69 conv -351w misfiring under heavy acceleration

Need some advice for a car that seems to stumble under heavy acceleration (this only appears once the engine has been running and warmed up). The issue is new this year after the car sat in storage for over a year.

It will idle fine and runs ok for the first 20 minutes or so, but once warm, the car seems to stumble and misfire under heavy acceleration. This persists for a few seconds then smooths out, regardless of whether I keep accelerating or feather the gas. It does not happen under light acceleration, nor when revving the engine in neutral. So far I’ve checked the following:

  • Drained and refilled fuel tank with fresh fuel and seafoam fuel treatment. Installed inline fuel filter
  • Cleaned and rebuilt carb (edelbrock performer)
  • Replaced coil
  • Checked spark plugs/wires

Any thoughts?

I confess I am not as familiar with Edelbrock carbs, so I can’t tell you precisely how to fix this, but it sounds like an issue with the accelerator pump.

The pump’s purpose is to enrich the mix when you step on the gas, and dump a bunch of extra air in. When you’re in Neutral and rev, the amount of fuel required is trivial, as the time it takes to rev and start drawing enough fuel out of the boosters is also trivial. You don’t notice hesitation.

But at full throttle under acceleration, you get the ‘squirt’ - throttle still wide open, flow is still low, and not able to draw enough fuel yet. Because the engine cannot quickly get to higher RPMs where there’s enough flow to draw fuel effectively (unlike a Neutral rev), you get a stumble/bog when the squirter is empty, and the mix gets lean. Slowly, the fuel/air mix catch up with each other from the boosters and the car accelerates at a more normal pace.

You may need to raise your floats so the carb can draw fuel easier? Or find some way to achieve ‘more squirt’. With Holleys, they have different sizes of squirters, so you can just add a slightly bigger reservoir to deal with this kind of problem.

Hopefully someone more knowledgeable about the Eddies/Carters will chime in!

It sounds like the engine isn’t getting the timing advanced when it should-Check the action on the vacuum advance at the distributor. Make sure the diaphragm hasn’t become solid, and the plates are still moving…Unless you have a non vacuum advance distributor. Also, along the same lines check and make sure both ends of the vacuum line from the distributor to the carb aren’t split. For some reason fuel vapors are a lot more damaging than they used to be, or at least so it seems…

Long shot, but check for vacuum leaks.
i recently chased some miss fires but caught some vacuum lines perished.

Bad coil. Starts failing once it is warmed up.

What is the timing set to? Is the vacuum advance connected and working properly?

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I don’t have any suggestions but just a question/comment; how does the symptom that is, this only happens after 20 min warm up, how to explain that? I’m assuming he can start it up, back out of the drive and mash the pedal… and it accelerates without hesitation. But 20 min later mash the pedal and this problem… ?

Codensor…shorting open …feels like your running out of gas

What type of ignition? Points or an electronic conversion?

I will have to double check what the timing is set at, as I had a family member assist me with that previously. Fairly certain its around 10-12 degrees.

The car has a Petronix ignitor 1. It was running a stock coil when these issues began (I’ve since replaced it with a 1.5 ohm flamethrower).

Did you wire the pertronix to coil positive, or run a new power wire or relay?

Is that with everything disconnected and plugged? What’s the total advance with everything connected? Is the vacuum advance connected and working?

With no insult intended here at all, if you are not SURE what your timing is, this is a very good place to start. Verify TDC on your #1 piston too, to make sure the weight ring on your damper hasn’t slipped. They get brittle, fail, move around, and then you have no idea what your timing’s actually at.

Once you verify TDC, set your initial timing, then rev the engine to see where it stops advancing.

Most of these stock engines seem to like about 10-12 degrees initial, with around 34-36 degrees of timing “all in” by 2900 or so.

Once you have it set, you can reconnect your vacuum advance, and go take some test drives! Ideally, you’d get your car nice and warm, then go drive uphill into the wind and listen for ping. If you’re getting any signs of detonation, back it off a degree or two and repeat. When you find the very edge of what your car likes, back it off a tiny fraction if you can (to account for varying temps and fuel quality) and you should be good to go.

It can only be one or the other: “short” is the opposite of open. maybe I misunderstood your comment.