# merc 25 hp fires on 1 cyl. when idle



## Trcothorn (Apr 6, 2011)

i took my motor to the shop last week to get a carb rebuild and figure out why it doesnt idle like it should, i went to pick it up today and the mechanic said he fixed up the carb, but the problem is when its idling it only fires on 1 cylinder. he said he doesnt know how or why and he had the thing in pieces trying to figure it out, but as soon as you give it any kind of throttle it picks back up and fires on both cylinders like it should, he said i shouldnt worry about it because it still runs and isnt hurting anything. anybody have any clue? its a 1985 mercury 25 hp 2 stroke.


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## crazymanme2 (Apr 6, 2011)

Switch coils around to see if that's your problem.You need a spark tester(or something like) to see if you have a weak coil.


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## Trcothorn (Apr 6, 2011)

i did switch the wires and still only firing on bottom cyl. when idling. but like i said as soon as you give it some throttle both cyl. fire and there is no loss of power or anything. do you think i should just let it go and use it the way it is? i mean as long as it still runs and gets me from a to b im happy, but i dont want to risk damaging the motor any more


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## crazymanme2 (Apr 7, 2011)

If you switch & it's still the same I would say switch box.

Your not going to wreak anything,just might get you stranded on the water.


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## cincinnati (Apr 7, 2011)

Compression check: Is bottom cylinder significantly lower than the top?

Since the same carburetor feeds both cylinders, through the same set of reeds, look for an ignition problem.

The switch box ("powerpack") in my '88 Merc 25HP, failed & the symptoms first showed as a low/idle speed miss. This eventually progressed to an any or all speed miss & finally to the point where the motor wouldn't even start until it sat in the hot sun & was warm to the touch.

You can test the other 3 ignition components - stator, trigger assembly & ignition coils - using an ordinary ohmmeter.


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## Bobg (Apr 7, 2011)

I've got the identical problem with the same year and hp merc. It's a 25 XD. Had weak spark on the top cylinder, replaced both coils. Got strong spark on both cyl's but no combustion on top cyl at idle. Trigger was broke on it's mount and replaced it. Still no combustion fire at idle on top. Thought maybe the trigger damaged the switch box, I heard that possibilty somewhere, so replaced the switch box.

In testing everything, the trigger that should only put out about .5 V is putting out 11 to 13 V and testing both trigger wires, the test on the other wire will kill the engine. Ohm testing the stator shows totally backwards. Black/yellow wire that should have 3300 to 3800 ohms is showing very little and black/white wire is also not up to specs but closer to specs. All this with great spark at idle, but the top plug stays clean and shows no combustion firing. Voltage out of the pack to the coils is in specs, and firing on time. New plugs, and wires.

I'm going to pull the lower unit and run without water to be sure the top cyl, isn't getting water intrusion from somewhere. If it fires on top then, it could be water getting in somwhere to the top. if it still doesn't fire in the cyl then, i'm going for the stator replacement. I won't be running it hot, just long enough to check if it combusts when no water is circulated.

I've checked everything I know to test, even replaced the exhaust cover gaskets, and sprayed around the engine with carb cleaner, checking for air leaks. Yup, the carb got cleaned first thing.

I'll get back after some further investigations.


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## cincinnati (Apr 7, 2011)

If your stator already does not test out correctly, why wouldn't you start there?


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## crazymanme2 (Apr 7, 2011)

From my Merc manual you need a DVA tester to check voltage on said componets.


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## Trcothorn (Apr 8, 2011)

wow guys all of this sounds a little too complicated for me lol. well i did put it in the water and rode around the lake for a bit and didnt have any troubles. just idles a little bit weird i guess. it starts fine and picks up speed good so im just gonna use it the way it is for now. like i said if it gets me from point a to b then im happy.i just dont want break something later on down the road, and the mechanic said he isnt sure if it would cause any damage by running it like it is
bobg let me know if you find anything out with yours and how much $$ it cost to fix lol


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## Bobg (Apr 8, 2011)

The good spark to the plug and it firing on time is why I didn't start there. Ohm tests will vary , and I'm just backing up all my other findings. The things I've done already, needed doing, and yes, I use a DVA, or peak reading volt meter. I've even borrowed another meter to back up my findings on my meter.

The next time I run into this situation, I may not need to do all this back up research, and can have a definite answer. I'm no engineer. Just a marine tech, and all the so called experts I can ask on the net, have not given me an answer to this problem. Everything they say to do, I've done, and then some.

I've found many times that if a componant doesn't test just like the specs, that doesn't mean it's actually faulty, but this stator is my last ignition componant to replace, if it's not waer.


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