HI, This is my first post here.

I have a 1972 240Z. It's story from my attaining it is chronicled here --> Linkhttp://www.cardomain.com/ride/2977492

The car has been at the mechanics, a Specialty Z Shop, since around Thanksgiving. The car has had a new engine installed and just about everything else one could think of mechanically, has been replaced.

The car won't rev past 4K. Carbs have been swapped, fuel system bypassed(for testing) coil, distributor,points/pertronix, all have been changed out, even the tach.

Wires to and from the coil and distro have been checked/replaced.

My Q: Has anyone experienced this problem before?

Follow -up Q: What was done to correct it?

Or to sum it up, WTF is going on?!?!?

$7,000 later and still no properly running Z wants to know.

Any and All Help appreciated.

Sean:bulb:

FWIW - I cured my similar problem this morning. This may or may not apply to your car, but might be worth a try.

With the electronic ignition on my car I had been using wider gap NGKs as specified for 280ZX - BPR6EY-11 V-Power, to be precise. This morning, after reading in an old generic troubleshooting guide from the '70s that a miss under acceleration is often plug gap too wide, I installed a set of stock BP6ES-8 instead. Both sets of plugs were nearly new, and the wide gap plugs looked beautiful coming out.

But with the narrow gap plugs, the car runs perfectly. Pulls smoothly and strongly past 6000 RPM. No issues at all.

I know that conventional wisdom is that wider gap is good if you have electronic ignition, and especially with a bit hotter coil (I've got a Pertronics Flamethrower in mine). But the wider gap does not work well in my car, for whatever reason. May have to do with the combustion chamber shape, or who knows what.

Anyway, it may be worth looking into on your car, Sean.

yeah all smog is pulled.

I will let the guys in the shop know about the gap solution.

Also. The engine was Top dead centered with all components in alignment........ not much good news so far here........

Update. Still no success. Mechanical all gone through(again). I'm back where i started suggesting that this is something "In" in the car ie. a wire, a bad crusty connection or an electronic component failing under heavier load.

Still full of hope that this issue can be resolved.

i guess check every piece of electronic that is associated with rpm readings. this just a random shot in the dark, but do you have another rpm gauge to test out? im pretty sure there is probably some kind of module that translates the sensor readings into rpm maybe that could be causing it?

I wish I had read this thread a long time ago... I have had a similar condition on a Datsun 1200... turned out to be the ballast resistor under the coil... There was a "near" open in the resistor.. ok at lower rpm but as it aproached 4500 rpm it began to sputter, progressively got worse untill it topped out at 5500 rpm to the floor. $1.99 could fix your problem.

:bulb:

HI Sean,

FWIW, I struggled through 3 SCCA weekends with a engine that would rev great til 5,000 RPM and then stumble badly..... Turned out to be a bad brake booster seal, allowing air to be drawn into intake manifold, screwing up mixture.... reversed check valve ran like a champ. Plugged off brake booster line, not a good fix for a street car, because pedal effort is quite high without the booster, but worth a try.....PS...would be kinda staring at you in the face as ur Dad said!!!

Good luck,

david spillman

Thanks Guys. The brake booster is new as are all the lines.

The Ballast resistor was replaced with a NOS part. The new pertronix(under suspicion now) doesnt require the ballast so I feel it should be removed however I think the last time I saw my car it was still wired in the circuit. I will Follow-up.

Thanks Again.

im on 3rd tachometer replacement. This one "seems" to act fine.

Has anyone given much thought to the damper springs in these round top SU's? We are all running these carbs on 2.8L motors when they were designed for a 2.4L motor. Could the 37 year old springs be worn out causing the damper piston to rise too quickly thus leaning out the motor or be too small in the first place (2.8L motor)?? I have this same problem on my 2.8L running 1972 round tops. The motor starts running rough at about 4500 and often will quit pulling at 5500 RPM. I have thought about calling Joe Curto and buying a set of the heavier damper springs for an HS6 SU but mine need rebuilt so I'm just going to send them to Z Therapy and fix the problem for good!

When the damper rises, it pulls the needle farther into the small end of the taper, thereby richening the mixture, not leaning it. So weak damper springs won't do this.

I realize that the original poster hasn't been on the site in a while, but since I've seen references to a few others with similar issues I thought that I'd report some recent changes on my similar issue.

Interestingly enough, I have been experiencing the same issue on mine. Felt like a 4000 RPM rev limiter. Just today I finally got a change in symptom by advancing the timing more. My current theory is that the timing pointer is not correct, and that I was running it very much retarded. Try advancing the timing about 10 degrees more than what it is now and see if that helps.

FWIW - I cured my similar problem this morning. This may or may not apply to your car, but might be worth a try.

With the electronic ignition on my car I had been using wider gap NGKs as specified for 280ZX - BPR6EY-11 V-Power, to be precise. This morning, after reading in an old generic troubleshooting guide from the '70s that a miss under acceleration is often plug gap too wide, I installed a set of stock BP6ES-8 instead. Both sets of plugs were nearly new, and the wide gap plugs looked beautiful coming out.

But with the narrow gap plugs, the car runs perfectly. Pulls smoothly and strongly past 6000 RPM. No issues at all.

If you review my earlier posts, you'll see that I claim to have fixed it with a plug change a while back. Turns out that fix was not permanent, the problem returned after some time.

To recap, my car would rev great in neutral, but under load (accelerating through the gears) it would reach a point where it would simply stop revving, much like it had hit a rev limiter. The point at which this would happen varied depending on my ignition config. At stock timing (5° BTDC) it would hit the wall at about 4000 RPM. If I advanced the timing to 15-17° BTDC I could get over 5000 RPM before it started to sputter and then stop revving.

Here's what I was running up until yesterday:

Stock late '71 L24 w/E88 head and all emissions equipment intact
Original 240Z distributor w/vacuum advance (cleaned, lubed and tested)
Mechanical advance curve quickened by removing one of the two governor springs
Matching Bosch cap and rotor
NGK plug wires
NGK BP6ES-8 (normal .032" gap)
Pertronix Ignitor
Pertronix 3Ω Flamethrower coil
Ballast resistor by-passed
Timing set to 15° BTDC (timing marker verified manually)

With this setup, I could rev cleanly to 5000-5300 or so, but the car was prone to ping under moderate load, even on what the oil companies are pleased to sell as "91 octane" fuel these days.

Since the car did not do this in the past, I decided yesterday to revert to the way it was when I bought it. I didn't re-install the original cap, rotor and wires, but did reinstall the points, original coil and re-connect the ballast resistor, and set the timing to 10° BTDC as it was when I bought the car.

Test drive showed it ran properly this way—pulls clean and hard to 6500 if I want. So now it was a simple matter of parts substitution to find out where the issue is/was.

My first move was to install the Pertronix coil with the points. And that immediately brought the problem back. With the newer 3Ω coil in combination with the points, the car would pull up to a touch over 5000 RPM if the ballast resistor was bypassed, or only about 4000 RPM with the ballast in place. So it was back to the original 38 year old coil.

Next I installed the Pertronix ignitor, but retained the original coil and left the resistor connected. This also caused the problem to return, this time at ~4800 RPM. By-passing the resistor while using the original coil and the Ignitor again allows the engine to rev clear through to 6500 RPM.

So this is where I am today:

Stock late '71 L24 w/E88 head and all emissions equipment intact
Original 240Z distributor w/vacuum advance
Mechanical advance curve quickened by removing one of the two governor springs
Matching Bosch cap and rotor
NGK plug wires
NGK BP6ES-8 (normal .032" gap)
Pertronix Ignitor
Original Nissan 1.5Ω coil
Ballast resistor by-passed
Timing set to 8° BTDC

The car runs great in this config.

My conclusion is that the Pertronix Ignitor changes the spark in such a way that a coil in the normal 3Ω range can't keep up for some reason. The stock coil is rated at about 1.5Ω, and the resistor is rated at about 1.6Ω, for a combination of 3.1Ω.

The points ignition works fine with the original 3.1Ω coil/resistor combo, but the Pertonix will not, it needs the 1.5Ω coil alone to keep up. This is despite the recommendation in the Pertronix documentation calling for a 3Ω coil with no resistor for 6 cylinder engines for street use. It may be related to the fact that the dwell angle of the Pertronix (which is non-adjustable) is borderline low, compared to factory specs for the points.

Worse, the Pertronix 3Ω coil doesn't appear to truly be 3Ω, since it won't quite keep up with the points ignition even with the ballast by-passed. And since the Ignitor appears to require less than 3Ω to run well, the combo flat doesn't work.

My ignition package was purchased from MSA about 8 months ago. I will contact MSA about this, and see if they can take the 3Ω coil back, hopefully exchanging for a 1.5Ω instead. But that might not be possible, considering that we're talking electrical parts here.

So that's where I stand right now. The car is running great on the Ignitor and original coil (no resistor).