Odd behavior with timing light on cylinder #1

Electrical & Ignition

  1. Gerald Morris

    Gerald Morris Senior Member

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    Greetings Moparians!

    I started getting some carburetor backfiring yesterday, and am checking This and That in search for a remedy. I thought this particular set of symptoms most likely caused by air horn warp, but NOW have a wholly different hypothesis.

    Just to make sure my timing had NOT slipped from the canonical 12.5 degrees BTDC I set this past March, I got out my timing light, put the pickup on the #1 plug wire, and got NO response from the timing light.

    So, I checked how well the old molex connector was plugged into the light, seated it firmly, STILL DARK until I goosed the throttle a bit and ECCE, FIAT LUX!

    So, I diddled with this a few times, consistently observing that #1 doesn't give me any inductive pulse strong enough to trigger the timing light until I goose the motor up to around 1000 rpm, twice what I like my curb idle to be. SOOOOO... I surmise that I'm not getting very good spark voltage on #1 at idle. This is consistent enough, seeing less than 12V on the dash voltage meter I normally refer to for the overall electrical health of the car. My alternator doesn't charge well until I get the engine turning over ~700 rpm. Those old roundbacks are like that, and I want to replace the thing some day.....

    So, just to check my timing, I put the inductive pickup on Plug #6, and saw that nothing had changed since my March timing job. THAT read JUST FINE at curb idle.

    As soon as the damned heads cool down enough, I'm going to inspect plug #1, to see if it has crapped up so badly as to have a resistance beyond the fuel-air mixture at power stroke pressure.

    Wise Thoughts VERY welcome at this time.

    Thanks in advance (12.5 BTDC of course)
     
  2. Davea Lux

    Davea Lux Old Man with a Hat FCBO Gold Member

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    Also check the connectors on both ends of the #1 plug wire to sure they are not corroded or loose. Grab your voltmeter and check for continuity and resistance on the same wire. For comparative purposes do the same test on another wire to see if the resistance is the same. (Note that carbon fiber wires will show fairly high resistance but should not test open) Pull the plug and check it for oil residue, carbon and proper gap. If you have a digital caliper, span the lobes on opposite sides of the distributor to see if there is a large variance. A flat cam lobe on #1 could be causing the weak spark, but more likely you are getting weak spark from a bad plug or a bad wire. If the plug looks ok, try putting a plug from a different cylinder in #1 to see if that improves the inductive performance.

    Dave
     
    Last edited: Jul 7, 2021
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  3. Gerald Morris

    Gerald Morris Senior Member

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    Thanks Dave! I DID actually check both ends of my plug wire, which I make up from Packard 440 w Rajah ends. FOUND THE IMMEDIATE PROBLEM! Sure enough, the plug just got fouled so badly there was no gap remaining. I wirebrushed it and picked it clean, as I do with any first time fouled plug, replaced it and everything ran better.

    I KNOW I need to OVERHAUL THAT ENGINE, and have known this for 5 years. Rings, valves, probably cylinder bores too, not to mention the rotating assembly. I've put ~ 50k miles on this engine since I bought it, and it had about 250k on it then.

    I'd say its well broke in.

    I REALLY want to build the 400 I have instead, IFF UNCLE SCHMUEL WILL RETURN OUR TAX REFUND MONEY!!!! Apparently, they're holding out on millions of working class taxpayers this year. Knowing this doesn't soothe my temper the slightest, far from it, but I'm NOT going down this road any further here.

    My idle vacuum returned to 17 inches at 500 rpm once I popped the clean plug back in. Mathilda should be good for a few weeks now. I'll have to watch #1 more often, that's all.

    I'm closing this thread. Thank you Dave for your prompt, thorough, and enlightened response.
     
    Last edited: Jul 8, 2021
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  4. Slap Stick

    Slap Stick Active Member

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    500rpm is too low for curb idle. I would kick it up about 150 or 200. Your battery will thank you.
     
  5. Gerald Morris

    Gerald Morris Senior Member

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    As specified by the FSM, Chrysler, Imperial 1966:
    upload_2021-7-8_9-47-17.png

    I've found that at the canonical Fast Idle speed, 700 rpm is where the alternator starts charging. While my 4 yr old battery indeed runs down into 11-12V when at 500 rpm, my TRANSMISSION is VERY HAPPY with that curb idle. Transmissions run an order of magnitude more in cost than batteries, and having purchased one of each for Mathilda, I KNOW WHAT I'll prioritize. In traffic, the old motor and trans sit quietly in 1st gear without the slightest strain at 500 rpm, then very smoothly deliver dynamic torque to the rear end. (2.91 I recently discovered, instead of 2.76) At higher angular velocity, I might have good charge, and yes, a cooler engine even, but the transmission does best to engage at low "w", then I can accelerate as needed.