RPM drops intermittently at high RPMs. Merc 4.3L

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sirchad

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My boat has had a problem that sometimes comes and goes and I can't pinpoint what is causing it.
Monterey 180 edge 4.3L 2002yr. Thunderbolt V ignition. I have had this boat for about 5 years. This problem has plagued me for about the last 3 years. It is intermittent.
When running at high RPM's or 3300ish or higher the engine will sometimes cut out for just a second. What I mean by cut out is that it drops immediately in RPMs to approximately idle and then after a second it goes right back to where it should be. This essentially causes a big surge when boating, not good for skiing. There are not any other symptoms to mention. Starts and runs good other than this.
I do change my fuel filter every year. This has made no difference. I also us Stabil fuel treatment each time, 1 oz per 10 gals. I do use regular fuel.
I have set my idle RPM to 650.
Because it cuts out and cuts back in so instantly I feel like it is more likely an electrical issue as opposed to a fuel issue.
 

alldodge

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These kind of issues can be a real bear to find as you know. Being intermittent and never continuing, it would have to be electrical IMO. You start with connections on everything from the key switch, man overboard, engine plug, coil, amplifier, shift interrupter, and battery to motor. Look close at the ground on the distributor.
 

bruceb58

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When you are running, if you hit your trim switch, does your engine stumble at all? If so, it may indicate a resistive connection somewhere in the power to your distributor.
 

Bondo

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Because it cuts out and cuts back in so instantly I feel like it is more likely an electrical issue as opposed to a fuel issue.

Ayuh,.... Welcome Aboard,..... I agree,.... I'd probably start with disconnectin' the tach,....
 

Scott Danforth

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in addition to all the above, could also be the ignition trigger, however worry about that later.

I would start with going over every connection in the boat. clean them (shiny metal clean), then seal with marine electrical varnish

start with the battery positive follow every positive connection in the boat - battery, cable, starter, power lead, big circuit breaker, engine harness connections.....etc, then move on to the battery negative and chase every connection on the ground side.

it will take an entire weekend
 

sirchad

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Thanks for all the replies. There is no stumbling when I hit the trim switch or backfiring or anything else. I am electronically very savvy so one of the things that sticks in my mind is "why is it that it only cuts out for the exact same amount of time each time this happens". Why exactly 1 second and not sometimes 2 or 3 seconds. If it were a faulty connection the cut out time should be more random. Also bad connections tend to get worse and worse, not always but typically. Also this has a carburetor.
 

sirchad

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Bondo I am curious why you say to disconnect the tach. Please elaborate. Do you mean just disconnect on the back of the tachometer itself and why?
 

Bondo

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Bondo I am curious why you say to disconnect the tach. Please elaborate. Do you mean just disconnect on the back of the tachometer itself and why?

Ayuh,.... The tach is driven by the coil's (-) terminal,...
A failin' tach can ground the ignition, killin' the motor,...
I'd disconnect it at the coil myself, as the wire 'tween the coil, 'n tach could also be failin', or groundin' out the ignition,....
 

Alumarine

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I once had a boat with a similar problem.
I only noticed it when trying to get a slalom skier up.
On occasion there would be a brief lack of power.
We used to ski all day and fall a lot so it was noticeable.


Turns out the hour meter was wired to the coil. Every 6 minutes the hour meter would click causing an ever so slight drop in voltage.
It was hard to trouble shoot.It had to click just as we were dragging the skier to notice it.
 

bruceb58

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Turns out the hour meter was wired to the coil. Every 6 minutes the hour meter would click causing an ever so slight drop in voltage.
It was hard to trouble shoot.It had to click just as we were dragging the skier to notice it.
I am guessing it was a coil being fed by a ballast wire or resistor. Not sure why someone would think that was ok.
 

bruceb58

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I would not expect the problem to be any interruption of spark because that usually causes a backfire of the un-burned gas in the exhaust when the spark returns. I'm not sure what that sounds like in a boat however.
The exhaust doesn't get hot enough in a marine engine to have that happen.
 

bruceb58

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I had my ignition drop out and never had a backfire issue. Most of the google searches shows backfire through the carb.
 

bruceb58

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The type in the intake would usually be from a too-rich fuel mixture
No...would be from a way too lean mixture. :facepalm:

Have you ever heard of the term "lean sneeze"? Maybe you should google it! :)
 

bruceb58

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And the backfire won't happen in the exhaust of a marine engine.
 
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