Jerking at High RPM

[bose213]

Hi everyone,

I’m having an issue with my 1979 Porsche 911 SC. When I accelerate and the RPM goes over 4000, the car starts jerking or hesitating. This only started recently.

I’ve done a few things recently: replaced the original Bosch W3CC spark plugs (it’s a Japanese import) with NGK BPR7EIX, adjusted ignition timing, tuned the fuel mixture, and installed new fuel injectors. I'm wondering if the issue could be related to any of these changes.

Another thought I had — could it be the clutch slipping? The car has a Sportomatic transmission, so that’s also a possible cause I'm considering.

Quick question – when changing spark plugs, is it necessary to replace the ignition coil too? As far as I know, modern plugs like the NGK BPR7EIX have built-in resistors. Do I need to change or adjust anything because of that? And are these plugs even a good match for this engine?

Has anyone experienced something similar? I’d appreciate any advice or ideas.

Thanks!

[47silver]

Original cdi? If msd them coil wire could be arcing.
Also i think stock wires have resistors.

[bose213]

I have the original CDI

[47silver]

Then doubt it is a coil wire.
Plugs, can you check their resistance against the new?
Is the dizzy advancing?.
Cause effect if it worked before you did tge changes then chances are one of the items caused it

[917_Langheck]

You upset the distributor gods when you adjusted the timing. Check your vacuum advance. May have developed a crack in the hose.

[David Inc.]

My car ran better with old style copper spark plugs than fancy new ones.

I'd also check the plug wires themselves, misfire at higher rpm is a classic symptom. Take the car out at night (or in a really dark garage), open the engine bay and rev it. If you see a sparkshow running up and down the plug wires (which is very cool) your wires are done. Buy Clewetts, not replacement Berus, which are ****.

[bose213]

Thanks guys for your replies — I really appreciate the input. I think I might have figured out what’s going on.

At first, the car started jerking at high RPMs, and I noticed that the fuel pump began making a loud whining noise. I assumed the pump was the issue and replaced it with a brand-new one. But the problem didn’t go away. After some driving, the brand-new pump also started making noise — even louder than before.

That’s when I started digging deeper and ran some tests. In 30 seconds, the fuel pump delivered around 250ml of fuel (and yes, the polarity is correct). Then I measured the voltage going to the pump — it’s only getting 9 volts.

This all leads me to think that there might be some kind of blockage or debris in the fuel lines that’s restricting flow and putting strain on the pump.

What do you think? What’s the best way to clean out the fuel lines, and how would you approach this situation?

[917_Langheck]

Pump terminals should register system voltage when in operation. Voltage doesn't care what the flow rate is. Clean your ground points for this circuit, including relays and fuses.

[Johnr352]

You could undo the bottom fuel line of the fuel filter and then at the "out" end of the fuel pump. then use a small amount of air from your air compressor to that fuel line end (at the fuel filter) and blow out the line. Replace your fuel filter, reattach everything. See if there's a change.

[bose213]

Originally Posted by Johnr352

You could undo the bottom fuel line of the fuel filter and then at the "out" end of the fuel pump. then use a small amount of air from your air compressor to that fuel line end (at the fuel filter) and blow out the line. Replace your fuel filter, reattach everything. See if there's a change.

That’s what I’m going to do. Thank you!

[Showdown]

9v at the pump isn’t enough-needs to be 12v. Check grounds and trace the FP wiring to ensure continuity.


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