kgillet1785
New member
Since there already weren't enough of these threads (sorry)...
Motor is a 3.7L 224 Mercruiser 470 in a 1981 Bayliner. It runs great across the RPM range with no issues until you bring it down to idle after ~30 minutes then put it in gear and apply throttle. Sputters a bit then dies. If i swap the coil (stock coil - 1.2 ohms primary, 9600 ohms secondary), it starts right up and runs without hesitation. Suspect it's an electrical issue and not fuel supply as so far I've replaced:
Fuel: Entire fuel supply from fuel tank pick-up, anti-siphon valve, all fuel lines, fuel pump, and rebuilt carb. No obstruction of fuel vent hose.
Electrical: starter, starter solenoid, distributor cap, rotor, spark plug wires, and spark plugs (gapped to .035").
It has a Pertronix Ignitor electrical ignition with the Flamethrower coil. Resistance wire has been eliminated. Distributor cap has been notched. All connections at coil and opposite end have been cleaned. Coil tests good: 3.1 ohms across primary and 10.6k ohms secondary. No resistance > .1 ohm from either yellow/purple 12v starter solenoid wire, grey tachometer wire, or 12v power supply connected to electric choke (since resistance wire was eliminated during Pertronix conversion). Spark plug wire resistance is within spec.
The boat has dual batteries. Both test good (12.5v and 12.8v). Once started, the voltage goes up to ~14.5v from alternator, so I don't suspect there's an issue with the alternator. No parasitic drain of batteries (.3 mA and .4 mA tested across negative battery terminal).
Here are some coil voltage and resistance numbers if it helps troubleshoot:
Pertronix coil (3 ohm):
Across +/- terminals on coil: Ignition 'on' position: 11.5v. Idle: 6.25v. (No values while under load of 2k or 4k RPM)
Positive coil terminal to engine ground: 14.25v at idle. 11.5v ignition 'on' position.
Negative coil terminal to engine ground: 9.46 at idle. 1v ignition 'on' position.
When cold, it tests at 3.1 ohms primary and 10600 secondary. After the boat died, it tested at 4.1 ohms primary and 12800 ohms secondary. Assume this is expected.
Stock coil (1.2 ohm):
Across +/- terminals on coil: Ignition 'on' position: 10.5v. Idle: 5.5v. 2k RPM: 4.0v, 4k RPM: 3v.
I don't have postive/negative terminals to ground values for this coil yet, but i'm surprised by these values given it's nearly 3x less resistance than the Pertronix coil.
Not sure what to do next; any suggestions would be greatly appreciated. More than happy to do any further testing that may help identify the issue. Thanks!
Motor is a 3.7L 224 Mercruiser 470 in a 1981 Bayliner. It runs great across the RPM range with no issues until you bring it down to idle after ~30 minutes then put it in gear and apply throttle. Sputters a bit then dies. If i swap the coil (stock coil - 1.2 ohms primary, 9600 ohms secondary), it starts right up and runs without hesitation. Suspect it's an electrical issue and not fuel supply as so far I've replaced:
Fuel: Entire fuel supply from fuel tank pick-up, anti-siphon valve, all fuel lines, fuel pump, and rebuilt carb. No obstruction of fuel vent hose.
Electrical: starter, starter solenoid, distributor cap, rotor, spark plug wires, and spark plugs (gapped to .035").
It has a Pertronix Ignitor electrical ignition with the Flamethrower coil. Resistance wire has been eliminated. Distributor cap has been notched. All connections at coil and opposite end have been cleaned. Coil tests good: 3.1 ohms across primary and 10.6k ohms secondary. No resistance > .1 ohm from either yellow/purple 12v starter solenoid wire, grey tachometer wire, or 12v power supply connected to electric choke (since resistance wire was eliminated during Pertronix conversion). Spark plug wire resistance is within spec.
The boat has dual batteries. Both test good (12.5v and 12.8v). Once started, the voltage goes up to ~14.5v from alternator, so I don't suspect there's an issue with the alternator. No parasitic drain of batteries (.3 mA and .4 mA tested across negative battery terminal).
Here are some coil voltage and resistance numbers if it helps troubleshoot:
Pertronix coil (3 ohm):
Across +/- terminals on coil: Ignition 'on' position: 11.5v. Idle: 6.25v. (No values while under load of 2k or 4k RPM)
Positive coil terminal to engine ground: 14.25v at idle. 11.5v ignition 'on' position.
Negative coil terminal to engine ground: 9.46 at idle. 1v ignition 'on' position.
When cold, it tests at 3.1 ohms primary and 10600 secondary. After the boat died, it tested at 4.1 ohms primary and 12800 ohms secondary. Assume this is expected.
Stock coil (1.2 ohm):
Across +/- terminals on coil: Ignition 'on' position: 10.5v. Idle: 5.5v. 2k RPM: 4.0v, 4k RPM: 3v.
I don't have postive/negative terminals to ground values for this coil yet, but i'm surprised by these values given it's nearly 3x less resistance than the Pertronix coil.
Not sure what to do next; any suggestions would be greatly appreciated. More than happy to do any further testing that may help identify the issue. Thanks!
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