79 175 hp intermittant spark

[DJones]

Good afternoon,<br /> Im hoping someone here can point me in the rite direction.I have a 1979 mercury 1750 175 hp,over this past season Ive had an issue with it only running on 5 cylinders.For a time it would plane out and seem to run on all six after warming up but the last time out it wouldnt.I figured Id start with the basics checking plugs and wires,and I found that the bottom coil that wasnt firing was cracked and corroded.I replaced the bottom two coils,cleaned the plugs and made sure the wires werent broken or cracked by ohming them out.Needless to say Im still not getting spark at the bottom cylinder,I am getting 12v to the coil when the key is turned so I dont understand.<br />Thanks <br /> Dave

 

[ebubeck]

Re: 79 175 hp intermittant spark

Swap the coils, if your missfire follows - you still have a bad coil.

 

[KCLOST]

Re: 79 175 hp intermittant spark

Since you bought new coils....<br /><br />My guess is a powerpack going out on you (or possibly a trigger problem)...<br /><br />Try this,<br /><br />Locate both switchboxes (powerpacks), they are on the starboard side of the block and have all kinds of wires of different colors going to them (about 3"x5" in size).. One is bolted on top of the other...<br />What you need to do is switch them around... remove all the wires from both and switch the packs.. There are color indicators for each wire lead location, but you should be careful not to mess them up... Other than that, it's a piece of cake...<br /><br />After all of the leads are connected back properly (including the ground wires) retest for spark.. If the non-firing condition moves to the other bank (port to starboard or visa versa), you will know it's a bad switchbox that is feeding the cylinder that isn't firing.<br /><br />If nothing changes you may have a trigger problem, which we can ohm out to test next...<br /><br />Link to switchbox location...<br /><br /> v-1750

 

[DJones]

Re: 79 175 hp intermittant spark

I finally got to look at the motor over the long holiday weekend,I want to thank you for your advice.I swapped allthe signal wires from the packs to the coils and the dead spark went somewhere else.At that point I opened up the harnesses and found a coroded wire that went to the bad cylinder.I made a new harness and checked for spark,everything seems o.k..started her up on the hose and it sounded good.Next I just want to check wire voltages with a clamp meter.Its hard to believe that a test so basic is so easily overlooked.Thanks again.<br />The stripers are here and hopefully Im ready to go.