Help! 454 bog/die on accel

[Dylan86]

I just recently got my 1986 c10 with a new 454, 9.5:1 compression high flow heads running. But my problem on accel it stumbles and almost dies, will not stop bogging till I return under 1/8th throttle. im running a 750 edelbrock, have the accel pump on highest setting and running 16 degrees base timing... any one have any ideas that would be a huge help...

[donut]

Fuel pump keeping up? Crack open the gas cap, still do it?

First thought, your running out of gas. If the fuel pump can't keep up you'll actually draw down the level in the carb and lean the mixture out.
Gas cap is if the vapor canister or some other vent is missing/clogged and your trying to pull a vacuum on the gas tank.

[Dylan86]

thats what i was thinking, i was running a small electric carter fuel pump, so then i just upgraded to an edelbrock 1791 pump with allot higher gph. did not change a thing..... the engine revs up perfect but as soon as you throw it into gear and try to accelerate even a bit fast it dies right out and you have to let off to basicually no throttle...

[notallthere]

Try switching to a larger accelerator pump nozzles. I have heard that out of the box the Edelbrocks are set up for a more economical and conservative cam and engine. Once modified they require a little larger nozzle to handle the off idle acceleration. Mine does this but it rapidly goes away after it warms up. I will be giving this a shot later on in the summer to see if it corrects the issue.

[Dylan86]

I've ordered 2 steps up for the nozzle and one step up for the metering rod. see if it does anything, it seems it is leaning out anything from slight accel to aggressive (dies on aggressive), so it must not be getting near enough fuel... you cant even smell fuel at all from the exhaust... I don't know if just changing the accel pump nozzle will change anything that drastic

[LynnJr]

On Rochesters that would bog so much it felt like the ignition was shut off we used to turn the screw clockwise that opened up the secondaries a 1/4 turn at a time until the bog went away.
If you gradually go up on acceleration do you still have a bog or is it only when you mash the throttle?
I ask because it takes the same amount of gas to go 4500 rpm slowly as it does to go 4500 rpm quickly but if the air supply is choked off the engine will bog then recover.
On Holleys they have a set of springs they sell to fix this but I haven't played with any Edelbrock's.

[rich weyand]

Check your spark. At low throttle settings, the charge is less dense and cylinder pressures are low. That's a very easy charge to ignite. Open it up, vacuum goes down, and the charge becomes denser, and cylinder pressures go up. That's a much harder charge to ignite. If your spark is marginal, you can have it run perfectly at low throttle settings, then act like the key was turned off when you step on it.

[Dylan86]

It occurs at pretty much any acceleration rate other then very slow accel then it does ok. And This Distributer was ran on another engine. It ran fine... But I guess it still could be that. The only fix would be to upgrade my Distributer/ig coil?

[rich weyand]

Originally Posted by Dylan86

It occurs at pretty much any acceleration rate other then very slow accel then it does ok. And This Distributer was ran on another engine. It ran fine... But I guess it still could be that. The only fix would be to upgrade my Distributer/ig coil?

What plugs are you running? What wires are you running? How recent are the rotor and cap?

[Dylan86]

New Factory ac deco plugs and wires. Almost new rotor and cap

[rich weyand]

Which part number plugs? Which part number wires?

[Dylan86]

I will have to look. Which ones should I have?

[rich weyand]

For the big blocks, I think most people would say an R43T or R44T. I know the small blocks with headers typically take a R45TS.

However, if you are running factory resistor plugs and resistor wires and have heads that breathe really well at low rpms, you may be punching a heavy enough charge in there to need stronger spark.

Also, there are people who say the BBC and ACDelco plugs don't play well together, and recommend the Autolite 144.

I would try replacing the resistor plugs with non-resistor plugs such as ACDelco 44TS or Autolite 136. I'm guessing here, because I don't know what threads etc you have.

Also switch to Taylor helical wound metal conductor wires.

These two tests have the benefit of being relatively cheap.

[LynnJr]

It occurs at pretty much any acceleration rate other then very slow accel then it does ok

So what you are saying is it will turn to 4500 rpm after it stumbles first or it will never get there?

If it never gets there think ignition

If it eventually gets there think carburetor.

On the ignition you can start the truck and let it idle.Now pull one spark plug wire off of the distributor at a time.The motor should lose rpm sputter or die.If it does nothing you have a bad wire or plug on that cylinder.
Check all 8 individually.

If you suspect it is the carb the quickest test is to swap it out with another carb or borrow one from a friend/buddy just to see if that cures it or atleast improves it.

[rich weyand]

Originally Posted by LynnJr

It occurs at pretty much any acceleration rate other then very slow accel then it does ok

So what you are saying is it will turn to 4500 rpm after it stumbles first or it will never get there?

If it never gets there think ignition

If it eventually gets there think carburetor.

On the ignition you can start the truck and let it idle.Now pull one spark plug wire off of the distributor at a time.The motor should lose rpm sputter or die.If it does nothing you have a bad wire or plug on that cylinder.
Check all 8 individually.

If you suspect it is the carb the quickest test is to swap it out with another carb or borrow one from a friend/buddy just to see if that cures it or atleast improves it.

A weak ignition will produce the symptoms he talks about. At higher vacuum, particularly at low rpms, the charge will be the densest, and the cylinder pressures highest. This is the hardest to ignite. Where a weak spark may work OK at lower rpms, or at lower throttle settings (i.e. higher vacuum), when you punch it, there's not enough spark to ignite the charge.

A higher energy coil (like an Accel 48000 volt) plus better wires (like the Taylor Spiro-Pro) can kick that spark up enough to handle the denser charge.

The issue is the free-flow heads. Better flow means denser charge means higher cylinder pressures means tougher to ignite. Stock engine, stock ignition parts are ok. Better flow, maybe stock ignition not hot enough.

[Dylan86]

I can make it to 4500 Rpm if I keep in in gear and slowly accelerate. But it won't climb rpm if I accel quickly. Just sorta stumble like there's no power bogging out. But when you rev it up. You can instantly rev it to full throttle with no load. No problems. It only bogs in gear. In gear if you step on it a bit and smell the exhaust you can't smell any fuel at all. Makes me think it's lean. But it could be spark.... If you really stop the throttle in gear it backfires and dies. Im just throwing parts at it guessing now...

[LynnJr]

Originally Posted by rich weyand

A weak ignition will produce the symptoms he talks about. At higher vacuum, particularly at low rpms, the charge will be the densest, and the cylinder pressures highest. This is the hardest to ignite. Where a weak spark may work OK at lower rpms, or at lower throttle settings (i.e. higher vacuum), when you punch it, there's not enough spark to ignite the charge.

A higher energy coil (like an Accel 48000 volt) plus better wires (like the Taylor Spiro-Pro) can kick that spark up enough to handle the denser charge.

The issue is the free-flow heads. Better flow means denser charge means higher cylinder pressures means tougher to ignite. Stock engine, stock ignition parts are ok. Better flow, maybe stock ignition not hot enough.

Rich
I think bad wires or weak ignition would prevent him from getting to 4500 rpm.I dont think a coil change will help unless the old one is defective.Plenty of big blocks with free flowing heads using points and a $10 coil without issue.
Also free flowing heads will make the charge less dense not more dense at his rpm level because they are not constricted or choked off like small peanut port heads would be at a given rpm.
Stumbles on acceleration usually mean wires plugs points cap rotor or carb not working together or defective.
He can run at 4500 he is just having trouble getting it there smoothly.

The key to my way of thinking is how did it run before the carb was swapped? If it ran good before I would look at that first.

[rich weyand]

Originally Posted by LynnJr

Rich
I think bad wires or weak ignition would prevent him from getting to 4500 rpm.I dont think a coil change will help unless the old one is defective.Plenty of big blocks with free flowing heads using points and a $10 coil without issue.
Also free flowing heads will make the charge less dense not more dense at his rpm level because they are not constricted or choked off like small peanut port heads would be at a given rpm.
Stumbles on acceleration usually mean wires plugs points cap rotor or carb not working together or defective.
He can run at 4500 he is just having trouble getting it there smoothly.

The key to my way of thinking is how did it run before the carb was swapped? If it ran good before I would look at that first.

No, free flow heads let more charge into the cylinder, otherwise why bother?

He can get to 4500, but not at low vacuum, ie high throttle settings.

The backfires (actually after-fires) are a big clue. Unburned charge is getting into the exhaust.

I'm still betting on spark strong enough for high vacuum charges, but too weak for low vacuum charges.

BTDT.

[geezer#99]

When you set your timing at 16 degrees, which port on the carb did you have the distributor vac hose hooked to?

[Dylan86]

It's a brand new engine... There was no before

[Dylan86]

It had a 350 before this. And my Distributer vacuum is hooked up to the driver side port. Like I've done with all my trucks. When I set 16° I removed the advance line and plugged the port on the carb.

[geezer#99]

So, did you plug the vac advance when you set your timing?
Did you confirm your timing marks with #1tdc?

[Dylan86]

You unplug vacuum advance and plug the carb port to set base timing. Then hook it back up after timings set at 16°... Is that not correct? But no I never confirmed the timing mark. I can do that. Just the fact that I can rev it up no load and it does fine so doesn't that mean spark timings good?

[geezer#99]

That's how you do it.
Just eliminating possible problems.
Why 16 degrees?
Did you prove your timing marks?
Have you tried pulling your timing back to 10 degrees or test with no vac advance hooked up?

[Dylan86]

Well the cam specs requested 14-16° advance. I've played with the timing. Tried retarding it. It just decreased the power even more. When I advanced it it made it feel a bit more peppy. But still had the same concern. I will check the dampner mark soon. I haven't tried to drive it with no advance no. Even at 16° it barely idles till you plug the vacuum advance in... What would running with no vac advance tell me...?