View Full Version : Merc 115 RPM drop and question on voltage

05-05-2003, 01:01 PM
" Randomly, a couple times a season, my merc 115 will suddenly drop RPM as if I throttled it down - no missfire, no stalling. I thought I fixed it a while back when I replaced an intermittant mercury switch that finally went & killed the engine.

Yesterday, running at full throttle it did it twice - the RPM dropped, I followed them down with the throttle, left it idling, and then took off to 5000+ RPM and then it did it again. I primed the engine w/ the bulb & tried to reproduce it, and wasn't able to. I am not convinced it's fuel.

The other odd thing is that the voltage usually sits at 12-12.5 when the engine is not running & 12.5-13 when it is; yesterday it was right around 15 volts all day.

I'm planning on replacing the stator, fuel pump, and primer bulb.. I hate shooting in the dark, but I'd like to fix it before it breaks. Any theories on what this could be, what else to replace (that's cheap), or how to test for it? I've been through the electrical system before w/ a multi-meter & had no luck finding obvious issues. Thanks for any/all replies,

Jon "

05-06-2003, 10:08 AM
" Jon,

It sounds like the voltage regulator/rectifier has gone bad. Fifteen volts sounds too high for the charging circuit. The only other thing that would cause this to happen would be a wiring problem. If the voltage regulator is not connected to the positive terminal in some way it will cause the voltage to rise like that; behaving as if the battery is dead.

The rpm drop could be connected to this, but it doesn't seem likely. The charge circuit and ignition windings in the stator are separate.

These stators are tough to diagnose. They can pass all tests and yet malfunction. If the high speed winding is going bad the rpms will drop to the 2000 range. This is only an estimate. In order to really test the high speed windings you have to connect a proper test meter and observe the voltage at full throttle. This is not easily done on a boat in the water, with the bonnet off, and yourself and an expensive test meter on the transom while someone runs the boat wide open. Even if you are crazy enough to do this, the water spraying all over the engine will probably cause a few problems that were not there in the first place.

Tony "

05-06-2003, 12:32 PM
" Thanks tony,

Ok, so I take this as test the rectifier to verify that it is connected to the + and "sees" 12 volts as required. If the wiring there is OK I should then replace the rectifier as a side issue.

As for the RPM drop - is there anything else that would likely cause this problem?.. I spoke to a mechanic yesterday who pretty much told me it could be almost anything electrical.. at that point it sounded like it would be best to just run it until something really breaks.

I figured that if it was one of the parts that there is 2 or more of, it would missfire or run unbalanced, he didn't share that theory. So, my question is, is it worth replacing the stator at this point? Are there other parts that would be worth replacing too? Could the trigger have anything to do with the RPM drop?

Jon "

05-07-2003, 11:15 AM
" Jon,

If one of the trigger coils were not working the rpm drop should not be that great, probably about 1000 rpms, but it can't hurt to check the resistance and test for shorts to ground.

I would not rule out a fuel problem, even though it seems unlikely. There could be some debris floating around in a carb that obstructs the main jet now and then, or a check valve in the fuel pump that is not working properly.

How far down do the rpms drop when this problem occurs?

Tony "

05-07-2003, 01:02 PM
" It's hard to say exactly, but way down, like in that 2000 RPM or less range, as if I pulled it down to idle speed - seems like it's going to just die out, but it doesn't, keeps idling. "

05-07-2003, 01:06 PM
" Next time it happens, I will not bring the throttle back and will instead look at the tach first.. my instinct is to pull the throttle back and see what happened... today I should be able to at least test that rectifier and hopefully check readings on the trigger & stator, the fuel pump is definitly working 90% of the time, is there anything else to test? "

06-20-2003, 05:31 PM
Jon I had the same problem with my 95 115merc .
the volt meter was reading 16 volts and the engine rpm dropped intermittantly. the week before it died . we found the voltage reg. fried and the stator bad.