Logo
FULL SITE MENU

1996 Mercury 115 dies randomly on the water

U

UTMBA95

New member
Hi,

My dad recently inherited a 1996 fishing boat with a 1996 Mercury 115. It had a fairly major serving not long ago. A few times recently it has died while running. Once it wouldn't start for up to 15 minutes, but other times it starts up fairly quickly. During the failure time, there was fuel in the priming bulb.

I'm learning about this engine to see if I can help him fix it. We took it out for a couple of hours today with the cowling removed so we could check for spark and look at the fuel filter, but it didn't fail. Perhaps it's a heat issue, or perhaps we just got (un)lucky today.

Being new to this engine, I may be making some bad assumptions. I'm assuming it's unlikely to be a carb problem since there are four carbs and losing 1 probably wouldn't completely kill the engine. Same for coils. Agree?

After we got back, I noticed that the fuel was at the middle of the fuel filter. Not knowing what these look like internally, I'm not sure if that's correct. Can somebody confirm? I also noticed that the fuel filter is reversed compared to the photo in the Merc manual. It also idled really rough running on earmuffs, but I'm not sure if this is related to the intermittent problem or something else entirely. Here's video of it running at idle until we shut it off. https://youtu.be/yLBs3N4QxWs You can't easily see the fuel level, but it's about halfway up the fuel filter.

Thanks for any help!
Mike
 
This is second hand, but he says it quits like you turned the key off. He also said that sometimes after the failure the engine appears to start a little and then die. The latter would seem more like a significant fuel delivery failure.
 
I'm confused by the wiring diagram. Do the low oil and temp sensors cut off ignition? They seem to cascade through the rev limiter and the warning module to the black/yellow kill wire. My plan was to disconnect the black/yellow wire on the switch box during a failure to isolate those, but I wasn't sure if the sensors killed the engine or not. I also noticed that the tachometer was jumping up from 1K to 3K sometimes at idle while the engine speed didn't sound like it changed. This could just be the gauge.
 
Last edited:
It would be a lot easier if you could get the motor to quit and then test for spark. This would eliminate a lot of guess work. I believe if your year is correct, your motor is a Yamaha designed "2 + 2 motor...you can be sure of that if only the top two carburetors have idle mixture screws and the bottom carbs do not. It is meant to run on just the top two cylinders until approximately 1800 RPM when the bottom two cylinders finally get enough fuel to fire. Ignition is constant on all cylinders...the bottom cylinders receive only enough fuel for lubrication but not enough to fire until the throttle is opened and the motor hits 1800 RPM. You said the carbs had been rebuilt....there is a procedure called synchronization where the carb linkage is adjusted so the carbs are synchronized to open at the correct point. You would need a manual to do this. When adjusted properly this motor should idle reasonably well and the transition from two to four cylinders should also be smooth.
 
Back
Top