BB512 started missing. Any ideas?

So I took my BB512 out for a nice long 3 day run covering about 1,000 miles. It ran great the first two days, but on the way home on the last day it started missing above 4,000 rpm. I was trying to diagnose whether it was fuel or ignition and decided it's probably ignition. It pulls very strong up to 4,000 rpm under full throttle then stumbles. It also does the same thing under 1/4 throttle, running clean up to 4,000 then missing. All else is as normal. My car still has the original Magneti Marelli box and the MSD Blaster 2 coil that was in it when I acquired it.

I'm thinking maybe the coil, which has been mounted sideways for probably 9 years. Also possibly the spark plug extenders. I'll give them all a good look over.

Has anyone here experienced a similar problem and solved it?

 

Yes, it's a carb car.
It also does it in neutral slowly revving up. Runs clean up to 4,000 then misses and barely revs higher.

The trip was RPM VT's "Spring Rally" My son and I took our BB512 and GTC from NJ to Vergennes VT Friday evening. About 20 cars participated. We had a nice breakfast Saturday morning at RPM's shop and checked out their current projects and cars in inventory. Lots of great stuff! Then we went for a nice 2 1/2 hour drive through the beautiful Vermont back country roads. Lots of fast sweepers and beautiful scenery. We had a nice lunch near Bromley, VT followed by another nice drive back to RPM's shop. There was ample swaping of cars which was very interesting and enjoyable. It was a wonderful event. Great cars, beautiful roads and fantastic people.

We spent the night in Burlington VT with some friends, then "Cannonballed" a couple of cases of Fiddlehead double IPA back home in record time. All in about 1,000 miles.

The GTC needs more front downforce!

Heres a link to some pics my son took.
https://www.facebook.com/cburke996/media_set?set=a.10153257044245903.1073741826.669330902&type=1&pnref=story

 

Thanks for all the great advice guys! I'll dig in today and keep you posted

 

OK I give up. What is the trick to get the plug wires out of the distributor cap? They seem locked in somehow, if I pull much harder I'll damage the plug wires...

 

I found an oily residue inside the cap and on the rotor. I have a fresh set standing by so I figured I'd just replace them. The Plug extenders look ok, no perforated holes from arcing. I also have a fresh set of those on standby as well, but I'll reuse the ones that were in there first. If the missing persists I'll swap in the new ones. The advance mechanism moves freely with appropriate spring pressure.

The tach was acting normally the whole time, no spiking.
The fuses are old style but clean. I'll give them a good brush up just in case.

Regarding the plugwires, I could easily peel the plug boots up, but the copper ends of the wires are extremely difficult to remove from the cap. They rotate and move up and down a little, but just wont come out. There is not enough copper exposed to pull on with needle nose pliers. Yesterday I got four wiresout then had to quit. Two came clean and two wires pulled free leaving the copper end pieces in the cap. I'll just go get a plug wire crimping tool and new wire ends and prepare for the worst. Incidentally, the contact point from the plugwire to the copper end piece is just a single fine filament folded back on the plugwire inside the copper end piece, very surprising. I'll continue Thursday.

308Mon: Nice pin trick!! I'll try that when I resume Thursday.

Turbo-Joe: How much resistance should I be looking for through the plugwires?

 

Well I swapped out my distributor cap, rotor and coil. I also checked the resistance of the spark plug extenders. Most were 12-13 Ohms but one was 0 Ohms, which is a closed circuit. There were also an 18, 15, 14 and 9. I also had a set of twelve extenders standing by on reserve and checked all of them while I was at it. One of the new ones had resistance of 1 (open circuit) so I threw it out. I created a distribution of all 24 extenders by resistance in Ohms . It looks like 12-13 Ohms is the sweet spot so I selected a batch of in that range to install. I also swapped in a set of NGK BPR6EIX Iridum plugs, even though the old BPR6ES looked fine. It started right up and ran great. No more miss above 4,000 and it also pulled cleaner off idle. It's now running better than ever!

I'm not completely sure whether the problem was the oily residue inside the distributor, spark plug extenders or the coil. For posterity though, I'm going with oily residue inside the distributor. I probably just could have cleaned and re-installed it. I did clean up the old cap and rotor, they look OK and I'll keep them as spares.

The last pic demonstrates the proper use of ignition repair fluid, the sequence is as follows: Install new plug, insert extender in plug wire, connect plug wire, verify boot is secure over tube, have sip of IPA, repeat...

Seriously, thanks for all the valuable input, you guys are great!
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