suspected erractic misfire/weak ignition

[jwgarvey]

first I want to thank each of you for taking the time to read/answer my thread as I know my statements are lengthy and wordy. I appreciate all your input.

tlorber,

I agree, but it is difficult to not start thinking of more complicated issues because its been about three weeks without positive results. thanks for pointing out that a stretched chain is less likey to cause wandering idle/timing variance as I was unsure about this (the chain will be looked at after and only after all ignition possibilities have been exhausted). there is no doubt that my set up could afford and upgrade to electronic. my primary concern is if the problem does exist outside the dizzy and I replace it with an electronic dizzy that covers the symptoms, will the underline problem come back to haunt me if the electronic dizzy fails to cover up the problem later on. for now, I will continue to persue the problem within the ignition system until I hit a wall or until it because not so cost effective at which point then and only then will I make the upgrade (that time seems to be coming very quickly at this point).

John

[jwgarvey]

adrian,

I have a test coil on the way this weekend as I refuse to spend the money unless I know for sure. In lue of this test coil, how do I fully test my existing coil at different rps while measuring primary current and secondary voltage? can I do this myselft? does this require a special tool other than a multimeter?

3-5 degrees of infrequent/flucuating retarding is definately a symptom. reviewing your comment produces the following- dizzy (see below), timing chain to be inspected after entire ignition system has been thoroughly and completely ruled out or problem solved which ever comes first. after all, timing chains do stretch over time and need to be addressed anyways.

these symptoms are definately random. based on your response, I have checked/rechecked/replaced points, verified dwell angle (no change of dwell angle at any rpm noted on dwell meter, dwell angle wthin spec). capacitor? do you mean condenser? if not what is the capacitor? the condenser has been replaced, coil (planned to do as stated above), ground connections checked cleaned and a additional chassis to block ground added, voltage supply to ignition (coil) withing spec as tested with voltmeter of course.

yes air and fuel have been ruled out at this point.

1. -bat to engine block has been checked.

2. variable spark tester has not been done, tool is cheap as I might pick one up, I expect the spark to be weak, this should be no surpise as least in my mind. things to do list.

3. yes dizzy is rock solid with no slop but cam has not been replaced and has lots of miles I am sure (cleaned cam with 1000 grit, just slightly I did not grind it). distributer test sounds like something that needs to be done by a skilled professional. does it require a special tool? can it be done at home? sounds like it may not be cost effective at this point. sounds like upgrade dizzy time (see above).( ironic, no dwell angle changes at any rpm see above, scratching my head).

4. I do not have an oscilloscope nor will I buy one, sounds like this will not be cost effective. sounds like dizzy time agian with regards to this intervention.

yes I am aware that vacuum advance is primarly relevant to economic fuel at low rpm. thanks for pointing that out.

so in review, test coil on the way, awaiting your response on how to further check my coil. may get variable spark tester but the results may not be surprising. distributor tester awaiting your response, sound like it may not cost effective, this pushes me towards dizzy upgrade. oscilloscope does not sound cost effective, this pushes me towards dizzy upgrade. new plugs in place. awaiting your response on capacitor? possible worn cam, this pushes me toward dizzy upgrade. chain will be inspected (see above)

thanks again for your input adrian.

John

[jwgarvey]

Had that same "wandering" timing on, if you can believe it, a 1973 Ford Pinto.

Turned out to be a worn distributor.

Replaced it with rebuilt and problem went away.

I am getting closer and closer to upgrading to electronic dizzy. thanks for your input mlc240z.

[alhbln]

John,

workbench testing of a coil requires an ignition pulse generator or a frequency generator to simulate different RPM ranges from 500 to 7000 RPM and an ignition/transistor module to trigger the coil. Not cost effective if you don't have the tools or know someone who has, borrowing a known good coil which is compatible would then be the best approach.

I meant condenser, capacitor is the technical term for the component, sorry for the confusion.

You can build your own spark tester with a plastic ruler, some tape and two screws to set the distance but take care not to get zapped. One screw goes to the output of the spark wire end normally connected to the plug, the other screw goes to ground.

A distributor tester is a specialized motorized tool to measure dwell, timing and variations at all speeds. A workshop handling cars build before the eighties should have one, a test of your distributor might cost anything between just a tip and $50.

The dwell angle does not change with RPM, but the dwell time (how long the points open before they close) does. The dwell angle is based on the shape of the cam and the gap of the points itself, so it remains constant at any speed.

The most cost effective approach after testing the coil would be to borrow a know good and compatible distributor and then decide if you have your original one rebuild or buy a rebuild/new one if the borrowed one fixes the symptoms. If the borrowed one does not fix the timing symptoms (which is IMHO somewhat unlikely given the tests you already did) you need to check the engines distributor drive gear or issues with the timing chain.

It seems that you have both a problem with the ignition electrics as the spark is weak (it should not, if dwell is set correctly and the supply voltage/wires/points/coil/spark wires/plugs are ok) and a timing problem. I would assume that the timing problem is most probably due to a worn distributor.

Best,

Adrian

[jwgarvey]

adrian,

guess I wont be workbenching my existing coil then, btw, very informative. test coil to be picked up this weekend .

ten four on the capacitor(condenser), I thought I was missing something in the system.

wow, diy spark tester very nice. will follow up with that for sure and keep one in my tool kit, thanks.

sounds like a shot in the dark for a dizzy tester, I may make a few phone calls but it is more likey I will move towards the dizzy upgrade. very informative though. thank you for pointing this out. u are a wealth of info.

again, the dwell time is a new term for me. what I have learned is do not gage dizzy stability on the steadiness of dwell angle. thank u.

if the coil is determind to be ruled out, I then have one more question regarding ignition. Ideally, I would like to maintain my current dizzy (points) and upgrade to a pertronix system. in your opinion, if in fact the dizzy turns out to be the problem by process of elimination and not by bench testing the dizzy, is it concievable that the pertronix can/will compensate for this presumed worn out dizzy and to what extent and probabability? Is it just a matter of rolling the dice? I would really like to keep the current set up and just upgrade to pertronix, but if it is just to iffy then I will simply go with a new dizzy namely 280zx e12-80.

thanks again for all your input on this matter.

john

[alhbln]

John,

if the dizzy turns out to be the issue of the timing problems then unfortunately it is not likely that an upgrade to an Pertronix Ignitor will solve this. Going for a good 280ZX E12-80 instead sounds like a good plan. I don't know if the 280ZXs advance and vacuum curve are identical, but as this seems a common upgrade path for 240Z owners it should be ok.

The 280ZX distributor uses a magnetic pickup to trigger a transistor for switching the current, this is the same principle as with the Pertronix Ignitor, and might even be cheaper than buying a new Ignitor. You might want to get the 280ZX coil too (a coil for a transistorized setup has different specs and coil ratio).

Good luck,

Adrian

[jwgarvey]

it is true that I can probably find a 280zx e12-80 cheaper than a pertronix so if the coil is ruled out then that will be my next move. as far as the centrifugal (mechanical) advance on that dizzy, it can probably be adjusted to suit my set up but I will save that for another thread . I will be posted again on this thread after this weekend so stay tuned. thanks again for your input.

john

[FastWoman]

John, you can get rebuilt distributors pretty cheaply from Autozone. Mine was less than $100 (not including the core, of course). The problem with used parts is that the breaker plate assemblies reliably go bad and stick/jam. There's a lifetime warranty on the rebuilt part, so when that weak link fails, the remedy is free and relatively quick.

FAIW, electronic ignitions were used prior to the ZX lineage. My '78 is electronic, and so was my '75.

I ultimately converted my ignition to a GM HEI module, which is still working great. It's a very inexpensive solution. If you were to go this route, an electronic retrofit (distributor and ignition module) would cost you less than $125. I have it on good authority that I should beef up the + supply to my HEI unit, so that will be one of my next projects when the weather turns better. I'll be powering it with a relay (another $5).

[jwgarvey]

John, you can get rebuilt distributors pretty cheaply from Autozone. Mine was less than $100 (not including the core, of course). The problem with used parts is that the breaker plate assemblies reliably go bad and stick/jam. There's a lifetime warranty on the rebuilt part, so when that weak link fails, the remedy is free and relatively quick.

FAIW, electronic ignitions were used prior to the ZX lineage. My '78 is electronic, and so was my '75.

I ultimately converted my ignition to a GM HEI module, which is still working great. It's a very inexpensive solution. If you were to go this route, an electronic retrofit (distributor and ignition module) would cost you less than $125. I have it on good authority that I should beef up the + supply to my HEI unit, so that will be one of my next projects when the weather turns better. I'll be powering it with a relay (another $5).

Funny you bring this up fast woman,

as you have already read, I will be getting a test coil this weekend. What is funny is that I am also getting a test dizzy (1978 the one that cam with this exact motor, I gave the dizzy to my father as a backup for his 1975 z as I wanted to keep my single point set up)(so much for his back up ). If it turns out that the coil is ruled out, I already will be purchasing the gm hei four prong for the upgrade(much cheaper and more accessible to find a gm hei control module in the event it goes bad). I have read many threads and have so decided to go this exact way if need be. my thinking at this point is not set on originality. If I have to get a new dizzy then might as well upgrade to electronic for same price range. ugh! funny though I might be using a gm product to run my car .

question. you stated " I have it on good authority that I should beef up the + supply to my HEI unit". how do you plan on doing this? power it with a relay? please explain.

oh, thanks for your input

[FastWoman]

I'm going to run a 12 ga (or maybe just 14ga) wire from a maxifuse/fusible-link (essentially from the battery) around the engine compartment to the vicinity of the HEI unit (near the coil). I'll remove the small +12 ignition wire from the (+) post of the coil and connect it to the coil of the relay. Then I'll connect the beefy +12 wire from the battery/maxifuse/fusible-link to one NO terminal of the relay. The other NO terminal will connect to the HEI. I'm told that the HEI is very power-hungry, but mine barely gets warm, so I am doubtful of this claim. That said, I can certainly see the benefit of heavier wiring to stabilize the supply voltage to the HEI. I'll also probably add a big capacitor on the battery side of the relay to quiet voltage transients and provide a bit smoother power to the HEI.

[alhbln]

There is no real standardization of HEI modules except the size and connectors, so the current uptake might vary depending on the product. Some HEI modules feature dwell adaption and current control but most don't. The original GM modules were below 4A, whereas newer high current modules can exceed that easily.

14 AWG should be quite sufficient for most HEI modules, and the relay makes a lot of sense. I am using 13 AWG for high current Bosch 137/139 modules which charge up to 8A to the coil, works great.

The inductive charge is relatively slow compared to a CDI ignition, so you don't really need a big capacitor to smooth things out (have a look with your scope, ideally with a current clamp if available). If you add a capacitor, do it as near as possible to the ignition.

[FastWoman]

Thanks, Adrian! The idea behind putting the cap on the battery side of the relay would be to avoid a current surge through the relay as the cap charges. That would be kinder to the contacts. ;-) Even with this arrangement, there wouldn't be more than 6" of wire between the cap and the HEI module.