1. 88_AHB

    88_AHB Well-Known Member

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    Hey everyone,
    Been trying to get my rebuilt 318 dialed in. Im having a issue with the car while idling. The rpm’s fluctuate about 300-400rpm. Ive checked for vacuum leaks nothing obvious, retimed and set idle air mixture. Im gonna check the float levels and see if they need adjusting. Is there something im missing to check/correct? Thanks
     
  2. volare 77

    volare 77 Well-Known Member

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    since you checked the obvious things then I would take a look at the distributor and check the movement of the weights, vacuum advance and the condition of the springs Have you put a timing light on it to see if the timing is bouncing around or staying steady?
     
  3. Aspen500

    Aspen500 Well-Known Member

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    I was thinking of the same thing but you beat me to it.:cool:

    Usually a vacuum leak will cause a rough erratic idle, not a 400 rpm fluctuation.
     
  4. 88_AHB

    88_AHB Well-Known Member

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    I haven’t put a timing light on it since this idle issue started. I’ll try that this weekend, the distributor is brand new MSD billet with matching coil.When I Initially fired it up and a few times after it never had this idle issue until a few days ago. Still haven’t driven it yet just put it into gear and fix fluid leaks etc. Would a blown power valve on the Holley might Cause this idle issue by chance?
     
  5. Aspen500

    Aspen500 Well-Known Member

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    A blown power valve would most likely cause it to run VERY rich, similar to if the choke was stuck closed.
     
  6. BudW

    BudW Moderator Staff Member

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    I don’t know much about MSD distributors, but I know the (newer) factory appearing Chinese made distributors that fit our car has flimsy lightweight counterweights that will do just what you are describing. Even the later model Mopar Performance distributors use the small weights and are also “troublesome”.

    Dist guts EI.png
    2642261 Dist Shaft.png
    The thick and heavy Chrysler style weights.

    Cheby Dist Weights.png
    The thin and troublesome weights used today.

    Another thing I run across is the grease used in the distributors get hard after a few decades and I’ve seen that cause the ignition advance to do funny things.

    The first thing I would do is either purchase, rent or borrow a timing light. If purchasing one, getting one with a dial-in advance is nice (but not a requirement).

    Then watch the timing at idle. If this is a fresh rebuild and you have a new timing chain, then the timing light should not vary much. Loose timing chains can cause the timing jump around a few degrees, at idle – which can affect timing a bit.

    On most cars (not just Chrysler), the (centripetal) weights in distributor shouldn’t start to move until over 1,000 RPMs (most cases at 1,500 RPMs).

    If you have a Mopar with the skinny weights in them or an older one with rock-hard grease in it, then that could be the problem. If ignition timing is varying at idle (because of mechanical or vacuum advance) then that would be a valid reason for your timing to vary that much.

    I don’t have an as extreme issue as you have, but the distributor weights on my ’77 wagon needs to be cleaned and re-greased – but I’m going to wait until something else in the area needs repaired, first.


    If you have the “skinny/wimpy weight style distributor (aka: Chinese built) – then the only recommendation I have is to find an original Chrysler distributor and install and that most likely will fix your problem (but your MSD might be a factor). As to which distributor (advance curve) to get depends on your current application. If you have a highly modified small block, I would look for a 340 or 360 HP version. The rest of us, I would look for a 318 (2-bbl) version. Either version need to have a vacuum advance on it. If not, stay away (for Lean Burn or computer-controlled versions, which also has no mechanical weights inside).

    If you purchase an older distributor, but before you put it in use, I recommend taking the top half apart (see top picture), cleaning well, add grease to areas the weights contact (pivot points, etc.) and a drop of engine oil where the two shafts touch (under the small felt under the rotor), test the vacuum advance (for leaks), install and set ignition timing, and call it a day (if test drive is good).

    I have two almost new small block distributers (and a 3rd brand new one) in my garage with the skinny/wimpy weights (of Chinese descent) that I took in trade (for the older ones) - that “I” refuse to use. The older ones fixed their problem(s). I suspect the same will work for you (if you have a newer distributor). I don’t have any more, older distributors, either (for small blocks).

    If the vacuum advance is disconnected when idle fluctuates, then there is not much else it can be to cause that drastic of a change.
    BudW
     
    Ele115 likes this.
  7. Ele115

    Ele115 Well-Known Member

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    Someday we will learn the full lesson on China......
     
  8. 88_AHB

    88_AHB Well-Known Member

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    So,I hooked up the timing light to the car after it has warmed up. The timing was NOT jumping around while the idle was still fluctuating.

    For the hell of it I replaced the power valve anyways since it’s very quick and easy to do.When I first fired up the engine after the rebuild the distributor was 180 out and popped a few times it idled normally. Then pulled it and reset it to TDC, thereafter the distributor was far out on timing and backfired through the carb.Got the timing where it needed to be, set the idle/air screws and tried to set the idle. After this time I first noticed the fluctuation in idle.

    I will pull off the distributor cap and checkout the weights in there tomorrow.I’m not running any of the lean burn system. The 318 has a mild Howard’s roller cam in it stock heads for a 88, eddy rpm intake,600 Holley vacuum secondary single feed, headers.
     
  9. M_Body_Coupe

    M_Body_Coupe Well-Known Member

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    So what exactly is the idle RPM itself?

    Transmission in P-ark or N-eutral?
     
  10. 88_AHB

    88_AHB Well-Known Member

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    I had it set around 850rpm in park and it falls and rises around 300-400rpm difference from that.
     
  11. M_Body_Coupe

    M_Body_Coupe Well-Known Member

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    How is your distributor vacum line hooked up? Carb ported feed or do you have this hooked up to steady intake manifold?
     
  12. 88_AHB

    88_AHB Well-Known Member

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    It’s hooked up to carb port feed.
     
  13. mgoblue9798

    mgoblue9798 New Member

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    Just a shot in the dark, but I had a power brake booster diaphragm fail and cause similar issues to yours. Unplug the vacuum line from the booster and put a bolt in the end of it and try it.
     
    78VOLAREWAG and SixBanger like this.
  14. BudW

    BudW Moderator Staff Member

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    I was thinking about the possibility of a bad brake booster. I don’t see a lot of bad Dodge brake boosters out there – but it does happen.
    An easier method than to
    is to grab ahold of the hose and plastic valve (where it goes into the booster). In most cases a good twist/pull and it pops out. Most cases, the valve has clamps on it - which makes disconnecting the hose, um, a pain. With engine running, you can place your thumb on the inside of the plastic valve and listen to engine run. If no change, then booster is not the problem.
    20161019_181846.jpg
    This is from my '86 Fifth Ave.

    Note: It helps to use a fingertip of soap on rubber gromment to reinsert the valve.

    Note: by listening to the air entering the booster (when valve is removed), after booster has sat for a while (10 min or longer) is one way to tell if booster is good (or not). It should hold vacuum indefinitely, or until you press on brake petal. There should be enough vacuum in booster to give you one complete power-assisted stop and maybe even a second power-assisted stop if car is in motion with engine off.
    BudW