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Mercruiser 420 Stumbles

Old 06-15-2020, 12:10 PM
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Happy Monday OSO!


I have a 1990 Formula 357SR1 with twin Merc 420's with ~338 Hours, all original and up to date on maintenance. Two weeks ago, while leaving a local restaurant, we idled out to the channel, got on plane and ran for ~4 minutes at roughly 3,800 RPMs and the starboard engine stumbled then died out without any alarms - all temperatures and pressures were good. We came off plane, I opened the hatch and hit the key and nothing happened. No clicks, no noises, it was like the key switch was dead. I jumped the starter solenoid and it fired right up, ran the rest of the day (6 hours) without issue, even the key switch was working when we stopped at the marina to drop a passenger off halfway through the remainder of the day.


Last weekend, we launched the boat and about ~4 minutes on plane (~3,800 RPM) leaving the dock after idling out of the no wake zone, the port engine RPM's stumbled and it died out. Once again, the key did nothing so I jumped the solenoid and it fired immediately and we ran down river for about an hour without issue. The key switch worked after jumping the solenoid once we got to our destination. The remainder of the day, the port engine would stumble and die after about ~4 minutes on plane at (~3,800 RPM) (three more times), jump the solenoid, and everything would be fine. It was occurring like clockwork. The final departure from our last stop, we idled out and this time, we got on plane and i kept the engines at ~3200 RPM and it didn't stumble or die.


Any idea where to start here? These engines are running beautifully with any problems or signs of an issue. It is odd that two weeks ago it was the starboard, then last weekend it was the port. They never did it at the same time, it seems like its running out of fuel but it fires right up when I jump the solenoid. Its odd the key switch is inoperable when it stumbles and dies but works after I jump the solenoid once. I plan on digging into this afternoon after I finish up some work but thought someone may have some insight here. Plan on checking the fuel system and the ignition systems first.


Thanks in advance!
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Old 06-15-2020, 06:01 PM
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Sounds electrical to me, I would check voltage to starter solenoid cold (when it works) then when it does it again, check and see if you are getting a good 12v. If so, probably bad solenoids or grounds (clean all ground anyways, canít hurt)

on another Note, I am building some 420 clones and there is really nothing online to reference. Could you post a bunch of pics of the 420s from all angles since you have a nice original set? Not many left out there

Last edited by sailtexas186548; 06-15-2020 at 06:29 PM.
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Old 06-15-2020, 06:16 PM
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You could ask BUP but he is BANNED. Try contacting Lavey 29 on River Dave's Place.
A friend told me he was very good at these things.
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Old 06-15-2020, 06:33 PM
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Originally Posted by sailtexas186548 View Post
Sounds electrical to me, I would check voltage to starter solenoid cold (when it works) then when it does it again, check and see if you are getting a good 12v. If so, probably bad solenoids or grounds (clean all ground anyways, canít hurt)

on another Note, I am building some 420 clones and there is really nothing online to reference. Could you post a bunch of pics of the 420s from all angles since you have a nice original set? Not many left out there
Not a problem! PM your cell number and I can text you some pictures when I open it up tomorrow morning.
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Old 06-15-2020, 06:36 PM
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Originally Posted by GLENAMY 242SS View Post
You could ask BUP but he is BANNED. Try contacting Lavey 29 on River Dave's Place.
A friend told me he was very good at these things.
GLENAMY,

I saw that thread today! It is extremely unfortunate as he was one of our biggest resources as OSO members. Iíll check out RDP and see what I find there as well.
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Old 06-15-2020, 06:50 PM
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Lavey 29 is BUP. OR you could wait and I am sure Griff will come up with the answer.
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Old 06-15-2020, 06:59 PM
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Could be as simple as loose battery cable on starter post, battery, ot ground on batt or back of engine/flywheel housing.

Do you have a test light and multimeter ?
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Old 06-15-2020, 07:18 PM
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Originally Posted by GLENAMY 242SS View Post
You could ask BUP but he is BANNED. Try contacting Lavey 29 on River Dave's Place.
A friend told me he was very good at these things.
A 420 is a carbureted 454 bbc. Right ? Your help is telling him to go on another site and ask Bup ? More of a modern fi technician, but IstillI donít get it. I donít get much I guess.
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Old 06-15-2020, 07:53 PM
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Originally Posted by SB View Post
Could be as simple as loose battery cable on starter post, battery, ot ground on batt or back of engine/flywheel housing.

Do you have a test light and multimeter ?
Yes, I have both. I find it odd that the problem jumped from one engine to the other and it only occurs when Iím just getting into the secondaries. The key switch becoming inop is odd as well. Iíll know more in the morning, hoping for a simple fix since this boat is, for the most part, one of the cleanest I have seen.
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Old 06-15-2020, 08:48 PM
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No crank is electrical.

Are your battery switches in positions where the two motors are tied together, by any chance ? Share a ground ?

Ignition switches share a power wire or ground ?

Any stumble/misfire when you use outdrive from or trim tabs ?
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