HELP! All Mechanical Eliminated- Wont rev past 4K

[ChrisZ]

Hmmm. have you tried to disconnect all that crappy smog stuff.

Chris

[Arne]

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.

[Seanw987]

yeah all smog is pulled.

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

[Seanw987]

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

[Seanw987]

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.

[overdrivex]

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?

[IBZINYA]

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.

[dspillman]

: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

[Seanw987]

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.

[Seanw987]

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

[Dwigley]

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!

[Arne]

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.