Spark plug gap

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    Spark plug gap

    Somewhere I read the proper gap for new plugs, but I can't find it now. And when i searched for "spark plug gap" on the GuestLog it didn't show up either. What is the proper gap for new plugs on an early model, and what difference does that distance do/make on the firing? What happens if it's too close or too far a gap? I'm sure this is a stupid question, but I know nothing about least I didn't until I decided to keep (against the grain of many others in my marina) the A4 in my boat, and read every post in this forum and the guest log, and FAQs. She hasn't run in many years, but I'm headed down to sweet talk her this weekend. Figured I'd start with- Clean gas and oil, new plugs and wires, compression check(and a couple squirts of Marvel while plugs are out.), fuel flow check, clip on a remote starter CLOSE THE THROUGH-HULL, and proceed with- "Checklist for a Troublefree Spring Startup:" in this forum and "21.Precautions when starting up an engine that has been idle" in the FAQ's. Sound reasonable? Any help is a HUGE help.
  • CowboyPhD71
    • Apr 2006
    • 1

    Nevermind, found it (.035) when I finished reading "Checklist for a Troublefree Spring Startup:". Still, any help/ideas would be greatly appreciated.


    • Don Moyer
      • Oct 2004
      • 2823


      The proper spark plug gap (early or late model is .035"). In general, increasing or decreasing the recommended gap will result in a weaker spark.

      In terms of additional help, here is a rather lengthy Q & A we prepared on the subject of starting up after a long lay-up that might be helpful:

      Q: What precautions should I take when starting an Atomic 4 after a long lay-up (several years)?

      A: Long lay-ups in the Atomic 4 fleet are quite common, and there are usually no serious consequences after an engine is brought back into service. Below are the most important steps in the process of "waking up" a sleeping Atomic 4:


      Insure that batteries are fully charged.

      Remove the old oil, and then add 30 or 10-30 weight detergent oil to the crank case until it reaches the full mark on the dip stick. On early model engines, the uppermost mark on the dip stick equates to the full mark on late model engines.

      Remove all the spark plugs and perform a "thumb check" of the compression in each cylinder. Compression should be strong enough to make it practically impossible to hold your thumb over any of the holes while cranking the engine on the starter. It's a good idea to add 5 or 6 squirts of Marvel Mystery Oil (MMO) into each spark plug during this step. Try to aim the end of the squirt can away from the manifold side of the engine so that most of the MMO reaches the cylinder bores for maximum benefit.

      With the spark plugs removed, crank the engine with the starter until oil pressure is noted on the pressure gauge. Oil pressure should be approximately 20 PSI at cranking RPM.

      Ignition check: Remove the high tension lead from the center of the distributor cap and hold the end of the lead approximately 1/4" form the head while someone cranks the engine on the starter. You should be able to slowly move the lead away while the spark continues to jump the gap to 1/2". If there is no spark between the lead and the head, check the contacts on the points for corrosion or other layer that would insulate the points from each other.

      Fuel check: Remove the fuel line from the carburetor inlet and pump some fuel through the fuel system and catch it in a clean quart jar. If you have a mechanical fuel pump, you can use the priming lever. If you have an electrical pump, you can jump across the oil safety switch and run pump by turning on the ignition switch.

      If your fuel filter is empty, it may be necessary to prime the filter chamber before the pump will be able to start to draw fuel. Electrical pumps are more of a problem to prime than mechanical pumps.

      NOTE 1: If fuel supply smells "stenchy", it may be necessary to remove all the fuel from the tank and refill with fresh fuel. In most cases, however, it is usually worth the chance to go ahead and at least try the old fuel. If the engine starts, you can consume the fuel normally.

      NOTE 2: In priming filters and the rest of the fuel supply system, it's very convenient to install a small rubber priming bulb between the fuel tank and the rest of the fuel system (like the ones used in fuel supplies for outboard engines). With such a priming bulb installed, you can pressurize the entire fuel system, even through the pump and carburetor.

      As soon as the fuel supply clears and is flowing strongly, reconnect the fuel line to the carburetor and remove the main passage plug at the bottom of the float chamber. Then hold a smaller jar under the main passage and pump fuel until a good flow is seen to come out the bottom of the carburetor. Replace the main passage plug, being sure to not over-tighten and damage the sealing washer. Prime the system a few more seconds to fill the carburetor.

      STARTING CHECKLIST: If the engine is to be started while the boat is still on land, refer to "Supplying water for running the engine on land" in the Misc. section of this Q & A.

      1) With the fuel and ignition systems reassembled, pull the choke, crack the throttle approximately 1/2", and attempt to start.

      2) Engage starter until engine begins to run.

      NOTE 3: One of the things that sometimes happens during a long lay-up is that the exhaust system becomes blocked. In some cases, some small critter builds a nest within the exhaust system, and in other cases the exhaust system may have already been quite restricted with crud and corrosion, and the lay-up simply added to an already bad scene. For the purposes of starting after a long lay-up, if a blocked exhaust system is preventing the engine from starting, you will probably observe droplets of gasoline being blown out of the carburetor intake as the engine attempts to start. If this is the case, you will have to disconnect the exhaust from the back of the manifold and attempt to start again. If the engine then starts, you will have to troubleshoot the exhaust system, starting with the hot section (the 1 1/4" pipe between the manifold and the muffler).

      3) If/when the engine starts, immediately check oil pressure for 30 to 50 PSI.

      4) Open choke as necessary to keep engine running as it warms.

      5) Check for water flow from the back of the boat. At idle, water should "batch" out of the back of the boat every couple of seconds.

      6) Recheck for an oil pressure of approximately 30 to 40 psi as engine warms.

      7) Monitor oil pressure, water flow, and temperature.

      8) When temperature reaches 140 to 150 degrees, the top of the thermostat housing should start to feel hot as thermostat begins to open.


      If the boat is in the water, functionally check the reversing gear as follows:

      1) Engage forward mode. A solid "detent" should be felt as the shifting lever is placed in and out of forward. While in forward, it should be possible to move the throttle to full power without having the forward clutch assembly slip.

      2) Pull the shifting lever out of forward, and then continue moving the lever until reverse mode is reached. There is no detent to hold the reversing gear in reverse. It is normal to hear a rather pronounced gear noise while reverse mode is being engaged.

      3) If either forward or reverse mode requires adjustment, refer to "Adjustment procedure" in the reversing gear section of this Q & A.


      Don Moyer