390 timing slack/slip

Discussion in 'Ranchero Tech Help' started by FordR500GT, Mar 19, 2024.

  1. FordR500GT

    FordR500GT In Maximum Overdrive BRONZE MEMBER

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    Workin on a 390FE motor (I assume 1974 or 1975). Was running pretty well except after about a 100 mile trip it was misfiring at idle. (Could feel/ hear the stumbling pretty well. Decided to change plugs, wires, cap and rotor because they were all at least 30 years old. Put everything together and boom, no start. Checked firing order a few times.

    Changed dura-crap box and pickup coil because the truck has had a problem with ICM failures before. Nice blue spark but no start. Brought up to TDC (compression stroke, I checked) Rotor pointing to #1 on cap. Timing marks are way off. I know balancers can “slip” but that much?
    IMG_3690.jpeg

    Now here’s what has me baffled. Looked back at a glimpse of an old photo of the engine bay. Previous owner (25ish years ago) had moved the positions of the spark plug wires on the cap by 2 to make up for what I’m assuming is an unimaginable amount of slop in the chain or maybe a couple teeth off. Did that, and truck ran again, just terrible.

    What y’all think? No stumbling or carb cough off idle, no backfires. Just a less than great idle before I changed the cap & extras.
     
  2. pmrphil

    pmrphil In Maximum Overdrive GOLD MEMBER

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    Timing chain cannot do that. Most likely someone had the distributor out and replaced it wrong, then moved the wires. Check by turning the crank clockwise to "zero" mark, then BY HAND turn backwards (with the cap off) until the rotor moves. The number of degrees should be less than 20.
     
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  3. FordR500GT

    FordR500GT In Maximum Overdrive BRONZE MEMBER

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    There is definitely more than 20 degrees of movement. I did order the chain and gears to make up for that. As far as the timing being so far off and the truck running normal, that I don’t understand.
     
  4. burninbush

    burninbush In Maximum Overdrive

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    We've been through that before ... If the truck is running normal, then you don't have a timing problem.

    You can remove the distributor, and then replace it without thinking about location; if you have room enough to move the distributor to fix the timing, it will all be OK. You might need access to a digital light to fix it to a specific location if that matters to you.

    Here's a way to set timing without a light, might be useful: find the timing light location on the damper, then rotate it backward until its ~11/16" before the TDC. Now pull your #1 plug wire, and with ignition on rotate the distributor until #1 fires. Lock the distributor in that position, that will be very close to correct timing.
     
  5. FordR500GT

    FordR500GT In Maximum Overdrive BRONZE MEMBER

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    Position of the distributor isn’t a problem, I know I can move that any which way as long as the rotor position corresponds with the wires & firing order. If I put engine up to TDC on the compression stroke, the balancer is nowhere near the 0 mark. Way off by somewhere around 30 degrees. Now if I use the marks on the balancer to set to “TDC” the #1 piston is further downward. I’ve had problems with a 400 motor previously, where it would only run with the timing fully advanced. I believe that is essentially the same problem with this truck. Thus why PO “advanced” the timing by reordering the plug wires. Dizzy was originally in the right position. I even pulled it out and reinstalled it, double and triple checked everything and truck still only runs with the wires moved over by 2.
     
  6. pmrphil

    pmrphil In Maximum Overdrive GOLD MEMBER

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    So just put it on TDC, and make a new pointer. You say it's well over 20 degrees of slack, verified, or figuring? Possibly someone installed the wrong damper, or pointer, for that matter. But the amount of chain slack you say there is MUST be addressed, then everything else should fall into place.
     
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  7. FordR500GT

    FordR500GT In Maximum Overdrive BRONZE MEMBER

    Messages:
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    Looking at the markings on the balancer, from rotor initial position to where it starts to move, that slack is easily within the neighborhood of 20 degrees, maybe a little less. Friction makes it a little difficult to get that measurement perfectly. Chain is getting replaced no doubt. I know some FE engines (no idea what years) used plastic gears and some nylons. I know for a fact that this engine has never been apart and it has sat much longer in the desert than it has ever been driven. Wouldn’t be surprised if some kind of damage/failure occurred on the last trip I took with it.
    Ordered another balancer and a double roller chain set. Waiting on parts to get here before I take it apart.
     
  8. handy_andy_cv64

    handy_andy_cv64 In Maximum Overdrive SILVER MEMBER

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    My guess, at first blush, is the timing jumped one tooth to cause the rough running, then a second tooth, to cause the no-start.
     

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