70 hp OMC gremlin

[hhilderley]

I have a 70 hp Johnson of likely late 70's vintage (electronic ignition). Runs like a charm but after periods of damp weather, either refuses to start or once started, looses power at full throttle.<br /><br />Have had carbs rebuilt and tuned, replaced power pack (under flywheel) and ignition box. <br /><br />When this gremlin emerges, spark is inconsistent to one or more cylinders. If I leave the boat for a few days and the weather improves...voila it runs like a dream again.<br /><br />Ideas??

 

[petryshyn]

Re: 70 hp OMC gremlin

Sounds like an ignition problem. Be sure the plug wires are all seated properly. If so check their resistance with an ohmemeter. If OK, it may be the insulation breaking down electricaly in humid conditions.<br /><br />>When its idling fine, take your girl friend's spray bottle, fill it with water and heavily mist each cylinders plug wire and coil (one at a time) until you hear the one effecting your motor.(misfire) Then you'll know which to replace...<br />The water you apply will short out the weak insulation and point to the gremlin....<br /><br />good luck!!!

 

[hhilderley]

Re: 70 hp OMC gremlin

Will give that a try once the ice melts. Another thought, could the problem be a sleepy coil? What's the test procedure to identify a coil (intermittent) problem?

 

[petryshyn]

Re: 70 hp OMC gremlin

Damp weather suggests unwanted path for electron flow....= carbon traces or insulation break down. If it’s the coil, it will be aggravated by the spray bottle water and short to ground causing very obvious misfire.<br /><br />Coils can be checked for a resistance value, but will not often reflect a bad insulation as suggested. Measuring them for resistance is really only definite if a open circuit or a value way out of spec is observed...<br /><br />>typical coil primary (small lead to ground lead) = .5 - 1.2 ohm<br />>typical secondary (plug lead to ground) = 5000 - 9000 ohm<br /><br />>see what I mean..... there is too much tolerance in manufacturing and ambient temperature to allow a specific reading to apply. A resistance reading will not check for a 'short' or 'leakage' to ground when high voltage is produced.<br />>the ultimate coil test is to apply a pulsating voltage to the primary and observe the secondary voltage on a oscilloscope. (not practical for most)<br /><br />check the resistance, if OK, then spray it down with water while idling. If no effect, presume its good.....<br /><br />****remember, this troubleshooting is based on your complaint of "missing when humid"<br /><br />my humble opinion...... <br />