Bad coil?

[Boatbuyingloser]

I believe I have a bad coil. I have a 1993 Mercruiser 5.7 Thunderbolt IV with the point ignition, does anybody have the specs for what my coil should be reading? I have a pertonix coil but it’s not labeled so I have no idea what OHMs is should be reading at.. I’m wanting to go back to OEM so if anybody knows the part number that’d be super useful as well

 

[Bt Doctur]

being a TB IV any standard coil will work for electronic ignitions

 

[Bondo]

I believe I have a bad coil. I have a 1993 Mercruiser 5.7 Thunderbolt IV with the point ignition, does anybody have the specs for what my coil should be reading? I have a pertonix coil but it’s not labeled so I have no idea what OHMs is should be reading at.. I’m wanting to go back to OEM so if anybody knows the part number that’d be super useful as well

Ayuh,..... You can have Thunderbolt ignition,.. Or Points ignition, but not both,.....
T-bolt is electronic ignition,....

If in fact you have T-bolt ignition, did you run through the diagnostics for it,..??
Odds are slim to none that the problem is the coil,....

 

[Boatbuyingloser]

Ayuh,..... You can have Thunderbolt ignition,.. Or Points ignition, but not both,.....
T-bolt is electronic ignition,....

If in fact you have T-bolt ignition, did you run through the diagnostics for it,..??
Odds are slim to none that the problem is the coil,....

Hmmmm now I’m not sure, points ignition means it has points and condensers right?
Honestly I’ve been assuming the whole time that TB IV was the points ignition.. here’s a picture of my distributor if that helps

No I haven’t been able to find anything like that, and it’s a Jasper rebuild so I can’t say 100% what engine I have besides it’s a 5.7 Carb. My Tach has been jumping up +1k -2k RPM and oil pressure readings have been off, maybe they’re related?
My problem is the boat will run at idle for ~20 mins before the RPMs slowly drop, it gets richer and richer before finally dying. Trying to get up on plane, it just falls flat and RPMs surge until I bring it below 2,500rpm, replaced carb, plugs, wires, distributor cap + rotor already. Only thing left I can think of is coil or distributor.

 

[Scott06]

Hmmmm now I’m not sure, points ignition means it has points and condensers right?
Honestly I’ve been assuming the whole time that TB IV was the points ignition.. here’s a picture of my distributor if that helps

No I haven’t been able to find anything like that, and it’s a Jasper rebuild so I can’t say 100% what engine I have besides it’s a 5.7 Carb. My Tach has been jumping up +1k -2k RPM and oil pressure readings have been off, maybe they’re related?
My problem is the boat will run at idle for ~20 mins before the RPMs slowly drop, it gets richer and richer before finally dying. Trying to get up on plane, it just falls flat and RPMs surge until I bring it below 2,500rpm, replaced carb, plugs, wires, distributor cap + rotor already. Only thing left I can think of is coil or distributor.

When it dies have you checked for spark with a spark gap tester ? when it dies check to see if you have no spark or no gas

 

[Boatbuyingloser]

When it dies have you checked for spark with a spark gap tester ? when it dies check to see if you have no spark or no gas

I don’t have a spark gap tester, but it is definitely getting fuel because I can smell it getting really rich a couple minutes before it dies, then I can’t start it for 10-20 minutes.
I’ve already gone through (I believe) the whole fuel system - I checked the pickup tube, anti siphon valve, checked all the lines, changed the fuel filter, checked carb filter, and vents are clear.
I had this problem last year as well and it’s somewhat intermittent which makes me think it’s a bad coil or maybe a bad ground because my tach & oil pressure gauge are acting up (No idea how to even start looking for a bad ground) Last year I had a buddy rebuild the carb, replace the distributor cap & rotor, new plug wires, and new plugs. This year I brought it to a marine mechanic who told me it 100% was the carb + plugs and they needed to be replaced (mechanic wouldn’t rebuild it), so we did that and it ran great for about 5 hours, then I felt a single sharp drop in RPM for half a second, 2 hours later it was acting up again. Same issue with a brand new carb + plugs.
I’ve run it off a separate fuel tank but no cigar

 

[ROY WILLIAMS]

Hmmmm now I’m not sure, points ignition means it has points and condensers right?
Honestly I’ve been assuming the whole time that TB IV was the points ignition.. here’s a picture of my distributor if that helps

No I haven’t been able to find anything like that, and it’s a Jasper rebuild so I can’t say 100% what engine I have besides it’s a 5.7 Carb. My Tach has been jumping up +1k -2k RPM and oil pressure readings have been off, maybe they’re related?
My problem is the boat will run at idle for ~20 mins before the RPMs slowly drop, it gets richer and richer before finally dying. Trying to get up on plane, it just falls flat and RPMs surge until I bring it below 2,500rpm, replaced carb, plugs, wires, distributor cap + rotor already. Only thing left I can think of is coil or distributor.

the gasoline is the gas filter and the gas tank vent .....
the engine is running and the gas tank is that cap OFF ... the engine is running 20-30 min ... OR the ignition TB IV ...
the carb is running of the engine ...
the gas fuel and the ignition issue ..gasoline is the water issue
the oil pressure 30-50 psi ....

 

[04fxdwgi25]

Sure sounds like a voltage or grounding problem. Would be best to get it to someone that specializes in electrical systems.

Too bad you're not closer to here.

 

[Scott Danforth]

Getting richer is fuel system

 

[nola mike]

How do you know it's getting richer? Intermittent/non firing cylinders will get you unburnt gas in the exhaust

 

[Scott06]

I don’t have a spark gap tester, but it is definitely getting fuel because I can smell it getting really rich a couple minutes before it dies, then I can’t start it for 10-20 minutes.
I’ve already gone through (I believe) the whole fuel system - I checked the pickup tube, anti siphon valve, checked all the lines, changed the fuel filter, checked carb filter, and vents are clear.
I had this problem last year as well and it’s somewhat intermittent which makes me think it’s a bad coil or maybe a bad ground because my tach & oil pressure gauge are acting up (No idea how to even start looking for a bad ground) Last year I had a buddy rebuild the carb, replace the distributor cap & rotor, new plug wires, and new plugs. This year I brought it to a marine mechanic who told me it 100% was the carb + plugs and they needed to be replaced (mechanic wouldn’t rebuild it), so we did that and it ran great for about 5 hours, then I felt a single sharp drop in RPM for half a second, 2 hours later it was acting up again. Same issue with a brand new carb + plugs.
I’ve run it off a separate fuel tank but no cigar

You need so e way to test for spark when it stops running. Would check the following when it starts acting up.
12 v on positive side of coil.
soark off coil to distributor and from dist to plugs
since you say its rich check for fuel dripping down boosters into carb throat, if so measure fuel pressure.

given instrument bouncing would think you may be loosing 12 v when key is in run which often daisy chains through gauges (12 v supply to gauges) on its was back to the engine …