115 hp Merc-no high speed power

[RRRandy]

I have an 1984 inline 6 cyl merc, 115hp. I was cruising on plane when it suddenly lost high speed power. It wouldn't rev over 2000 rpm. I brought it to a shop and they found the carbs full of rust etc. They cleaned the carbs and I replaced the fuel tank, hose and primer bulb. No fix! I brought it to another shop and they told me to change the reed valves. I changed the reed valves but still the same problem. I trouble shot further and found no spark on #2 cyl. I changed the lower switch box so now I have good spark on all 6 cyl. Also replaced all spark plugs. Timing checks at 6 deg at idle, 20 deg at WOT. The trigger and coils check OK with resistance tests but the stator is slighly over range. The manual says 5400 to 6200 ohms between the Blue and Red wires and I read 6300 ohms. The other stator resistance checks are within range. Is this close enough to be OK or should I replace the stator?
Compression checks at 93 to 98 psi on all 6 cyl.
I have been unable to test the High Speed Spark Advance Black Box because the manual says to rev the engine to 5600 rpm at which point the timing should advance by 6 deg. This engine only revs to 5500 rpm and what's the point in advanceing at such extreme rpm. Is this a typo errorr? Is there a safer and more accurate way to test this Black Box?
Any other ideas as to the cause of suddenly losing high speed performance?
I would really like to solve this problem before the water becomes stiff!!

Thanx from Canada

 

[Jeff_G]

Re: 115 hp Merc-no high speed power

First of all get rid of the spark advance module if you like your engine. Throw it away.
Also remove the air jets from the float bowl covers and throw them away.
Secondly who ever said reeds is...oh well.
If you suddenly lose power like you say then the chances are the high speed side of the stator has gone bad. You need a DMV reading peak volts to confirm the stator.
If you can take the boat back to the river or lake. Back the trailer in until the boat is just in the water. Leave the boat on the trailer. Start the engine and let it warm up. Put the engine in gear with a timing light on #1. GO up to about 4,000rpm in gear prop on. Flash the timing light so you can see the flashes. Watch the flashes as you go up in rpm. Do this on all 6. Above 2000 rpm or so if the stator is breaking down you will see the flashes breaking up.
It can be a little disconcerning the first time you do this but it is actually quick and easy.
Do no rev the engine out of the water or without a load on it.

 

[Vairpilot]

Re: 115 hp Merc-no high speed power

Follow Jeff's advice. Last year my stator went, and yet it ohmed out ok when I tested it. Since that was the last culprit I replaced it, has run great ever since.

 

[RRRandy]

Re: 115 hp Merc-no high speed power

Thank You Jeff & Vairpilot for your quick response. I just quoted the price for a new stator @ $383.38 plus TAX. This coming weekend I'll try the timing light in the water on all 6 cyl. I'm an Electrician so I do have 'peak meters', maybe even a data logger from work. I'll also unplug the black box on the front frame and observe the effect. I don't know what voltage to expect but I think I can look for consistency.
Anyways, I'll reply again after I run more diagnostic tests.

 

[Jeff_G]

Re: 115 hp Merc-no high speed power

You should have about 180 V from the stator. The spark advance box, although a good idea and sometimes works does go bad with some frequency and can over advance the timing above idle speed and cause severe engine damage. So disconnect it and toss it.