I took my dads 67 out for a drive today and it was running great. Then about a half hour into the drive, it started stumbling pretty bad. It seems to happen on strong acceleration. It doesn’t have to be WOT, but probably happens when getting into the secondaries. And it’s pretty bad. The engine jerks around and stumbles hard. It is an L79 (327/350) with an Edelbrock carburetor, vacuum secondaries. It is otherwise almost completely stock.

My mind goes to vacuum, but I don’t see anything obviously out of place or detached, like vacuum advance. I didn’t check if the secondaries open evenly, but the throttle revs up pretty quick when parked. And like I said, it accelerates pretty normal under light load, partial throttle.

is there anything obvious I should look for? Timing maybe….

And yes, I know this is the C3 forum. So if it makes the question more relevant, pretend it is my 68 L79 that is having the problem. They both have the same Edelbrock 1406 carb.

 

Start with the easy, cheap stuff first: Ignition.

Heat can cause coil issues. You say it's stock, but does it have HEI anyway? HEI modules can be heat sensitive. What is the timing set to?

You can plug the vacuum ports you don't need, and see if you can isolate any vacuum leaks. You probably don't have any thermo-vacuum switches, but you are likely looking for something that is temperature related.

After that, vapor lock, or a fuel issue? Easy stuff first, though.

While we are pretending, what Corvette came with an Edelbrock 1406?

 

Well, at least my dads L79 has the original intake manifold. Because the 67’s came stock with a Holley carb. Whereas the hypothetically broken 68 originally came with a spread bore quadrajet. So I had to get a new square bore intake to make the 1406 fit. I could never get my quadrajet running properly; so I gave up and put on the Edelbrock. I’m not sure why my dad had trouble with his Holley, although he still has the original carb in his basement.

I’ll have to check things over maybe next weekend. Just looking for ideas on where to start. Thanks for the tips.

 

The fact that it doesn't act up until 1/2 hour into the drive makes me think Ign. Coil.

 

Is it repeatable? If you take it out tonight will it do the same thing?

 

I’ll have to check that out. It happened just once yesterday. So I don’t know if it’s repeatable. But once it started happening, it didn’t stop. My initial thought was maybe old gas. There was about a 1/4 tank of gas of unknown age (probably 6-9 months old), so I filled it up to dilute with new gas. But that changed nothing.

I’ll take it out for a spin again sometime this week (dammit, if I have to!) to see if it is repeatable. If it goes away, then perhaps vapor lock. We will see.

 

Pour a bottle of fuel system cleaner like BG44K and that will clean up your carburetor. However torn accelerator pump diaphragms won't be fixed. Old gas with ethanol can make any engine run poorly, sometimes fresh gas really helps.

I would second the coil mention as they tend top start acting up after they get hot and are on for a while. This is especially true with the old canister type coils. I have replaced 2 MSD coils in the past few years, I bought an Accell Yellow coil that was DOA. Try a Delco coil and see what happens. The Delco coils seem to be the best one for many older Corvettes.

The Previous owner stuck a Quadrajet on my Corvette and that was the first thing that got pulled off. I sold it for enough to buy a New Holley and that was the smarted thing I did. I replaced the intake and put a Holley double pumper on the car and it lived happily for many years. Then one day I called AED and they built me a custom Holley 850 double pumper that flowed 1100cfm+. It was missing the choke tower so I put the choke Cold start hardware on with no choke plates. It allowed it to bump the idle up while it warmed up.

6-9 month old Oxygenated fuel will not burn at all in some engines. You need to put Ethanol stabilizer for it to last more than a few weeks. I had a half tank of year old oxygenated gasoline and I pumped it out so it would not plug up the carburetor or other parts. My truck has a Ford V-10 and it really runs poorly with old gas with ethanol in it. Misses at anything above idle and will not rev smoothly. I always use ethanol stabilizer in the tank when I fill it.

 

Don’t rule out the points. I’d check the condition just to check, condenser can act funny when warm too. Is your choke opening? And last of all coils I’ve used the Bosch blue coil is the most durable. I work on VWs for a living and all else is junk other than a nice fresh blue coil. It has resistance built in so no ballast needed. Might be worth a look. Good luck.

 

Well, I’m embarrassed to admit that it took me this long to figure out the problem. I tried replacing the fuel filter last week, and that changed nothing. Then I took the car out, and the engine just died. Fortunately it was down the road a few hundred feet, so I pushed it back into the garage (with help). But nothing I did got it to restart I ran out of time and finally came back today. Thinking it was possibly a bad coil, I took off the chrome ignition cover, and found this. So I’m guessing this wire was loose and was getting pulled away from the coil on acceleration. Then it just finally pulled off, which is why it wouldn’t restart. I’m just glad it was an easy fix.

 

Whooo Hoooo! it's great when it's something simple and you found it fairly quickly.

 

Thats great!. Back to driving without spending a dime!!!!