First chance to put a few miles on my L68 '69 in a while. Drove it about 70 miles and it ran great and even had a chance to open up all 6 barrels a few times. Ran really strong and outside of a slightly high idle, it was perfect. Then, out of the blue cruising at 70mph and about 2500 RPM it started to stumble and lose power. Whenever I drove the RPMs above 2000 it would start to stumble. Limped home and a block from home driving low speeds it just shut off. Would start, run for a brief second and then shut off. Tried this a few times and boom, it started right up like normal and ran great. Drove 5 blocks and shut off again. Got it home and this morning it fired right up.

I changed the fuel filters recently when I redid the fuel lines (they were leaking). I'm thinking alternator, it's original. Any ideas?

 

I believe a failing alternator wouldn't have this effect.

What is your ignition system: points/condenser or Pertronix-type?
(I'm thinking it's an ignition problem that has become heat sensitive.)

 

This is typical for when a coil starts to go bad.
They will fail when they warm up, then work fine when they have a chance to cool.

 

Definitely ignition related. Ignition coil breaking down from heat?

 

Does the ammeter show negative at any time? If not no alternator problem.
Do some resistance tests on the primary and secondary circuits of the coil.
Do you have a points distributor?

 

OldCarBum is very likely correct here. That is a typical way the coils start to fail. I have had a few in my life and learned about coils. Look at the bright side the Corvette did not strand you with a total coil failure.

I have a MSD Distributor, MSD Ignition Coil, MSD 6AL Ignition Box, MSD Digital Retard box along with my cool MSD "Super Conductor" Wires. In the past 30 years I have had three coils fail on my 1968 427 L88 clone. When I test fire the ignition the spark plugs sounds like a welder. Due to my high compression I have to open the spark plug's gaps to compensate for the higher compression. The .055" to .060" gaps that I run are likely the killer of ignition coils. I called MSD and their answer was to buy a more expensive (therefor better) coil and mount it away from the engine. They claimed that vibration was a killer for these older coils. My coil has been where GM put them on the engine. Our C3 came with the factory T.I. system which was on the car working fine when I bought it. After a few thousand miles I switched it for a complete MSD Ignition system to try and get a stronger spark.

The factory T.I. was rebuilt by Dave Fiedler of T.I. Specialty who has been in the T.I. restoration business since 1982 and he made it look and work like new with his improved design for the T.I. circuitry. He makes the T.I. Systems function perfectly and he set the ignition curve for the 427 engine. I put it on the car and it ran great and everything was perfect so I took it off and bagged it in a vacuum sealed bag for storage. I then started to install the MSD ignition system and since then, the (all msd) ignition coils don't last but a few years before choking up above 2-3k rpm, two failed like yours and the third just flat out died.

The enemies of the ignition coils are Heat and Vibration and they are fairly picky about getting a good solid voltage to make things work. I have an Edlebrock RPM Performer Air Gap Intake manifold and there aren't any hot spots around the coil. My engine was balanced for up to 7k rpm operation but I do not want to be the guy revving a BB up to above 7000 rpm. I am pretty sure it is smooth enough. In my C3's case it is likely the large spark plug gaps more than anything.

I would definitely not buy an inexpensive Chinese Coil and expect it to last awhile. Spend a little more and buy a quality coil and it should last you many, many years.

 

Agreed. All coils should be lifted off the intake manifold ~1/4" to 1/2" to minimize direct heat transfer.
NGK makes coils. I like their plugs and have had no issue with their coil either. NAPA carries them as I'm sure other places as well.

 

Actually your larger plug gap for higher compression is the exact opposite of what is recommended.
https://ngksparkplugs.com/en/resourc...mp%20the%20gap.

 

Very typical especially older cars with the coil mounted right on the intake manifold. It's better to mount the coil on the firewall

 

I was taught that a larger gap would work better in a cylinder with my closed chamber cylinder heads and the high compression of the L88 engine. I read somewhere that they suggest that you use a more powerful coil for a higher compression ratio. For the 427 I have learned that larger gaps work better getting all the air/fuel to burn fast and allowing me to run 36-38* of total timing.

When I test the compression I see numbers in the 245-260 p.s.i. during a compression test which is a very unfriendly environment for making a spark. My engine makes more power with a larger gap over the smaller gap which is another reason I use the big gaps. I tried different combinations and experimented with different plugs on this engine. My favorite spark plug was a Bosch FUSION spark plug. They make a really powerful spark and work well with the MSD multi-spark system.

To test the differences in my engine's power I use a GTech Pro which is a very fast reading accelerometer built into a box with a display and a shift light. This device starts measuring the power when it has the correct vehicle weight and you simply start driving. I get measurements from runs in a parking lot. The GTech makes tuning your car easier as you can sense even small increases and decreases in the HP and torque.

Keeping the bottom of the coil off the intake makes a big difference in coil temperature and coil life. I have my coil about 1/2" above the intake but the brackets are connected to bolts threaded into the intake. Maybe time to go to the firewall and use vibration isolaters.

 

This is what I use mounted to the firewall probably 12 or more years old now. No phoney round oil filled.

https://www.a1racing.com/fast-electr...tion-coil.aspx

 

From info provided on this topic here before,
ALL oil filed type coils are now cheap chinese made coils. Theres been a few threads on this in the past but NOS or just a plain ole used but working coil is a better gamble than any new coil of that type which will be a gamble regardless of the name on the box or sticker on the side

 

points and condenser

 

Have you checked the points to see if they are burnt and correctly adjusted?
Condensers just like coils can cause issues when heated.

 

points and condenser

 

I agree, it sure sounds like a bad condenser.

Jason

 

sounds like a failing fuel pump to me, especially if its a mechanical.

 

Could have been a lot of things really... trash that found its way into the carb... or water in the fuel that did the same. If it was trash in the carb it could have effected a particular circuit in the carb that came into play around that rpm.

 

Gentlemen, with all due respect, the symptoms the OP describes are intermittent. Engine runs very well for a time then, for no particular reason starts running rough. OP waits X amount of time and engine starts and runs fine. IMHO, the scenarios you both present would occur all the time. I still feel the problem is electrical.

 

My mechanical pump failed just like that for over a year before I put a gauge on it and saw the pressure drop when it got hot then bounce back after a short cool and then fail again when hot. spent hundreds on new electronic ignition, new carb, leads , spark plugs, tracing wires. Put a new pump on and fixed. Fricken 39 bucks at lordco. lasted 5 months, new pump did the same. All new mechanicals are crap, get rid of them. Now have a 7psi electric back by tank. Never done it since, even starts quicker, pressure doesnt bounce and never drops. But oK, wouldnt want to question an expert