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DIY inboard outboard boat motor parts & accessories

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Testing Outboards with Minimal Test Equipment

All Engines
Intermittent Firing:
  • This problem can be very hard to isolate. A good inductive tachometer (The Fluke 88 is highly recommended) can be used to compare the RPM on all cylinders up through WOT (Wide open throttle). A significant difference in the readings can help pinpoint a problem where there is no other easy means to do it.
Engine Continuously Blows Power Packs/Switch Boxes:
  • If an engine repeatedly blows ignition modules, especially on the same cylinders replace the ignition coils on that cylinder. A coil can have a problem internally where it has an inductive kick back which can destroy an ignition module — And still check good with all known tests. If the packs totally lose fire on all cylinders, the stator could be sending voltage spikes into the pack destroying it.
Visually Check the Stator, Trigger, Rectifier/Regulator and Flywheel:
  • Cracks, burned areas and bubbles in or on the components indicate a problem. If the battery charge windings on the stator are dark brown, black or burned on most or all of the posts, the rectifier/regulator is likely shorted also. Any sign of rubbing on the outside of the stator indicate problem in the upper or lower main bearings. Cracked trigger or outer magnets can cause everything from misfiring to no fire at all. Loose flywheel magnets can be dangerous, check the tightness of the bonding adhesive.
  • These can cause problems ranging from a high-speed miss to a total shutdown. An easy check is to disconnect the stator leads to the rectifier (Make sure to insulate them) and retest. If the problem is gone — replace the rectifier/regulator.
Johnson/Evinrude Engines
Open Type Timer Bases:
  • When all cylinders fire with the spark plugs out, but will not with them installed, try re­gapping the sensors. (See the section on OMC ADI lgnitions).
Engines with S.L.O.W. Features:
  • If engine won’t rev up and shakes real bad, the S.L.O.W. function could be activating. If the engine is NOT overheating, this can be caused by a temperature sensor failing early. Try replacing the temperature sensor with the Tan and White/Black wire. Make sure that all of the Tan wires are located as far as possible from the spark plug wires. Also check the blocking diode in the engine harness.
Mercury/Mariner 6 Cylinder Engines with ADI Ignitions

If more than one cylinder is not firing:
  • replace BOTH switch boxes unless you can pin the problem down to the trigger.
Always check the bias circuit:
  • Disconnect the White/Black jumper between the switch boxes and check the resistance from each boxes White/Black terminal and engine ground. You should read 14-15,000 ohms on stock switch boxes and 9,200-9,800 ohms on racing switch boxes. MAKE SURE THE READING IS THE SAME ON BOTH SWITCH BOXES! Any problem on the bias circuit and BOTH switch boxes must be replaced as a set.
No Fire on 1, 3, 5 or 2, 4, 6:
  • Swap the stator leads from one switch box to the other. If the problem moves, replace the stator. If the problem remains on the same cylinders, replace the switch boxes. If the stator is replaced and the problem is still there, try another flywheel.
No Fire on One Cylinder:
  • Since this can be caused by a bad blocking diode in the other switch box, disconnect the White/Black jumper between the switch boxes and retest. If all the cylinders are now firing, replace the switch box that was firing all three cylinders originally. To verify this condition, swap the trigger leads around of the switch box that was firing all three cylinders originally. If the misfire moves to another cylinder, the switch box is bad.
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