470 Mercruiser dies after 30 minutes

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!

When it dies did you check it for spark?

Jeff

You really cant measure voltage (negative terminal) on coil when running. That is where the signal inputs to discharge coil. meaning it is not always to ground. it pulses.

+ coil terminal to engine ground only.



When engine dies you need to process of elimination right away.

1. check for spark at spark plugs,
2. check for fuel in carb (if you move throttle is there sufficient fuel to discharge from accelerator discharge ports?)
3. is coil very hot? I am not sure but why did you remove resistor wire? Is the coil internally resistored?
4. check distributor shaft after fully warmed up for side to side movement. Distributor is on an angle and the housing bushings do wear out.

What is temperature when running and then pulling back to idle? Should be around 170*

I'm new to troubleshooting electrical issues, so that's good to know. Appreciate the information.

To answer your questions:
1. Couldn't diagnose
2. Couldn't diagnose
3. Coil was hot but nothing out of the ordinary. Resistor wire was removed when the Pertronix coil was added as it has an internal resistance of 3.0 ohms.
4. Didn't see your response until we were off the water. I'll check next week.

Temperature usually increases to around 165 after a 20 minutes under throttle. Once back down at idle, it drops down to around 145.

Took it out this afternoon to check for fuel/spark once it died, but it ran pretty well except for dying once under full throttle from standstill. After dying, it fired right up and ran the rest of the day. Had it out for ~75 minutes.

Hadn't tested it out since replacing the cap and rotor as well as the plugs and plug wires but wasn't very optimistic that those would fix the issue and wanted to use the opportunity to test whatever was mentioned here. Also filled it up with ethanol free gas today before taking it out, but i don't expect that had any impact.

The issue may have been either one of the plug wires which had a resistance of 30k ohms whereas its replacement tested at 1.8k ohms or - something I hadn't noticed before - the rotor didn't have enough clearance in the cap and had worn away at the base of the contacts and into the plastic a bit causing some burn marks (pictured below).

Supposed to get some rain in the next few days but plan to take it out again next week. I'll follow up with any updates if there are any.

Appreciate the responses!

" but it ran pretty well except for dying once under full throttle from standstill. "

Please clarify this?

Do you mean when you gave it full throttle from idle it stalled?

If that is the case then possibly you have a couple of issues.

1. your accelerator pump discharge is too lean. Not long enough or not enough fuel throughout the duration. When the throttle is moved gas should be discharged at the EXACT same time out the accelerator discharge nozzles!

2. your mechanical advance (under point plate in distributor) is not working freely. Look for corrosion or bad springs or interference and double check shaft side to side play

Yes, full throttle from idle.

I'll check the accelerator pump output next week.

Happened to take a photo of under the point plate in the distributor (below). Looked to be in good shape and felt as if it's moving freely.