motor only runs with coil ignition wire unplugged and about 1 cm away from coil tower

alright this is a strange one (at least for me). I have a 1996 bayliner capri 2052 ls with a mercruiser 5.7l alpha 1 gen 2 thunderbolt V ignition 2 barrel mercarb. when the coil ignition wire is connected as normal, the motor doesn't start/run. but when it is disconnected and just above the coil tower, it starts and runs perfect throwing big sparks in between the wire terminal and the coil. I tested the coil wire and it has 4k of resistance and is 1 foot long and no breaks in continuity. I have a new coil, plugs, cap, rotor, ignition sensor. any ideas would be greatly appreciated, thank you.

Did this engine ever run for you? IFSO, when did the problem start?
Did you gap the plugs to spec when you replaced them?

sandkicker said:

Did this engine ever run for you? IFSO, when did the problem start?
Did you gap the plugs to spec when you replaced them?

Click to expand...

yes the engine has ran and runs great with the coil wire slightly off of the top of the coil. all plus gapped to spec. it started acting up after i pulled it out of storage a few weeks back. it would have a hard timefiring up but would eventually after trying a while. i had the carb rebuilt and put in new plugs and fuel filtersand new fuel. still acted up. then yesterday, no spark at all. diaged andreplaced ignition sensor (replaced cap, rotor, coil as well for good measure).now i get good spark but only when the coil wire is disconnected and just abovethe top of the coil. not that its related but it also has new manifolds/risers,both water pumps, tstat.

start with a compression test of the engine

i shouldalso add that there is a small black ground wire loop maybe 16 gauge that comesout of a small wire harness bundle that isnt connected to anything in betweenthe carb and the distributor. ive looked around and tried to see if it couldreach anything that could need a ground wire and it doesnt reach anything. itsonly about 3 inches long. i thought maybe it was just an extra for something idont have on my engine set up. its definately a ground wire (it gets continuityto battery -, and tests 12v to thepositive side of battery with a meter). the new ignition sensor came with a 3wire set up and it is grounded to the engine block. the old one just had the 2wire style.

What does the inside center of the Dist. cap look like? Does it have a carbon rod, or something else? Usually "old school" style was a spring loaded carbon rod that the center spring of the rotor rode on, delivering spark. So, what does yours look like, is anything touching like this with the cap on?

Sometimes you had to stretch the rotor spring a bit to touch the carbon rod.

To avoid confusion clearify "ignition coil wire"

Is it the purple ignition wire to the + of the coil
Or the HIGH tension wire from the center of the coil to the distributor cap.

Is there a grey wire going to the - side of the coil?
If so diconnect and retry starting. Grey wire is tachometer conection.

Also try a new coil wire...

Standard wires should have about 1k per foot.
If your coil wire is less than one foot long I would say it is a bit high.

You replaced almost everything else why not the wires?

Try to find any plug wire and try it.... Even one that is longer should work for testing.
Anyone you know have any spare plug wires?

Rule of thumb if you dont know exactly the type of wires you have measure the length
And divide by 12... So for example... a 1 ft or 12 inch wire divided by 12 equal 1 which means 1000 ohms per 12 inches

its the high tension wire in the center (spark plug type wire) of the coil that goes tothe center of the cap. the purple is on the positive side and there are 2 graywires on the negative side. didnt change the plug wires because they didnt havemuch run time on them (although they are at least 2 years old), but i am goingto change them out asap along with a new high tension coil wire as wellincluded in the sierra kit.
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im hoping itsa too high resistance in the high tension coil center wire problem because i amgetting 4.10 reading on the 20k ohms settingand its only 1 foot long. youre saying it should be around 1.0. thanks for allthe input from everyone. ill report back once i get the new wires installed andretest. ill also disconnect the 2 gray wires on the negative side as well whentrying to fire it up because i heard that a shorting out tach could causeignition problems (tach seems to be working fine though).

don't believe the cap had a spring loaded center rod. the rotor spring was high and level. i did bend it up just slightly to be sure it would touch though. even felt the cap springing up a little as I screwed it down.

just a quick update - I bought a new set of sierra magforce hi performance plug wires and the new coil wire has only .98k ohms of resistance at 1 foot long. kghost was dead on with where it should be. that's over 4 times less resistance than my old wire at 4k. im going to install the new set as soon as I have a free moment and report back. hopefully this solves the problem. but even if it doesn't, it should make a significant difference in performance.

Change the cap and rotor. It's leaking high voltage.

Resistor spark plugs , like ACMR43T, or MR43TS (extra long) help to eliminate arcing, or crossfiring.