Help Misfiring >5K Rpm

[Gertvr]

After spending many hours tuning, I felt like a dog chasing my tail. I am having a difficult time getting my initial tuning right and manifests badly above 5k rpm. I think i might have found a problem while trouble shooting but i need to double check with the experts on the forum.

Here is the setup, everything is new except for the engine and the distributor: 78 SC with, SSI HE, M&K 2-2, MSD 6AL, new PMO 46, new magecor spark plug wires, recurved distributor, new cap, new rotor new NGK BRP5ES spark plugs gapped at .042".

What i have done so far:

1) Timing set to 35deg @ 5K Rpm had difficulty setting the initial timing @ idle
2) PMOs set according to instructions. All barrels pull same air and idling is stable at 900 with AFR of ~12.0
3) Tach is stable at idle and up to 5K Rpm

Symptom:
1) While settig the timing @ 5k Rpm noticed misfiring i.e. strobe light misses a few "beats"
2) testdriving pulls fine up to 5k and then starts misfiring with intermittend short spurts where it would pull fine up to redline but for most of the time it would just misfire, this is the same at part throttle and WoT.

Trouble shooting done so far:
1) Checked all earth connections MSD, engine (fine)
2) Checked all B+ and switched + (fine)
3) Checked Distributor pickup coil measured @ 0 Ohms between 2 connector points on distributor ( should be 600 Ohms according to this thread) I checked the coax wire and it was fine, no short circuits between the core and the braid.

This raised a few questions:

1) is the 600 Ohms mentioned in the thread measured with normal AVO meter at normal resistance or is this measured with some inductive capable resistance meter.
2) if it is static resistance then the measurement indicates that the magnetic pickup coil is short circuit. Could this be the reason for the misfire, why would it run at all if this is the case?
3) would the 0 Ohms resistance measured have fried the MSD magnetic trigger input, it still delivered +4.5V when i turned the igition on and measured between the violet and green wires?
4) can i replace the pickup coil with one of these napa units. If not where can i get a replacement pickup coil?

Please let me know what your thoughts are on this.

Thanks for helping

Gert

[Steve@Rennsport]

Gert,

1) Thats normal resistance measured with a DVOM.

2) Magnetic PU coils, as well as the green trigger wire and its connectors, can become intermittent and needless to say, somewhat frustrating to troubleshoot.

3) As long as you used the green & violet trigger wires, no harm, no foul. Hopefully, you connected the green coaxial cable correctly to those grn/violet MSD wires as that affects ignition phasing and thus, timing.

4) I've not used a NAPA version but that looks like the Bosch ones we buy.

[Gertvr]

Thanks Steve,

Is this the correct Bosh order# 1 234 211 010?

The pickup wires connected to the MSD is a coax type wire which i removed from the BOSH CDI plug and had a white sleeve: braiding to green; core again with white sleeve to violet.

Gert

[Steve@Rennsport]

Originally Posted by Gertvr

Thanks Steve,

Is this the correct Bosh order# 1 234 211 010?

The pickup wires connected to the MSD is a coax type wire which i removed from the BOSH CDI plug and had a white sleeve: braiding to green; core again with white sleeve to violet.

Gert

Yessir,..thats the one.

I think your trigger wires are reversed. The shield gets connected to the violet wire and the white center conductor gets connected to the green wire.

[Gertvr]

I hope its any easy fix to the problem. I will revese the wires and start tuning again this afternoon. Isolated the pickup coil tested it again and showed 560 Ohms. Works better if the scale on the DVOM is set corretly

Gert

[Steve@Rennsport]

Let me know how everything turns out.

[Richard Burns]

Does the '78 have points? You don't mention checking the gap.

[Gertvr]

Originally Posted by Steve@Rennsport

Let me know how everything turns out.

Steve,

All I can say is, you're da man . Thank you, your experience is worth gold to us who are new to Porsche.

Reversed the wires and all was well. It now pulls all the way to rev limit with not so much as a hiccup and is much more responsive to tuning input on the carbs.

Originally Posted by Richard Burns

Does the '78 have points? You don't mention checking the gap.

Richard '78 does not have points, magnetic/inductive pulse generator.