. you can see it idling black smoke. i love this truck but it runs like crap! everybody i talk to wants to set it up differently.
when i put it into 2nd and floor it it'll pull fairly decent, but still when i let back on the throttle it starts to pull harder. this truck is supposed to run like hell broke loose, but instead it runs like garbage.

I didn't catch the blowing black at idle. Choke or float. Same with the other symptoms. What rpm are you running when it won't accelerate at WOT!

[yellow400ex05]

im not sure what rpm im at at wot but if i have 3.08's and mash on it at 50mph or 45 i'd say like maybe 2300 these trucks dont come with a factory tach most of the time.

and my choke is fine my floats were adjsuted properly and are below half on both the sight glasses

[yellow400ex05]

my new plugs are nice and black already with carbon and if i let it idle it'll leave black marks on the pavement.

[Gumby]

not PSI but maybe fuel volume problem, its drinking it faster then it gets it whne your on it hard?

[geezer#99]

A motor with your cam likes lots of timing. Both initial and manifold vac for the dizzy.
Usually when you hook to manifold the idle speeds up. If your vac pot is shot then you're likely opening the primary further which uncovers the transition slot and you're idling on the power circuit. That's why it idles rich.
Your secondaries are also adjustable. You can open them a touch and close the primary to get it back on the idle circuit.
You need to check out that vac pot.
Pull the vac hose off the carb and use your lungs. Suck on it real hard and let the suction go and you should hear the pot pop. Or you can reach under the vac pot close to the dizzy and try to feel the rod move. Or pop the dizzy cap off and see if the plate moves with suction on the hose.

[INSIDIOUS '86]

With that much cam I'm sure it's idling super rich and your vacuum secondary's are opening. But it's getting over carbed at low rpm like that your losing air speed and signal at the carb. Set your timing without vacuum advance plugged in to 38* at 3000rpm. This is called power timing. It's loud. Kinda scary but it works. Run the rpm up till the mech adv stops adding timing. At that point run timing to *38 you'd initial will be where it's at and there's not much to do to change it without dissecting the distributor. Once you have total timing set plug the vac adv into full manifold vacuum and set your idle and mixture

Now tune the carb. Take your primary jets and run them down two sizes at a time until you get a surge or hesitation at part throttle/light cruise. Once you do go back up two sizes. Once that's done add ten to your primary jet number and that will ball park your rear jet sizes for you. Now tune the secondary spring to your liking. It will prob be either the second heaviest or heaviest spring in a Holley pack but if you can get your kickdown to work you may be able to go lighter on the spring.

If you have issues with setting idle and getting it to run clean let us know

If your distributor is stock then it needs to be recurved. Get a re curve kit but DONT use the **** weights. Change the springs till you get adv all in by 3000 or as good as you can get it but not so week that it adv at idle.

If you need a rpm gauge harbor freight sells some cheap ones $10 that you can quickly wire in under the hood.

[yellow400ex05]

well i adjusted and fine tuned the mixture screws and it seems much better, still rich but no leaving black marks on the pavement as of now i and a buddy of mine realized that my harmonic balancer is spun and using a timing light to adjust timing would prove to be entirely inaccurate.

also my plug wires are ancient and only 7mm wires, which may not be enough of spark either to help burn the fuel efficiently. what size wire is good for a BBC? 8mm or 8.5mm? i have HEI ignition as well.

[donut]

HEI, 8mm wires. I believe 7mm were for the older point systems.

[INSIDIOUS '86]

Originally Posted by yellow400ex05

well i adjusted and fine tuned the mixture screws and it seems much better, still rich but no leaving black marks on the pavement as of now i and a buddy of mine realized that my harmonic balancer is spun and using a timing light to adjust timing would prove to be entirely inaccurate.

also my plug wires are ancient and only 7mm wires, which may not be enough of spark either to help burn the fuel efficiently. what size wire is good for a BBC? 8mm or 8.5mm? i have HEI ignition as well.

The size of the wire is really only based off the insulation thickness. Inside most plug wires are the same. A set of tailors will be fine.