New CDI coil issue above 3000 RPM

motorstereo · motorstereo's Garage

I am baffled by a problem with a brand new CDI coil from Parts Klassik. My old Bosch blue coil (oem for the 914, I believe) works fine, but on the advice of people who know these cars, I decided to replace it with a "correct" coil.

My car: '81 911SC ROW with Bosch CDI, Bitz Racing EFI conversion, NGK non-resistor plugs, old style Beru plug wires (I ohmed them--3.8, 3.8, 4.7, 5, 3.9, and 4.0 k Ohms).

The new rotors have 1k Ohm resistance, so I also installed a coil wire with 5k Ohm resistor from Parts Klassik to bring it into spec ("CDI require 5k-ohms on the distributor rotor. However, rotors sold today lack the necessary resistance, so this wire will ensure you have enough")

The engine runs great cold. Then after it gets warm, it stumbles and misses past 3000 RPM.

I put the old Blue coil back on and it runs fine.

I have installed it twice now, thinking my installation was the issue, with the same results. Blue works, new one = poor running above 3000 RPM when warm.

What would make the car run poorly with a brand new CDI coil but run fine with an old blue coil? All I can think of is the new coil is defective, but I'm open to ideas.

blucille · 01-06-2023, 02:31 PM

I bought two coils from parts klassic. For a new cdi system and twin plug setup for blucille. They both failed pretty quickly. I wouldn’t bother using their coils.

Jonny H · 01-06-2023, 02:51 PM

Poor choice of plastics on the 'nose cone' leads to cracks around both m5 terminals when molded. Spark tracks along crack and grounds on the casing. You can sometimes see it happening in the dark.

911MANN · 01-06-2023, 04:27 PM

Hey, Jonny- I recall a couple years ago your development of an improved/high quality ignition coil. It proved quite challenging, but you were still pursuing it back then. Any update for us? Thanks!

Zuffenwerker · 01-06-2023, 05:11 PM

I’ve never seen a coil fail like that but plenty of cdi boxes. Brand new bad coil … that sucks but not a huge suprise. Seems like new parts nowadays are a total crapshoot. Call Mike tell him you got a dud and he will hopefully send you another. I’ve had great luck with their coils and I like that they look like the originals. You are doing everything the right way and being very diligent with your ignition system!

motorstereo · motorstereo's Garage

I will ask Mike to send me another one. Hopefully the next one will be a good one!