50 Mercury ignition problem?

[Owen]

G'day all,
I have a 50HP mercury 2 stroke (about 1990 vintage).
Just before Christmas I was steaming along when all of a sudden the motor died.
I found that the motor would run OK up to about 1800 RPM and then just die in the arse (missing & spluttering like two plugs had come off).
Anyway took it down to the local Mercury dealer and said give it a full service and find the fault.
After the service it still wouldn't pull more than 1800RPM under load (starts fine and will rev right out with muffs on).
So they pulled it apart and found the stator was out of spec on the high speed circuit (still working, just not right), so they replaced that.
Once again, starts first go. Runs perfect in the tank.
On the water gets to 2,000 RPM and just won't go any further
Does seem to be running sweeter and doesn't seem to be breaking down and spluttering as much, just not revving.
So now they think that the switch box (black box) is cactus too, so we get another one of them too.
Still same stuff.

So here we are about $1,000.00 in parts later and still haven't fixed the problem.
The head mechanic is off on leave and the ther guys are not sure what the problem is.

I'm not crooked on the mechanis (yet).
I do heaps of repairs on welding equipment and it's not uncommon to have to replace one part before you can tell if another related part is stuffed too.
On the other side of the coin I won't be paying for parts that need not have been replaced.
But anyway, I haven't yet discussed that with the mechanic so I'm not bagging them, nor looking for sympathy or advise on how to handle the costs.

What I would like is advise on what else may cause those specific symptoms that is perhaps being overlooked.
Spoke to Mercury Qld and their tech said based on the symptoms, he would have said stator and that about the only other thing is the trigger, but he thought that'd usually exhibit the fault right through the rev range.

It's almost like a rev limiter is being activated or something.

Any ideas??

Owen (looking forward to another boatless weekend )

[DaneCross]

You are describing the exact problem I had with my Merc 50hp (4 stroke). The guys I took it to couldn't replicate the problem in the shop, so they'd give it back telling me they'd tuned it up and that it's running fine. I'd get it on the water, 2000rpm max. This happened 3-4 times. Turned out to be a kink in the fuel line! Enough fuel was getting through to run at low revs but couldn't keep up when the throttle was wound up. For some reason (I'm guessing the angle the motor was being run at) the line wasn't kinked at the shop. My advice, check your fuel line.
Good luck,
Dane

[familyman]

Owen I had exactly the same thing on an old blue band 50,gear shift stop had been busted out of the crankcase by some goose forcing the lower unit on without lining everything up . >
My guess is look for air leakage into the crankcase around the carbs or similar.
The other possible is a weak coil,had one of them too
This will do the same thing.Try getting a mate to steer while you pull the plug leads one by one ,
This will isolate the dud coil as no difference will occur when the lead is pulled.Good luck
cheers jon