Removal of Spark Plug Wires from 339 5288 coil on 1976 7.5 hp Merc

[WarthogMan]

My 1976 7.5 hp Merc is only firing on one cylinder. I'm trying to trouble shoot the ignition system and to check the resistance on the secondary winding on the 339 5288 coil, I'd like to remove the spark plug wire from the coil but am not sure if just pulling on it will damage anything or if it is designed to come out. If I can get it out, hopefully I can isolate the problem to the coil or the spark plug wire. Any other ideas are more than welcome if anyone has dealt with a similar problem. I'm not sure it's either one of these but it seemed like a good place to start. Thanks

 

[racerone]

The wire simply pulls out of the coil.

 

[merc850]

If it has a rubber boot over the casing peel that back first then pull the wire with needle-nose pliers.

 

[WarthogMan]

I appreciate your quick responses. I just wanted to make sure I didn't damage anything. Both the coil resistance and the plug wire checked OK and it has a good spark on both plugs when I do a static test with a timing light. I'm still trying to figure this out though. It starts up and idles fine when I run it in a barrel but when I put it on a boat under load it only develops about half power with occasional surges to full power. It sounds to me like an intermittent short somewhere that affects the top cylinder based on that plug coming out cleaner than the bottom one after running on the boat. I am stumped at this point. Is there a particular grounding point that I should check that has been troublesome based on your experience? I see that each coil has a little wire on the back that probably provides the ground to the frame for the coil. Each of those appear to be OK. Any help would be appreciated. Thanks so much.

 

[racerone]

Clean / sand the surface where the coil ground wire goes.----------Test for strong spark.--------------Does spark jump a gap of 3/8" on both leads, yes or no?.--------Sparkplugs are not used for that test.--------Have you done a compression test ?.

 

[WarthogMan]

Thanks Racerone! For the 3/8" spark gap test, can you just use a screw driver inserted into the spark plug boot and then hold it about 3/8" from a ground? Is it not a good test if you gap an old spark plug with the max gap possible (probably will not get to 3/8") and then do a visual spark test? I did a compression test last night and both cylinders were balanced at about 62 psi on my cheep old hand held compression gage with a rubber tip. I don't have confidence that 62 is the correct reading but I think the gage is good enough to tell me the cylinders are balanced. I've used this gage on my omc 8 hp motor and I get about the same 60 psi readings and that motor runs perfectly. I think we probably both have the same hunch at this point - if cleaning up the coil ground doesn't fix it and the 3/8 gap test is failed by one of the coils, it's probably a week coil that should be replaced. Would you agree? Thanks for all your help, sir!

 

[racerone]

Using a screwdriver for that test works just fine.

 

[Chinewalker]

If the power pack is original, check all the wiring thoroughly, as Mercury electronics of that era are noted for crumbling wires.

Also, it's easy enough to swap the coils around (simply undo the green wires and swap them, then swap the plug wires) to see if the issue follows the coil. If the issue follows the wiring, I'd lean toward the power pack being bad. If the same coil fails to fire in both cases, then try a different coil.

 

[WarthogMan]

Thanks Chinewalker! I did the 3/8" gap test last night and the top cylinder failed while the bottom cylinder passed so I'm suspecting a bad coil even though when I do the same test with the spark plug inserted into the boot I get spark.on both plugs. I'll switch the coils and do the 3/8" test to confirm and also double check the condition of the wiring. It all looked good from an insulation standpoint on first look. Thanks again for all the help!

 

[WarthogMan]

Installed a replacement coil that I got off of ebay and it passed the 3/8" gap test. Ran it in a barrel last night and it ran fine. Now it just has to pass the boat test at full power and I'll have a good running motor. Thanks again for all the help to get this running!

 

[WarthogMan]

Final Update: The motor ran great on the boat. Tested idle, mid range and full power. Replacing the coil was the key. What I learned was that you can have a spark on the plug during a static test but the coil may still be bad. The 3/8" gap test was the key to solving this. Thanks again for all the help.