Ignition Misfire

[Randalla]

I’m having an issue I can’t seem to solve. Car starts and idles great but as soon as there’s a load on the engine it acts like it’s running on three cylinders. It doesn’t back-fire, the ignition just seems to break up at higher rpms and under load, though the problem is somewhat sporadic. A little background is probably in order. My car is a 67 ½ 1600, recently restored, but with a few enhancements. Specifically, I have one of Curtis’s new wiring harnesses, a resistor coil (no ballast resister), an internally regulated alternator, a non-standard voltage regulator (simple 2 spade, in and out version), I’m running Pertronix rather than points. I’ve also had my distributor re-curved and have total timing set to 34-36 degrees. I should also say I have new wires and the correct plugs gapped at .020. My suspicion is one or more of these components don’t play well together. In an effort to isolate the problem, I’ve switched back and forth between the points and Pertronix, verified timing and checked for loose connections. Tomorrow I’ll be taking off the alternator and voltage regulator to have them tested. I'm thinking I could have a failing diode, perhaps. If everything tests well tomorrow, I’m at a loss of what to try next. Any thoughts?

[notoptoy]

Open up your spark plug gap to .42 with the pertronix.

[AidanDawn2000]

Other than gap your ignition sounds well sorted, are you sure it's not a carb problem? Like no oil in one of the dash pots or something?
Once I was able to confirm i had an ignition problem by seeing missed flashes on a timing light.

[JT68]

Would not bother with the alternator if it shows even a small positive charge on the amp meter. The engine should run perfectly on just the battery (like a race car) - does not need the alternator for ignition if the battery is charged. Not a diode failure symptom.

Does sound like ignition system, still could be tuning,fuel delivery or carbs. cap and rotor? If identical behavior with points, then isn't the pertronixs. coil failure is relatively rare-especially intermittent like you describe, but possible.

running overly rich?

If I was trouble shooting, I'd swap out major sub systems to hone in on it. (1st swap entire dist & cap/rotor/wires at once, 2nd coil+ballast, that would rule out ignition) then I'd start looking at fuel system. I realize this can be difficult if you don't have the stuff to interchange, but very effective method.

Monitor Fuel pressure while driving etc. swap carbs etc. Bad ignition switch (failing at rpm due to vibration?)--you can add a hot wire circuit battery-to-coil to rule that out. Same with a flaky Ignition connection under the dash.

[Linda]

Fuel pump OK or need a rebuild?
Any vacuum leaks at carbs?
Fuel valves sticking?
Floats sinking or not set correctly?
Plugged fuel filter?
Dizzy advance stuck?
Maybe...lol
Linda

[itsa68]

Last year mine was having the same problem and found out that the new distributor rotor was defective and had me blaming the plug wires.
Hope your problem is simple as that.

Cheers
Ray B.

[Randalla]

Thanks for the responses. You've given me a few more things to try. Small correction and a few more pieces of information. The points are gapped at .020, not the plugs (my mistake). I am seeing some flashes on the timing light which to me indicates an ignition issue, though I'll double check the carbs. Cap and rotor is an easy swap to eliminate one more thing.

[redroadster]

Sounds like a classic case of a fouled plug unless they are very new , replace
Set them more like .030 ,the farther the gap the more spark needed,
@90-110 compression the spark is super small
New points and condenser?

[Randalla]

Well, the Roadster is still missing under load, sporadically. I've now ruled out a bad condenser, distributor issue, diodes in the alternator, the voltage regulator and timing. Still feels like an electrical issue, so I'm trying to exhaust all the possibilities before turning to fuel. The ignition switch is a new 280ZX switch and seems to work fine unless something is happening when the engine is under load. No carbon tracks in the nearly new distributor cap and the center spring loaded button is not jammed up inside. The plugs are brand new and gapped correctly. The wires are brand new as well. Total timing is set at 34 degrees of advance. anything else I'm missing before I switch to looking at the fuel system?

[Linda]

How good is the coil?
How about the coil connections? I have had those wires be dirty and/or loose and cause problems.
Otherwise might be time to move on to fuel

Linda

[Randalla]

Hi Linda. Should have mentioned that the coil is brand new and tests fine with a VOM. Connection have been wire brush at the coil and the harness and connectors are new as well. Thanks for weighing in. Going to try a few other things yet before jumping to fuel.

[nismou20]

Is the state of tune of your carbs pretty much the same now as before the restoration? I found recently after a Cylinder head refurb that the car would idle and and purr, only to miss like yours with no backfire when driven under load. Carbs were way lean on both. Richend up and problem went away. Plug color?

[Randalla]

Good thought nismou20 and easy to check. I'll drop the jet 1/2 turn this AM and report back.

[Gregs672000]

As you do your testing, see what happens as you approach your area of problem. In my experience, an ignition miss is a more clear "jerk" of a problem as one or more cylinders drop out, where as a lean condition has more of a "hhhhaaaaaaa" sort of sound and engine feel... like it's out of breath or starving. Can you get it to rev fully if you approach it slowly? Or does it go fine but then "bang!" it misses? The first would suggest that it's fuel, as the engine is able to keep going despite a leaner condition than it wants unless you overwhelm it with a sudden or higher intake volume demand that is too lean (and engine may knock or ping). The other still suggests electrical to me. Be careful, as a lean condition is not good for our more high compression "no knock sensor" engines if it's super lean.
This stuff can be hard to determine, but spend some time exploring the symptoms and conditions (more he says? Argh!). Of course, a wide band A/F meter would tell you immediately if it's lean or too rich, and that would likely eliminate fuel questions. But unless you're going to want one for ongoing tuning etc, probably not worth the price. A run on a dyno with a wide band O2 sensor hooked up would likely cost about $50 bucks or so, might be worth it if it continues.

[Randalla]

I did richen the carbs about 1/2 turn and there was no difference, still stumbling under load. I find more often than not, the solution for these things that have you totally baffled and ready to slit your wrists, is something simple you've overlooked. So...to that end, tomorrow I'm going to start with the obvious stuff I've discounted because everything is new. Going to start with substituting ignition wires one at a time, then spark plugs. If they check out that's two more things I can cross off. Next I'll swap out distributor cap and the rotor. Thinking I'll next double check float levels to make sure I'm not starving the engine under load. If all of that checks out I'll borrow my buddies portable A/F meter to see where I'm at. It does still feel to me like electrical, but I'm quickly running out of things to try. I just know when I find it it will be something stupid...if I don't slit my wrists first.