SupremeDeluxe

Have been chasing an intermittent random miss at idle on my 67 427/435. It runs great off idle and under heavy acceleration. Carbs are adjusted properly, no vacuum leaks, steady 15" vacuum at idle (except when miss occurs). I recently read R66's thread on PCV with great interest, as my symptoms are exactly the same.

Car is running stock TI, known-good pickup coil in a freshly rebuilt distributor. 10" initial timing, 34 total (conservative). B26 vacuum advance cannister with full manifold vacuum. Carbs are set up correctly, stock 64 primary jets, accel pump adjusted proper, idle emulsion screws out approximately 1-1/8 turn (strongest vacuum and most even idle).

Plugs are R44XLS, which I have run in both my street big blocks with great results. Plug wires are Lectric Limited repros made around 2001 before their quality seemed to decline.

I think my first issue is the plug wires. They measure between 6-20K Ohms. I will address those with some quality aftermarkets. Welcome recommendations (I plan to harvest the braiding off the current wire set).

Today as I was troubleshooting this problem further, the engine suddenly died (ignition cut out--not fuel). After confirming a no-spark situation, I started to go through the TI troubleshooting in the service manual and the Chevrolet News TI bulletin.

Finding #1 & Question 1:

Primary resistance on coil is 0.4 Ohm, within the factory spec. Resistance from primary + to secondary is 14.1kOhm. Above spec, which is 8200-12,400. Is this coil bad? (original 263 coil).

Finding #2 & Question 2:

Performing the key on coil voltage diagnostic, 0v at coil. According to the flow chart, this indicates open circuit between battery and coil positive. Next I verified 12 V at grey wire to TI amp, which is there, but no 12V on black wire terminal (to coil). I believe this indicates a failed amplifier?

It seems odd (or very lucky) that the amp would fail just as the car was idling in the garage. I have a new K&B amp I've kept waiting on the OEM amp to fail one day, but I'm not going to swap parts before I understand whether the high coil secondary resistance is possibly a root cause.

Thanks in advance for any and all direction

DansYellow66

I used 8 mm black Taylor wires inside my braided sleeves - a tight fit. I hear Pertronix makes a black 7 mm ignition wire kit that would probably work better.

I installed repro wires on my BB in the mid-90s and they only lasted about 100 miles so good chance yours are bad too. I'm actually amazed they lasted this long. My symptoms were similar to what you have described. I think my wires had bad connections at the plug terminals. My experiences with bad plug wires are the engine misses at idle or light load such as a steady state cruise. Accelerate hard and put a load on the engine and often it will run cleanly. Not sure of the reason light loads put more stress on the wires - one of those electrical things I guess.

Randy G.

I had a similar experience with repro wires.

SupremeDeluxe

I took a closer look at the wires. The printing has a date code of 1Q96. They were on the car when I got it 5 years ago and I assumed they were repros, but based on this printing are these possibly Delco replacements?

Either way, they are 20 years old and I will replace them with new. If they indeed are genuine Delco wires, that would explain why they have lasted so long.

I went through the TI flow chart from the beginning again today, and today I DO have voltage at coil from amplifier. I believe the harness connector was not fully seated on the amp, as it went on much further than it was before I took it off yesterday. So for now, amplifier is alive but I still haven't ruled out an intermittent issue from it.

R66

Looking in my GM service manual trouble shooting section, the TI coil has a resistance of 0.35 to 0.55 ohms on the primary side and 8,000 to 12,500 on the secondary side. Yet when I go to the specification section of the same manual, it says a resistance of 0.41 to 0.51 ohms on the primary side and 3,000 to 20,000 on the secondary side. :total:
I don't know which is correct
I don't believe the higher resistance on the secondary side would affect the TI amplifier, but you might want to check the K&B specs for the coil to be used with it.
An open or shorted coil on the other hand might blow the TI amplifier or pickup. I read this in the Mallory troubleshooting manual.
My problem ended up being just too much idling. I took it out and drove it after a week of troubleshooting and it idled good for a couple of minutes before it started the intermittent miss again. Drove it again and it came out of it again. One of the very knowledgeable guys on the forum stated the Holley 3367 runs lean at idle and thus seems to have been the root of my problem along with the original weak point type ignition. Until I get a TI unit (on my long list of wants after the list of needs is done), I am just going to drive it on the highway more and idle less.
Good luck

SupremeDeluxe

It's hard to differentiate normal idle/lope characteristics from an actual miss on these cars. In my case, I am sure there is an intermittent miss. I recently rebuilt the distributor and had the shielding on and off a few times, and the wires are old. I just got a set of new Pertronix 7mm wires I will retrofit with the shielding off the OEM wires this weekend. I also went ahead and bought a new USA-made repro coil, mostly for the fact that the coil on the car now is a 49 year old original and not something that should be relied upon IMO. Will update this thread after I replace the coil and the wires.

Incidentally, the troubleshooting section and the specifications section in the 1967 manual both cite a secondary range of 8200-12400. So by that spec, this coil is out of tolerance.

SupremeDeluxe

Update to this. I think plug wires were MOST of the issue. I made a set of new wires, following the suggestion to get the 7mm Pertronix kit and slip the braiding over them from the original Delco set. Looks 100% stock. The Pertronix wires seem to be of good quality, though the first set came missing parts (which Jegs quickly corrected for me).

The coil wire had over 50kOhm resistance! The individual leads were between 8K-15K Ohms. New wires are 1200 Ohms on the longest.

Idle is significantly improved, though idle quality improvement is a misnomer on a L-71 car with so much cam.

I'm still using the original TI amp (though I carry a spare in case) and original 263 coil. I ordered a so-called correct replacement coil expecting to change the coil for good measure, and it came looking nothing like an original so I returned it.