88 Merc outboard power

[Jack Hutchison]

I have an '88 Mercury 135 that starts and idles smooth, but will not develop low speed power. It strains to get the boat up on plane. Once over 3000 rpm the engine rips like new.

Compression is on spec, new fuel. I just had the carbs rebuilt, fuel filters replaced, new plugs, new electronic module, you name it. I've spent a fortune trying to get this fixed. So far, I haven't found a shop that addressed the issue, but spent my money replacing parts that I thought were ok.

Any ideas.

Jack

 

[Alumarine]

Re: 88 Merc outboard power

Hello and welcome.

Did it ever work well with this or any other boat?

 

[sschefer]

Re: 88 Merc outboard power

Sounds like your low speed stator coils might be going south on you. There are 4 ignition related coils in the stator, the remaining coils are for the battery charging circuit. Two of the igniition coils are for low speed (approx 0-2500 RPM) and two are for high speed (approx 2200-6000 RPM). One coil of each set feeds cylinders 1,2,3 and the other coil feeds 4,5,6.

The initial test is to use a spark gap tester to check the condition of the spark at cranking speed. Spark should be bright blue on all six.

Because of the 60 deg design of the Merch V6 it is not at all uncommon to loose three cylinders and idle smooth because the crank journals are non opposing.

If one or both low speed stator coils are weak it will run rough since it might fire randomly.

 

[Jack Hutchison]

Re: 88 Merc outboard power

Thanks much. This makes sense. The stator coils may be weak. I didn't realize that there were high and low speed coils and a charging circuit. I noticed yesterday that my ammeter is reading 10 volts. Maybe related to stator failure.

I'm hauling the boat into the repair shop tomorrow.

Jack

 

[verado7]

Re: 88 Merc outboard power

maybe but a bad regulator or rectifier will give you low charge - try disconecting either first and see if it improves .

 

[sschefer]

Re: 88 Merc outboard power

maybe but a bad regulator or rectifier will give you low charge - try disconecting either first and see if it improves .

This is true, hard to believe but a bad reg/rectifier could be shorted to ground and causing premature firing of the switchbox. It took me a long time to figure out how that could happen. I really didn't believe it until CDI started including the procedure of diconnecting the two yellow wires, (A/C input to rectifier from stator), to the rectifier as a test procedure for rough idle and miss fire.

I don't think that's the problem here but ya never know.

 

[Jack Hutchison]

Re: 88 Merc outboard power

If my stator is bad and needs to be replaced, would I be better off buying an aftermarket unit, like a CDI vs OEM?

What are the tests?

I had a high speed circuit go out on the original Merc stator five years ago. Sounds to me like the Merc stators are marginal at best, as one would think they should last a lifetime under normal recreational service.

Also, how difficult is it to replace a stator? I understand that the flywheel needs to be pulled using a special tool.

 

[sschefer]

Re: 88 Merc outboard power

If my stator is bad and needs to be replaced, would I be better off buying an aftermarket unit, like a CDI vs OEM?

What are the tests?

I had a high speed circuit go out on the original Merc stator five years ago. Sounds to me like the Merc stators are marginal at best, as one would think they should last a lifetime under normal recreational service.

Also, how difficult is it to replace a stator? I understand that the flywheel needs to be pulled using a special tool.

You do need a flywheel puller.. Nothing you buy at an autoparts store will work. http://www.promarineusa.com has the correct puller. Might have to dowload the catalog to get the part nr.. If you find one on eBay make sure it has the bearing, washer and pad. If it doesn't you risk splitting your crankshaft.

The Merc Red stators have a horrible reputation but the Black ones are fine and every bit as good as the CDI's. It's hard to tell the 40 amp stators appart and if I knew for sure I'd tell you they were both made by CDI.

 

[Jack Hutchison]

Re: 88 Merc outboard power

Thanks Commander Shefer. I'll avoid the red stator for sure.