1978 firebird 350 v8 suddenly developed a miss

Hi,I have a 1978 Pontiac Firebird Esprit with a 350 v8.

I have taken extremely good care of it since I purchased it from its original owner.

It has never been altered,or damaged,even the original GM stereo and 8 track player are still intact and working.

Yesterday I was driving the car and it was fine,I parked it and went inside for a few hours,came out started the car and drove away only to discover it now has a miss.

when I accelerate the engine “shudders” but then smooths out once it gains speed.

Upon hard acceleration like entering the freeway you can feel the loss of power and the miss is pretty bad.

once it gets up to speed you cant feel it.

I am very analytical and have been over the vacuum lines,there are no loose or broken hoses.

The carb was rebuilt by one of the best shops out here and installed by me about 5 months ago,so I doubt that could be the problem.

I pulled the spark plug wires off the cap one by one with the motor running,and they all seemed to make the engine run worse to some degree.

When I first bought the car it did something similar but it turned out to be a loose spark plug wire.

this time that isnt the case.

I am baffled,like I said this just cropped up out of the blue after the car sat parked for a couple hours,so I doubt very sincerely it could be anything major.

Again,I take extremely good care of this car,I never drive it hard like a moron or do burn outs,etc.

The car is babied.

Since I use this car as my daily driver I need to get this handled asap.

Anybody out there have any ideas?

The car has over 125,00 miles on it,but the oil,and tranny fluid,etc are all changed and maintained at the proper intervals.

Oh yeah another thing I checked was the catalytic convertor.It is the original bead pellet type to the car,but I had it inspected at a reputable muffler place and they did a test and its fine,so that can be ruled out.

Can you simulate the miss with the car stationary ?

If not the only way you could pinpoint the problem may be on a rolling road (Dynomometer) under load. But this sounds like an ignition problem. If your car has the original spark plug wires, I’d start there along with a new set of plugs a dizzy cap, rotor arm and points / condenser if it doesn’t have the HEI system. I’d drop in a new coil for good measure while I was at it, these parts don’t cost a fortune.

The spark plug wires maybe okay at idle but may start breaking down under high load, and a blown condenser or failing coil can give you the same symptoms, since you need to get it fixed it quick just do it all in one shot, it’ll save you a great deal of frustration and repeat visits to Napa or wherever.

Of course if you have a dynomometer locally you could get them to diagnose the problem, though it’ll probably cost you $2 - 300 for that benefit anyway.

Alternatively you could just pull the plugs, they might give you a clue to a weak mixture, that might be hard to disgnose though since your engine runs okay under other load profiles.

Hi,yes it will do the same thing at idle in park if you slowly depress the gas.It does not have to be under load at all.it will do the same thing in neutral as you slowly depress the accelerator.
What I dont get is how this just happened out of the blue after the car was parked!
If you drive away at even a slight speed it misses until the engine gets to a certain speed.
I bought a set of spark plugs I a going to install tonight,but as I said I already tried disconnecting the spark plug wires at the distributor one by one to see how adverly it would affect the cars idle.
The car does not have points GM by 78 had switched over to its new HEI setup,but if that went bad wouldnt it just quit running altogether?
I have never had to replace one of those units.I know how to do it,I would just hate to flush 80 dollars down the toilet if that isnt the problem

Ah ! Okay, then before you disturb anything ask someone to press on the gas while you look for arcing spark plug wires - a job better done in darkness. If you’re getting an electric light show under the hood, that’s your problem. It could just be a plug breaking down or a crack in the dizzy cap but difficult to pin down remotely.

Let us know how you get on.

Hi,I didnt have a friend to gun the motor for me,but I checked it running at night and worked the throttle at the carb myself and didnt see any lights.
The spark plugs are a real bitch to get to on the passengers side toward the back.some genius at GM put the a/c dryer righ there so it looks like you might have to go from underneath the car just to change the plugs back there.
I honestly doubt if they have been changed because the represent such a challenge.
I looked at the distributor cap and it isnt cracked,is there anything else i should be on the lookout for?

One other thing to look for in a HEI ignition is burn through of the rotor. It allows the spark current to ground to the distributor shaft.

I’m a little rusty on this, but I assume you have a carb with a vacuum advance. You’ve checked the vacuum lines, but have you checked the vacuum advance canister attached to the distributor. If it can’t hold a vacuum and advance the timing, the car will stumble under acceleration. I had this problem with a 69 Buick Skylark with a 350 Rochester 2 barrel. I also found with my older non-computer cars (69 and 71 Skylarks), advancing the timing a few degrees past the factory settings helped smooth out the acceleration. I have no idea if a 78 used a computer or not.

Ed B.

Hi,I dont know if the vacuum advance would be the problem or not,there is a small metal diaprham attached to the side of the distributor which is the vacuum advance,I see no “canister” of which you speak.
How would you test the vacuum advance anyway? and is it likely to be the culprit since the car delvoped this problem when it wasnt running? ALSO the car has the miss at idle as well which would lead me to think it isnt a vacuum advance problem.
Man,I really hate problems like this where you have to trace the thing down.
Whats funny is that it occured after the car was shut off,which makes no sense.

The “small metal diaphram” is the vacuum advance, If it can hold vacuum it’s probably ok, but it is 30 years old.

Ed B.

Hi Ed,yes,I was well aware of what the vacuum advance looked like,i was curious because you said canister which led me to believe there was a larger tank type vacuum unit somewhere.
Anyway,Its the strangest thing,It fixed itself and is running just fine again!
A friend of mine said it might be that the station I bought fuel from watered there gas,he said that it happened to him recently

the screen name is cool jim rockford, meaning the rockford files ?, I would assume