weak spark --140hp mercury

[chevymad]

Does anybody know how to figure out whats causing the weak spark? I need ohm and voltage figures for each part and wire. Bought a book, says take it to your nearest dealer. Some help there. Motor ran fine then my brother let it sit in the water a couple of months. Went to pull it out and there was no spark. I took the trigger box in the house overnight. Hooked it back up next day now I have enough spark to jump a 1/4 in. But still not enough to make the engine fire. We took the boat to the dealer and they said it needed an overhaul, $1800, because 2 cylinders were lower in compression than the rest. They have 105 lbs vs. 135. But I think this is cause it was sitting and that it might come back if we can get it to fire. I beleive it should run even if those 2 cylinders were bad. (they do come up with some deep creep sprayed in the plug hole)<br /><br />This motor has a thunderbolt II ignition. Single coil, distributor with electronic trigger-not points, and trigger box.

 

[Mark-C]

Re: weak spark --140hp mercury

Get a better manual. Sounds like it could be low voltage at the coil, which can be due to bad wires or connections from battery, ignition switch, etc.

 

[mbb]

Re: weak spark --140hp mercury

Im guessing here since I dont know the ignition system exactly. Sounds like a late 70's inline 6 140. From my personal experience with older mercs, the outside of all the wiring might look OK, but all the copper inside can be completely corroded. I had to replace a stator due to this, motor was running fine, but wires were held only by a few strands inside. It idled about 300 rpm faster in gear after the stator was replaced. The switchbox/converter and/or coils steps up AC voltage generated by stator, if wiring is bad, it can lead to weak spark.

 

[Walter]

Re: weak spark --140hp mercury

I'm guessing your Merc is a 1972 or 1973 inline six...I don't think they made a 140 hp any other year...but I may be mistaken. It would be helpful when posting questions regarding your engine, you'd include the year and model. <br /><br />Many marine tech's call the aftermarket repair books "pay me now or pay me later" manuals because they're so general. If you're intent on maintaining this engine yourself, you do need to find the factory repair manual specific to that engine... With a little mechanical/electrical saavy, the proper manual should help you confidently troubleshoot most problems.<br /><br />Because you said this engine ran ok prior to sitting out on the water for months tells me your problem might be something less serious than one of your major charging system components going south. Be sure to check all connections starting at your battery and working upstream from there. Like MBB said...moisture/corrosion/poor connections can all lead to poor ignition performance. <br /><br />Your local Merc tech was right though...If all your cylinders were with in a few psi of one another you'd be ok...but have two of them down 30 psi?? That's not good!<br /><br />Good luck...let us know how you fare...<br /><br />Walt

 

[chevymad]

Re: weak spark --140hp mercury

The motors a 1978 1400 model. From the wiring diagram that is in the book it looks like the only way the stator is connected to the trigger box is through the main battery wire. The box has 1 switched bat. connection, 1 constant bat. connection, and the green to the coil. On the other side are just the 3 wires to the distributor trigger.. I have 13 volts at both the switched and constant power connections with the key on. I've tried to trigger the box with a ground wire and get the same amount of spark as with the distributor trigger. This makes me think the trigger is allright. I'm leaning to the coil. Need to borrow the scope i guess and see what the voltage is to the coil. Suppose to peak at 180 v dc right?<br /><br />The book i have says mercury used this ignition from 1970-78 on the L6 1150,1350,1400, and 1500 models.<br /><br />I may have more problems but would like to get this to start first. Cant beleive that even if 2 cylinders are down it wont run. After runnin then i'll recheck compression and see what happens.