You need to find someone with a propshaft dyno! Makes it easy to sort through issues like this. I have one but Texas is a bit far from you.

Bob

Final Update - Replaced the Coil, Wires and the HEI Ignition Module with a AC Delco GM/Merc module (old one was 1 season old in a new distributor and was supposed to be a Merc module and was not). Seems the problem is solved, knock on wood! NEVER get a cheap knock off Module, they will fail for work inconsistently!

Also, in the root causing I found the T-stat was a 160 and the little dingle balls in the bypass were worn out causing high idle water and oil temps (as well as lower oil pressures). 140 degree stats and proper bypass operation makes a huge difference in lowering idle oil temps and almost going from 20 psi oil to 40 psi during a long idle up the river.

Thanks Again Smitty, the tune and injectors are working great! No soot, great throttle response and no issues on long runs or idles.

Glad to hear you found it! In my past experience the the chinese copys of ignition modules (standard , duralast,autozone, advance auto etc) are weak, usually overheat and break up on dyno and are root of many problems in boats, usually once past 4000 rpms. The ac delco 8 pin module that merc used originally that has a 399 at end of number on it, , accell, davis unified ignition dyna mod, and pertronix versions have all worked fine for me for most part. I found that if you want to rev any of them real high, you need to lower the dwell in ecm so they dont overheat as easy or add a msd box (5800 to 6200 and up range). I tested their ability to carry rpms with msd signal generator on bench one day, the chinese ones broke up much earlier. I mounted one of the good gm ones on a piece of aluminum as a heat sink and use it for diagnostic purposes in boats and on dyno using a MSD signal generator, Smitty
https://cimg7.ibsrv.net/gimg/www.off...0798ae09ef.jpg

Since the GM modules are discontinued/NLA, what do you recommend for a replacement module?

Ive made 1100 hp using a Davis unified ignition replacement module, used quite a few accell versions and a couple pertronix versions on NA and boosted engines with zero ignition issues on dyno or in the boat. The standards, duralast and other 20 to 40$ ones are junk, they overheat, breakup, quit on top of random issues. I have a accell 8 pin in my 1974 TA in a Pontiac thats seen 7000 rpms with no issues. When I dynoed that, we had issues with some no name one that randomly broke up around 5800, 6000. As it got hotter, it broke up progressively earlier. thats when my son figured out how to use a 100$ MSD tester as a signal generator without even running the engine. We literally took cap off and attached msd tool to module pins from pickup coil and dry fired the ecm which fired the coil. Thats when we saw the spark breakup earlier and earlier while triggering it . I lowered the dwell setting in the mefi program and it went to 6 k before breaking up, I drove to advance auto and bought accells version , went back to dyno shop and we banged rev limiter at 7000 fwiw. Ive used this msd tool on a ton of stuff since then as a signal generator, Smitty

Great info as expected, Thank You.

What is the "driveability" symptom when a module is "breaking up" at high rpm? Is it a noticeable miss / cut-out feeling? Or is it just a flattening of the powerband?

When I was dynoing my Pontiac, it acted like it was going on a rev limiter at 5800, 5900. Except rev limiter was set at 3500 x 2 (7000)in the mefi 4 ecm. We were using a 1981 Pontiac 301 large cap locked rotor distributor that we grafted a 8 pin tbi style module in (same exact piece these mefi boats use). If we made a quick change and pulled it again, it would break up at 5600,5700 like it was on a rev limiter. Ive seen them break up in midrange under a load as if fuel pressure was low too when it wasn't too, completely quit. my 94 tbi plow truck had some Chinese distributor in it and of course some cheap 8 pin module (again, same module style a 454/502/500efi uses), it started coughing and breaking up when I unleashed all 210 hp it had when pushing snow or backing up quickly (prob more like 105 at half throttle). changed module and coil because i wasn't pulling it back in the shop twice, problem went away. I will say though, Ive seen more 2 wire distributor module jumper to coil harnesses go bad then modules or coils on these. Im no electrical expert but also, mercruiser has a resistor in that jumper and a gm motorsports mefi doesn't and have never found the answer as to why or why not, I can say Ive used the wire with no resistor on some boats and on dyno and it didn't do anything 'strange" . I have seen a couple bad coils too, I built and dynoed two identical 454 mags, iron heads I touched up, decent parts, Bob M cams, modded mpi intakes. They both made around 560 on my dyno NA and ran flawless. When I bolted the blowers on, (whipple sidemounts), motor one made 785 or so with unsustainable boost, lowered it and made 725 or so. bolted blower on motor 2, it made 40 or 50 less and was flat on top, zip stripped the coil off motor one on and problem went away. Put Dui coils on both of them, smashed the bad one and threw it in scrap so I wouldn't accidentally put it on something in the future! Both those engines have DUI ignition modules in the original distributors too and still run flawless 3 years later in customers boat.

Thanks for all that. When those jumper harnesses "go bad", is it an open / break in one of the wires? Too much resistance? Or something else?

ive seen boats breakup, miss randomly, cut out when hitting bumps and quit completely (break in wire). on a twin, you have the luxory of swapping parts side to side to find problems. When a guy has me tune his boat on water with a mefi, I bring a spare jumper, spare coil, spare ecm, spare ign module etc in case theres a intermittent issue, same on my dyno, you can eliminate parts as being a problem quickly then

all my ignition coil jumper have been truck harnesses,secure them up right and as my engines show,,they can go many years(16 now) w/o a problem.as far as ignition modules,ac delco 399's,work well,,I got a base tune w/ jacked up dwell times and burned out modules.I got real good and fast at changing them,even while going over a big azz Pershing wake in hyannis harbors channel.:D

Yep, the higher the rpm, the lower the dwell time needs to be to keep from cooking modules! the gm motorsports mefi4 harness I used to buy when they were a deal of the century comes with that coil connector and it has NO resistor in it like mercruiser does, I think the truck wire does, Ive gotten a few off junk vehicles thru the years. Whats your opinion of the inline resistor?

my guess,its to limit the current to the module,not that I see a need for it nor did gm.

i use a lot of these delco external coil ignitions its often not the modual if its a good modual is almost always the pick up coil you take the dist gear off and take the shaft out and the coil is right there its around 20 bucks try to use a good one as the cheap ones the wire connectors fail .........we use these because they idle with a lot of initial timing and that really smooths out an engine idling for 8-10 hrs everyday