AF: Noise & Shaking After Ignition System Maintenance - 1975 Plymouth Valiant | Allpar Forums
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Noise & Shaking After Ignition System Maintenance - 1975 Plymouth Valiant

Discussion in 'A Body: Duster, Valiant, Dart, etc' started by Squeeb, Jul 7, 2020.

  1. Squeeb

    Squeeb New Member

    Jun 14, 2020

    I just recently performed an overhaul of the ignition system and the engine sounds and shakes worse now. I can’t figure out what the problem is. See below for an overview and a link to a video.

    1975 Plymouth Valiant Brougham 225 1 bbl slant six
    <35k miles


    I did the following to the ignition system:

    Installed new distributor cap, rotor, magnetic pickup, vacuum advance, spark plugs & wires, coil.

    The magnetic pickup was set at a gap of 0.008”. The spark plugs installed were NGK Iridium IX. The timing was set and is hitting 0-2 degrees advanced @ around 775 RPM. We adjusted idle RPM up and down to see the affect. Really no change in the issue close to idle speed. When you accelerate and engine RPM is greater than 1250, the shaking/noise goes away and is not exacerbated. I used a thermography gun to measure the exhaust temperatures at each cylinder. Cylinders 2-6 were about 400-450 degrees F and cylinder 1 was about 330-350 degrees F. The coolant return is at cylinder 1. I think that is why it has a lower exhaust temperature. We also checked all the spark plugs and they are firing correctly. It does not appear that a cylinder is misfiring.

    Why the poor noise and shaking? What do you hear? Why was it running better before and worse now? Could it possibly be a intermittent misfire? It's driving me nuts.

    Some thoughts & questions:

    1. I only checked one gap between the reluctor & pickup, then I set it. I did not check every gap between the reluctor and pickup. If I did this and confirmed a variance, would this be an issue and/or cause? Is it better to favor a tighter clearance if needed to bring them all within spec?

    2. Are Iridium spark plugs a bad idea? Should I go with something closer to OEM?

    3. The ignition maintenance wouldn’t require me going into the engine tappets and setting lifts/adjusting. My uncle wants to go into it and I don't want to. The car has <35k miles. Is it possible that a tuned up ignition system will run better with the engine only after the tappets are adjusted? This is a lot of work and I just don't want to do it atm.
  2. valiant67

    valiant67 ...

    Jan 7, 2003
    You need copper core spark plugs. Iridium is a waste of money on this car. But I doubt that’s the problem. Nor is it lifter adjustment (though is still do that later even with the low miles).

    It’s time to go back and check your work. Was the timing set to the decal under the hood at the specified RPM with the vacuum advance plugged? Double check all connections and the plug wiring order.
    Bob Lincoln and ImperialCrown like this.
  3. Scrounge

    Scrounge Got parts?

    Apr 30, 2009
    Second the copper plugs, though your problem seems to be elsewhere. How do the motor mounts look? Also, if the car has been running on E10 gas, your carburetor might need a rebuild. When I owned '60's and '70's era Mopars, my practice was to change the gas filter every fall. Yours might be due. Check for vacuum leaks. The EGR might need cleaning or replacement.

    I think adjusting the tappets is normal maintenance on your /6, but I don't know on what schedule. You might check the shop manual.
    #3 Scrounge, Jul 7, 2020
    Last edited: Jul 7, 2020
  4. Bob Lincoln

    Bob Lincoln "CHECK FAULT CODES"
    Level 2 Supporter

    Jul 10, 2002
    Did you set the reluctor gap with a *non-magnetic* (brass) feeler gauge? If not, it will be inaccurate and WILL cause poor starting and operation.
    It helps to check the gap as you rotate through all 6 reluctor teeth, to see if there is too much side play in the distributor. This will cause the same problems as above, in extreme cases can shear off the pickup.
    Use only Champion copper plugs as stated above.
    Did you install the coil with the correct polarity, and install the capacitor to the correct terminal?
    Did you install the wires correctly, firing order 1-5-3-6-2-4?
    Did you set the timing with the vacuum advance hose disconnected and plugged?
    Are you sure you installed the vacuum advance correctly and reconnected the vacuum hose to it? Is the hose in good shape with no cracks, and connected properly at both ends, to the proper ports? It should show no advance at idle. If you unplug the hose and block it with a golf tee, do you get correct ignition timing at idle (2 degrees), and a steady increase in advance from the mechanical weights when you rev it?
  5. Squeeb

    Squeeb New Member

    Jun 14, 2020
    Thank you everyone for your input. I will do everything you recommended. I will follow-up with what the issue was. Thanks again.
  6. AHBGuru

    AHBGuru Active Member

    Mar 22, 2015
    Definitely valvetrain noise, I wouldn't hesitate running the overhead on it, especially it isn't known for certain whether it was ever done, or done correctly. That is considered routine maintenance on older Slant 6 engines.

    What oil are you running? If you don't have enough ZDDP/ZDTP, you run the risk of wiping out a cam lobe on cold start (true of any legacy non-hydraulic roller Mopar engine).

    You could also check for a failed harmonic balancer; they were of poor quality on the older cars.

    As noted on a similar thread, these old Mopars required everything to be in "balance" to run right - the fuel, air, and spark components had to be more or less "tuned" to each other, or they would never run very well. Go through everything again, make sure you have the correct and properly functioning carb, fuel pump, and distributor, correct coil, plugs, and wires, a functioning ignition control box, and oddball stuff like a good fuel pickup/sending unit, vapor canister is Ok, vacuum leaks, primary ignition wiring condition, head/head gasket condition, etc
  7. GaryS

    GaryS Well-Known Member

    Aug 24, 2004
    FWIW, a few years ago, one supplier sold a lot of slant six distributor caps with offset contacts that caused major problems. They were so bad that when you looked inside the cap, the misaligned machine cuts were apparent to the naked eye.
  8. ImperialCrown

    ImperialCrown Moderator
    Level III Supporter

    Jun 8, 2008
    Are they the correct spark plugs for your year and application? There was a plug tip reach change when the head was changed around 1974. The "noise" part concerns me if the plug tips are hitting the tops of the pistons?

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