4.3 v6 THUNDERBOLT V barely idles

[aayjay]

190hp (carbureted) Thunderbolt V ignition barely runs now... but today was going fine for four hours, then when approaching large waves I throttled back to avoid being pounded. Water stacks up on the back of the boat as she puts her butt back in the water as usual, but this time it died and would not start. I checked for fuel and the pump runs fine and squirts in both venturi's (barrels) when throttled (not fuel issue). So I go to check spark and there seems to be none. Thus, I clean the cap inside contacts with a bit of emory cloth and scuff the rotor connections on both ends. Inspection of the cap shows an elbow that looks like a vacuum hose but I see no hose to re-attach to it, but assembled It starts again and planes out. this time the wife is yelling "somebody is telling you to STOP) so I power back to see and as soon as the water pressure builds up on the transom again it dies again. After Sea Tow I get it home and turn the key and now it idles but dies when I give it any throttle, Also - I think there was a hose attached to the elbow nipple on the distributor cap but don't know what or where to look at/for. Other idiot thing I did was that I i tried scuffing some rust off of the electrical component under the cap (the Thunderbolt module?). First Question... What parts besides plugs, wires, rotor and cap should I check/replace? Second,question... where would that hose come out of the block that was connected to the cap and what purpose does it serve? (It has all new risers and manifolds and was good on oil pressure and water temp. now she barely lopes and touch the throttle and dies.)

 

[Bondo]

What parts besides plugs, wires, rotor and cap should I check/replace? Second,question... where would that hose come out of the block that was connected to the cap and what purpose does it serve? (It has all new risers and manifolds and was good on oil pressure and water temp. now she barely lopes and touch the throttle and dies.)

Ayuh,..... I'd buy the plugs, cap, 'n rotor,.... I'd Test/ Inspect all of it,.....

Have ya got Power at the coil's (+) terminal with the key "On",..?? While crankin' on the starter,..??
Could be as simple as a bad, corroded ground or power connection,...

The brass elbow in the cap is a spark-proof vent,.....
Sparks can't jump 'round corners, especially in a metal fittin',....
No hose was ever on it,....

 

[aayjay]

Thanks for your quick reply, and yes there has to be power to the coil because it does run, but I will check for any corrosion and clean shiny and grease to keep it that way, but until I get the the basic parts (Plugs wire rotor and cap) I cant really say what it could be. could I have hurt the ignition module by trying to clean the surface of the sensor behind the rotor?

 

[aayjay]

yes to power to coil while key on and yes to power when cranking

 

[Bondo]

Thanks for your quick reply, and yes there has to be power to the coil because it does run, but I will check for any corrosion and clean shiny and grease to keep it that way, but until I get the the basic parts (Plugs wire rotor and cap) I cant really say what it could be. could I have hurt the ignition module by trying to clean the surface of the sensor behind the rotor?

Ayuh,... I doubt it,.... That's why I'm thinkin' yer loosin' power to the coil's (+) terminal, when ya pound down off a wave,.....

A Bad ground, or power connection in the primary ignition circuit,....
Start at the battery, 'n clean the connections on Both ends of Both cables, 'n move on from there,.....

 

[aayjay]

yeah, i feel you on the possible loose connection, but it starts and idles and dies when given throttle, which sounds like weak spark (either old cap/rotor as the plugs looked dark and sooty like they weren't burning hot enough. I will perform the tests to find out whether it is a coil, sensor, or amplifier issue. I dont think it is sensor cuz it does fire, just not very strong.

 

[aayjay]

it all looked so old and ratty i c hanged the coil as well as the plugs cap and rotor. The wires to the coil were gummy to say the least, After sanding them clean with fine wet/dry paper I bumped the key after two plunges of throttle and Viola! It runs well again. It popped once through the carb but the timing seems close as idle is stable, but I should check it to make sure the P.O. didn't bump it too far advanced. It seems to grumble (make bumping clunks quietly but periodically, almost but not quite rhythmically) when the outdrive is tilted/trimmed up. is this common for an I/O?

 

[achris]

Grumbling is usually a sign that unis or gimbal bearing are on the way out... And the engine may need an alignment check.

If your poor idle comes back, clean out your TKS carb. They are known to give problems like that...