Chasing down a miss I get when the car is warmed up and under load. Car starts, idles and runs fine when cold. Idles fine when warm. All at once, after driving 1/2 hour, it starts to miss strongly every 10-20 seconds. No backfiring. If I drive with the revs below 2000 it misses gently. I have a Petronix.
I am first checking for loose connections. I changed the cap and rotor. I swapped in a new Bosch Blue Coil. Just how loose should the insulators be on the plugs? Three of the four insulators sway 5 to 10 degrees on the plug but the insulator on #4 sways 30 degrees (I still feel the spark plug threads when putting the insulator on the plug).
Is there supposed to be a vacuum advance / vacuum line on the distributor? I see none.

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I read the title to this thread and thought I'd be reading a sad story about how you parked the car in the WalMart lot and couldn't find it when you came back out.

Can't really help with your problem except to say...check your grounds. Good luck.

Carbs or FI?

It has carbs. I am new to the car and to 914's so bear with me. I would love to throw points and a condenser onto the distributor but I believe I need a vacuum advance and a vacuum line to one of the carbs?

I think that you are on the right track. You mentioned just about every ignition component except the plug wires and plugs. The spark plug connectors should snap firmly in place. Im not that concerned whether they flop around or not, but there should be some force required to push the connectors on the plugs or to remove them. Also, since your issue is related to a hot engine, the insulation on the wires may be breaking down and may open up cracking with heat.

So, since you are new to the car, spend the money to replace the wire set to eliminate that as an item causing your issue.

Also, when were the spark plugs last changed out? The little engine is really inexpensive to rune up. So, good time to renew.

INcidentally, the miss sounds like ignition, not fuel related.

Is your voltage drooping?

Sounds like your pertronix is cutting out.
Maybe your alt is not charging.
Does it get worse if you turn on the headlights?

Not having a vacuum advance is not uncommon with carbs - depends on the dizzy you are running. You can always throw points and condenser back on it, but I'd suspect wires first.

When it's missing, does it get to the point of being regular and increasing with RPM, i.e. does it appear to be one plug is cutting out? If so, disconnect one plug wire at a time and see if anything changes. Also move the wires around when missing to try to determine if they are arcing.

It is not regular. Let me try new wires.

I doubt that it's the Pertronix. When they fail they just stop working instead of limping along. Is the timing set correctly? Valves adjusted? Which carbs do you have? Can you post a larger image showing more of the engine bay?

Off Topic: I would not run a Pertronix w/ a Bosch Blue Coil. I read many horror stories about how they are really not compatible with each other & can be prone to failure when combined. I believe the instructions for the Pertronix even specify what output the coil should have & the Bosch Blue does not match that. When I installed my Pertronix II I also purchased the matching coil that goes with it to insure compatibility & have had no issues.

Hmmm, mine was installed by the PO and has ran several years sans problems with Pertronix and Blue coil. I've never dug into the documentation, but if this is the case, thanks for the heads up.

Check your valve clearances, and try a compression test. I hope I'm wrong, but my car exhibited similar symptoms the last time it dropped a valve seat.

--DD

Another item I forgot to mention, the tach dips and bounces quite a bit at the same time as the miss. The miss always starts the same time after I start from cold (I drive the same test loop) at about 190 on the console oil temp gage. I will be receiving the plugs (WR7CC) the wires, the points and the condenser shortly. I noticed I have WR7DC plugs (longer electrodes) that are not recommended, however that figures into the equation (I will swap out plugs first by themselves out of curiosity).

My next question (if you verified that you have good voltage) was "Is the Dist getting very hot?"

In addition to low voltage, high temps could make a questionable pertronix start acting up.
You DEFINITELY have an electrical issue (Pertronix dropping out).

No problems with Pertronix and the blue coil in almost 4 years.

Yes, the distributor cap is very hot.

Definitely an ignition problem then.

Unplug the black/purple tach signal wire from the coil and see if that fixes the misfire. If so, then your tach is shorting internally and needs to be rebuilt.

--DD

Wow, that's outa left field. Never would I think to check the tach like that.

Not the tach shorting. Waiting for the condenser and points to arrive and will update.

Rats. At least it was an easy test!

Double-check everything that connects to the coil's (-) terminal. Make sure none of it can short to ground.

--DD

Well....It was the "Hot Spark" ignition system. Swapped in points and a condenser and it now runs great.
Thanks all for the help.

interesting! ive had the same unit for 5 years no issue, but im sure it wont last for ever so i have another one in the glove box! glad you figured it out, enjoy driving it!!