Technical Why is my car running like crap suddenly? "Bad gas"?

Bone-stock 1978 SBC 350 with about 75,000 miles on it. When I dropped it into my '53 Chevy 12 or so years ago I put an Edelbrock Performer manifold and 600cfm carb on it. Otherwise it's totally original.
It's no powerhouse, but it has always run smooth and quiet.
In early November I filled up with 91-octane gas from a place that claimed their super unleaded had no alcohol, and at that time I added Stabil as well.
Since then I've driven the car to work once every few weeks. Always starts right up, always runs great. This morning when I fired it up (and it fired right up immediately), it was idling a little rough. By the time I got the main road a mile from my house it was barely running. The whole rest of the way to my son's daycare (a couple miles) and then the rest of the way to work (another 10-15 miles or so) it was running lousy: Shaking and bucking. It was so bad at idle I had to put it in neutral at red lights so it wouldn't shake the whole car and so I could give it a little gas to keep it running. Going down the road I could feel it surging and bucking, which it has NEVER done in 12 years and around 20,000 miles.
Any idea what is going on? I was hoping my choke had stuck closed but I pulled the air cleaner and it is wide open as it should be by the time the car is warmed up. I did stop and add 8 gallons of gas from a Sunoco station, hoping the off-brand gas I put in my car? It seemed to smooth out a bit just as I got to work but I didn't have time to drive around to really see what was going on.
So I'm at work now with no time until the end of the day when I have to pick up both my young kids and take them home and for the first time in years I'm worried that the car will die on me. HELP!

 

had a similar thing happen, literally overnight started to run bad, Mine was a broken valve spring.

check plug wires, maybe one or more came off or burned on exhaust? Dist cap loose, dizzy loose lost timing?
check the oil see if metal on dipstick or milky color...let us know what you find

 

My small block developed that problem on the way home class one night. I got home and took my trusty mini mag light out and looked. I found that one of the little block off nipple thingies had popped off the carb. Major vac leak.

 

If you have HEI could be the module.

 

Hi. Maybe keep it simple & look at the fuel filter. Just a guess.

Jimmie

 

HEI's make good suppositories.

 

I'm going to point at something simple,,check your rubber gas line hoses & filters.

Make sure to check the hose from the gas tank to the hard line. HRP

 

It could be anything....valvetrain, ignition, carb, etc. Maybe even bad fuel.

 

Happened to me, was a seized choke cable.

 

Thanks for all the quick responses, everyone!
I'm a little bummed out by the "it could be anything" responses. I was really hoping it would be obvious to those more mechanically literate than me! (Even if it's something bad, I'd rather KNOW.)
I don't think it's a fuel line or filter problem because it is not acting at all like it's starving for gas. I am real familiar with that sensation - I coasted to a stop on the side of the road several times with this car when I first got it going until I finally bought a new plastic gas tank. And I just replaced the fuel lines and filters not too many miles ago.
The choke is wide open, so that's not the problem either.
It really feels like it's only running on a few cylinders, and even then it's like hit-and-miss. The engine isn't stalling and catching, it's shaking like hell (at idle) and bucking or surging going down the road. Unlike anything I've ever experienced; not like fuel starvation and not like a stuck choke.
I do have the HEI distributor, so if it continues or gets worse this might be my excuse to get a new distributor.

I'll keep my fingers crossed that it's the gas. I know I've had some lawn mowers run like $#!* with old gas in the tank. I just thought non-alcohol gas with fuel stabilizer would last more than 3 or 4 months.
If the problem gets worse or stays the same I'll come back and update when I figure out the problem. I'm hoping I'll just be saying "it got all better all by itself"!

Just on a cost basis, it seems like the logical steps would be:
check plug wires and vacuum lines
replace fuel filters
disassemble and check carb
replace HEI module and/or entire distributor
and if none of that works start shopping for a new motor or a good rebuilder
Am I leaving out any steps?

 

Its not hei, they either work or they don't. I'm going with a vaccum leak.

 

One thing that happens with old gas, is that it makes the intake valves stick (it gums up the valve stems). and this causes pushrods to bend. But this usually only happens if the car has been sitting for a long time, and it doesn't take long to happen after you get it running again.

Anyways, it's worth a looksee under the valve covers to make sure the valve train is happy. Broken valve springs are hard to spot sometimes, and they can cause real bad running. same with a bent pushrod or flat cam. A compression test is helpful, before you get too far along.

Easy things to check include the ignition, and looking into the carb to see if there's anything obviously loose or foreign stuff stuck in it. Once there was a piece of sand next to a jet on a carb and it caused interesting problems.

The "it could be anything" reply usually comes from guys who've been doing this for years, and have seen a lot (but not everything) that can go wrong with an engine.

 

Okay, so probably not the HEI, probably carb or valvetrain. I can work with that.

 

I would not rule out the HEI. They can get flaky. Look at the cap and rotor, look at the advance mechanism, look at the pickup, look for loose wires. The modules "usually" die completely, but sometimes they get flaky first. Just because something never happened to one guy, doesn't mean it never happened ever.

 

Had the same problem on an Avanti stored over the winter. Ran like crap, so I did everything other than draining the fuel tank. Finally, that fixed what wasn't ignition or carb or stuck valves.

jack vines

 

Most of the "ran fine last time" problems are relatively easy to find with some simple trouble shooting stuff. If you have a recent trouble with mice they will make your car run like crap all of a sudden. Usually plugs don't go bad over night but I've seen a float decide to stick overnight. Vacuum lines can fail one second after they were working fine. Parts can break and valves can stick at any second too.

 

Does it smoke? run rich? flood? Check float valve in carb.

 

31 vicky beat me to it with the float valve thing

 

I've had a HEI module go flaky before it died before and also had a near new "Chinese" made canster fuel filter disintegrate in our modern "good" gas and plug up the small screen filters in my Edelbrook carb which made it run like crap until I found it....

 

The reason I pointed out the rubber hose is ethanol will absolutely break down the rubber.

My car ran like a top and then all of a sudden I started having the same problem,,running like crap.

After spending money replacing the fuel pump,rebuilding the carb I I then found the rubber hose connection at the gas tank had cracks in it and it was sucking almost as much air as gas. HRP

 

It fixed itself!
I just ran out after lunch to take it around the block and it idled fine, went down the road fine, and only hesitated slightly twice on throttle tip in. I assume this means one of two things:
1) dirt in the carb that worked itself through or lodged somewhere harmless for the moment just as I got to work, OR
2) it really was crap gas and sitting a couple hours allowed the 8 gallons of new Sunoco to dilute and mix with the 5-10 gallons of swill.
Assuming it keeps getting better I think it must be the gas.
Do you think I should throw some Marvel Mystery Oil (which I already have) into the tank, throw in a can of Sea Foam (which I can buy on the way home), or just leave it alone and top off the tank with new gas every time I drive the car for a while? I don't really know what happens to gas when it "goes bad," so I don't know how best to deal with it. The old gas had red Sta-bil stabilizer in it.

 

The MMO definetly wont hurt anything,i use it in all my stuff,mower,chainsaw, even my diesel Tractor that i use to blow the snow out of my driveway,which i just spent 2 hours in this morning!,I wish i had a problem with my old car not running right,it is in storage for at least another 3 months.hope you get it fixed this easy.. Harvey

 

Morning sickness. You may have fouled some plugs before the choke opened.

Check the Johnson rod too.

 

sounds like moisture under the cap . that time of year for it and Hei's if not driven daily will do this if the cap is older and not clean ( carbon tracks or the red dust ) problem disapears once it sits and the motor is warm to dry it off .

 

Moisture under the cap can do it, worth a look under the cap to see if anything looks green or oxidized. Change the fuel filter of course. If your tank is old and is at all suspect, I would put an inline filter close to it, something that is easy to get to. I use those old red Holley filters, they take the Ford CG-20 element. Look over all rubber gas hoses, all vacuum lines and the spark plug wires. Run it at night in the dark with the hood open, look for arcing. Always do the easy cheap stuff first.

 

Change all your rubber fuel lines with fuel injection hose. Todays gas eats those older rubber lines. It happened to me after changing the carb, distributer and other stuff just to find out it was the rubber lines.

 

Moisture under the dist. cap sounds like a possibility - we have had crazy temperature swings lately and it's parked in an unheated metal shop when I'm not driving it. Sometimes when I open the overhead door the car fogs up. The cap's not old but it's not brand new either. Will check.
Meanwhile I'm tossing in some MMO in the gas tank and keeping my fingers crossed and driving it unless it acts up again.
THANK YOU ALL.

 

All that matter is did you get the kids home safe?

A wet cap usually leads to a no start condition, at least a hard start. I've never seen a wet cap go with a statement like this - This morning when I fired it up (and it fired right up immediately),

 

Yup, made it home just fine. Started up just fine four times today, maybe five or six. It is running much better now, although maybe 90 or 95%, not quite 100%.

 

If it is due to "Bad GAS" K-100G is the best stuff on Earth. (www.k100.com).

VR&C.