Continuing ignition problems

[konish]

Fellas,

Car info. 1971 240z with ZX distributor mods done (ballast resistor removed). Here is my issue. My car has the craziest idle and it is reflected in both my timing light inductive rpm function and the wild swings in my AFRs according to my LM-2. The rpm jumps +/- 300 rpm and then throws the AFR numbers are *all* over the place. So basically the car rpm surges up and down at idle in particular pattern...it's crazy.

Timing is set to about 15* BTDC static and the vacuum adv is disconnected. The timing under the light seems pretty stable and doesn't really jump around with the fluctuations in rpm (I guess it's not enough to get the mech adv to cause a shift).

I just dropped in another stock electronic distributor (ZX dizzy with E12-80 module) that was known to be working well. It replaced my suspected bad ZX dizzy that had a broken reluctor rotor tooth. I also replaced the cap, rotor and coil with new units. I also replaced the stock 50 amp alt with a 70 amp internally regulated alt. I used a bypass plug that allowed me to remove the mech VR.

I have pretty much confirmed that it is *not* a vacuum leak (tested with carb cleaner, alcohol spray and stethescope). If it is, it's so small that vaporized alcohol (or carb cleaner) doesn't affect the idle at all so I'm ready to move onto to a different step in diagnosis.

New spark plug wires are next but I'm betting that they aren't the issue.

Questions:

1. Could a slowly dying tach cause running issues since the circuit is dependent on the circuit being completed in the tach itself? I'm guessing not since it's inductive in nature so it's either working or not, right?

2. Power issue? Could I run a wire directly from the battery to the + side of the coil essentially adding another power source to the coil in addition to the stock harness (for troubleshooting purposes only)? I figure the car would keep running even with the ignition shut off, but of course, I could just disconnect the other power wire during testing.

3. Grounding issue?

Seriously, this is driving me nuts...any suggestions.

[tlorber]

I'm not totally clear on the symptom. You state that there is a "crazy idle" but then say that you have 15* static advance (idle speed) and it is stable. Are you saying the idle wanders or that it is stable? If it is wandering AND your AFR is fluctuating I would be more inclined to look at carburation than ignition. Did you shoot the brake cleaner in near the throttle shafts? This is where the carbs will leak air if the bushings are worn.

[konish]

I know, it's weird. The static timing looks stable under the strobe, i.e. it doesn't bounce around any more than a degree or two at most.

I have literally soaked the carbs in both carb cleaner and alcohol (Weber DCOE 45 152s). Fluid at my initial vacuum leak caused the idle to increase dramatically as well as any fluid near the throats.

The rpm jumping is hard to describe. The car can actually run stable for 5-15 seconds where the AFR and idle are pretty stable. Then it'll start to miss and the AFRs go all over the place and the rpm is erratic. It may clear itself and run stable again for a few seconds and then start up all over again progressively getting worse.

I agree it could be the carbs, but I want to eliminate the ignition first. I'll be installing an electric fuel pump soon which will eliminate fuel starvation from the mech pump and reg. from the equation.

R/

D

[Walter Moore]

First off, the static timing with any ZX distributor should be set at 10 deg BTDC unless the vacuum advance is totally disabled. (Not that timing is causing the trouble.)

If the timing isn't changing, but the car is surging, It doesn't sound much like an ignition issue. The high static timing worries me however because the ZX distributors are infamous for producing too much spark advance when fitted to the older cars.

[konish]

Okay, I'll bump it down to 10*. I moved it up because the headers were getting *really* hot at idle. Also, I'm inclined now to believe it's probably carb related.

However, since my Taylor wires have been problematic about boots pulling off the ends, they're getting replaced which will basically finish my refresh of the ignition system.

I'll pursue tuning my carbs, and see how that pans out.

I think my next step is to get a color tune to see what each cylinder is doing and adjust the mixture from there. With the crappy quality control on the new style Weber's, I feel lke it's impossible to adjust each mixture with any accuracy by using the number of turns from seated method.