73 firebird rev bumps

I have a 73 Pontiac firebird with a small block Chevy 350(TBI converted to Carb) with an Edelbrock 1406 carb. Its in pretty good condition. I’ve been slowly restoring it and its almost mechanically sound. However, an issue just popped up. When the vehicle is at operating temp, running in neutral or driving, The revs will jump. It stays strong at 1k and it will bounce to about 1.5k 2k and then back down and it does this every two to three seconds. I recently replaced the alternator, and serpentine. if it helps, the break booster was replaced, headers were added, and a new clear fuel filter was added, new distributor cap and rotar, and new spark plug wires.

Does just the tach jump? … or does the engine actually jump up in revs from 1-2K? What transmission does it have?

The engine. I’m thinking it would have something to do with fuel but cant figure out how? It is a manual. It will do this while driving which actually makes it hard to drive as it will do about a 500-1k rpm jump. So trying to shift into first isn’t fun. So I have it sitting for now.

Are you touching the gas pedal everytime this happens? Or does it just happen?

Just happens with foot on or off the peddle.

Maybe another note, when first starting, the engine revs high then drops to 1k rpm and is steady until it warms up. Then the jumping starts to happen.

Some sort of a vacuum leak? Loose carb/leaking gasket?


I will check all the vacuum lines, and see if the gasket is leaking. Ill report back after.

If it will jump at idle, remove the air filter and shine a light into the carb’s secondaries to make sure they aren’t trying to open. A 1000 rpm jump takes fuel and air. The Edelbrock carb is based on the Carter AFB carb. I have never played with one but I think they have vacuum operated secondaries and you don’t want that opening until you are ready.

While a vacuum leak could be an issue, so could the carb.

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Advance weights sticking in the distributor? Timing running retarded and the weights pop loose which in turn advances the timing suddenly is what I’m getting at. Easy enough to check and cure.

Maybe the transmission is trying to downshift?

Vacuum problems to the modulator valve? Manifold vacuum to the modulator: NOT ported vacuum?

I’ve run some of the 1406 carbs and they’re great; and simple. It’s difficult for me to think of something in the carb that would cause this kind of problem seeing as it does not have vacuum operated secondaries.
Just some random theorizing; sometimes known as WAGing.

He says it is a manual trans…

The AFB doesn’t have vacuum secondaries? Are they airflow actuated like a Quadrajet? Or purely mechanical?

I missed the manual trans; either initially or while thinking. My bad. :slight_smile:

By no vacuum secondaries I was referring to the Edelbrock 1406 which has mechanical linkage only to the secondaries. They have electric chokes but no choke pull-off diaphragm.
Both my Roadrunner and Superbee had AFBs but that’s been so long ago I do not remember if they had vacuum operated or not. I think (?) that early AFBs were mechanical linkage and at some point (late 60s) vacuum operated came along but that bit of trivia is long forgotten.

One of my cars actually has 2 1406s on it with progressive linkage and it works great. What was surprising to me is that initially I figured I would be lucky to get 10 to 12 MPG on the open road.
It gets 21 consistently at a 65 to 70 MPH cruise with 3:36 gears; go figure.

The advance weights are somethng to consider. I’ve seen this a few times in the past although not with this particular symptom. The ones I’ve seen were rusted and the weights stuck in either the retarded or advanced position; which caused problems either way. However, weights that re back and forth could possibly be behind this.
The rust is caused by condensation inside the distributor cap while the car is sitting. That condensation is also the cause of no-starting or rough running until the moisture is gone.

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Does frequency of surging change as you apply more throttle?

What’s your fuel pump set up?
What’s the pressure? Regulated? Is it steady during surging?

Did it ever run right with this set up?

I keep going to lean issue. Rule out base gasket leaks when warm, fuel delivery pressure/volume and float settings…

  1. It is the same
  2. I will need to get back to you on that
  3. i dont believe it is regulated
    it ran well after everything was set up and randomly started doing a week later.

The advance weights seemed a bit slow to me, but didn’t think it was bad enough to cause an issue. Should I recheck? I could always upload videos to youtube and record me opening the weights up.

No video is necessary. Just put a little lube at various points until they have free movement. And the springs are not broken.

Working with these is ancient history to me but something else has occurred to me. The engine is a 350 TBI converted to a carburetor. Does this mean an aftermarket or stock intake manifold change? I assume it does.
No issues with the carburetor base pattern and the intake pad where the carb sits? Any spacers, etc. used?

My fuzzy memory (very fuzzy) seems to recall an intake fitment issue that can happen with different generations of SBCs. Maybe someone more knowledgeable than me will weigh in on this.
I’ve got a detailed book on old engines involving blocks, heads, manifolds, etc, etc. I will thumb through that this evening to see if I can find something and post back later.
Just wondering if the engine is inhaling air from some place it should not be.

The old Carter AFBs had vacuum secondaries. As I recall, they had a second set of throttle plates above the mechanical secondary plates, the plates above being counterbalanced by weights. As more air flowed past the plates it would overcome the balance weights and open progressively. You could affect how fast they opened by shaving the weight off the counterbalance.

Having said all that it sounds like a vacuum leak to me …

Early SBCs had the inner 2 intale manifold bolts on each side threaded straight into the heads. Later…86 up, I think, had them straight up and down with respect to the engine. You can’t just leave them out.

Later aftermarket manifolds usually have both versions available

replies will be slow until Tuesday as I wont have any time off until Tuesday to really sit down and start tinkering. It is most certainly an aftermarket intake manifold from summit racing. The carb has no fitment issues and no spacers being used.

A quick look showed for instance an intake gasket for a 78 carbed 350 as being totally different from an 88 TBI engine.

Something you might consider if do not have one is the use of a vacuum gauge. They are cheap, easy to use, and will tell you many things about what is going on with an engine. To me anyway they are invaluable.
Yeah, I was stupid back in the day and handed the Snap-On guy a 100 dollar bill for a vacuum gauge but back then options were a bit more limited. There was no eBay, online suppliers, Horror Freight, and so on. Even the local auto parts houses did not carry them.

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