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cyl 1 and 2 firing @ 2tdc

zaschf

Member
Hello all,
here is a really difficult on, I guess.

I got this motor earlier this year when I bought a classic boat that needed some TLC.
The motor is a 80hp 4 cylinder 2 stroke Mercury from about 1980 of which the wiring was in a pretty poor state. (wire rot) I have replaced the wire harness.

Anyway, when I got the motor 1-2 fired okay, 3-4 didn't fire. (no spark)
Checked the trigger module and found a nearly broken wire, which I replaced with no change for the problem.

Swapped 1-2 with 3-4 trigger wires. 1-2 still fired okay, 3-4 still not firing.
Trigger module must be okay.

Swapped the stator wires (red/redwhite) (blue/bluewhite). 1-2 still fired okay, 3-4 still not firing.
Stator must be okay.

Swapped 1-2 with 3-4 coil wires. 1-2 stopped firing and 3-4 came right.
Coils okay.

There is only one thing left and that's the switch box.
Bought a new switch box, wired everything up.

Now here is my new problem that completely mystifies me.
I've marked the flywheel with all cylinder's TDC so I can verify that each cylinder gets a spark at the right time.
Strobe from No1 shows on No2 TDC mark
Strobe from No2 shows on No2 TDC mark
Strobe from No3 shows on No3 TDC mark
Strobe from No4 shows on No4 TDC mark
Well, the sparks are just before TDC of course.

Disconnecting No2 coil wire from the switch box makes No1 fire at the correct timing.
As I'm writing this up I realise that I've been prodding and pulling on just about all wires and maybe I have a short to the coils between 1 and 2. (edited)
I'll check that tomorrow morning because now is beer'a'clock :)

Anyway, anyone got any idea where to start with this problem.
 
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Just keep looking and trying different things.

Here's a post that Graham wrote up on Switchbox/ Trigger Failures:

It's that peak (pulse) that tells the SRC switch in the switchbox which cylinder to fire (positive for #1 and negative for #2).

That fact that you can get at least one to fire means the (trigger coil) winding is at least intact - so resistance check will look good.

What the ohm meter doesn't tell you is "voltage" that pulse is coming out at. So you may have a "phantom" (short) in the trigger feed for your number 2 cylinder.

The trigger can be properly tested with a DVA meter (or DVA adapter on a normal volt meter). You can't use a typical multi-meter because it can not accurately capture and display that very short burst of energy that the trigger sends which is just a "chunk" of Alternating Current.

However, a phantom spark issue would also be indicative of a bad switchbox.

A quick test you can do, however, is to swap the trigger leads where they feed into the switchbox. Yes, the timing will be totally out of whack, but if you use a timing light while pulling it over you can see if the problem moves or not.

If it moves, your trigger is toast; if it doesn't move your switchbox is the issue.


Jeff
 
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Was this a new or used box? The phantom short in above post cannot happen as if either leg of trigger is shorted in anyway or period of time you will have a no spark situation...Double check your wiring to switchbox and make sure timing light isnt close to primary wires from box as it will pick up from them.
 
It's a brand new switch box.
I have a small hand held oscilloscope with which I can use to measure the trigger signal.

I'll let you know how I get on.

Frits.
 
The motor is up and running. Turns out that the phantom spark was related to the sensitivity of the pickup from the strobe.
Nevertheless, it was a good exercise for fault finding.

But I'm still not sure of the charging circuit.
The rev counter shows no rpm but it did before I started working on the motor.
I've had the stator out to get to the trigger unit.
What should the impedance be on the two yellow wires from the stator? The book (SELOC) makes no references to it.
 
I guess the 0 ohm reading between the two yellow wires from the stator is very well explained by the attached photos.
As I'd cut back the sleeve it became obvious. More wire rot to fix. The flywheel need to come off again and hopefully, closer to the stator, I find wires in a somewhat better condition from where I can rewire them...
 

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What kind of crap wire insulation Merc used!

One thing you might keep in mind: For some fool reason, my tach wouldn't always work off one of the yellow wires. Switch wires and it works fine! For the life of me I can't see why this happens--seen it twice now--but it does.

Jeff
 
As I peeled back the harness the full extend of the wire damage was obvious and causing a full short. See the bare wires on the attached photo.IMG_260365 (Small).jpg

The rev counter is working and the battery is getting charged.
I'm a bit concerned about the high charging voltage of about 15.5V. The battery will not last very long with the high voltage.

Should I look for a charging regulator?
 
Okay, I went out for a sea trial today and it went pretty good.
Here are the numbers:
sea flat, no wind,
boat weight (approx) 1.5t,
motor Mercury 80hp 2 stroke (1980) not sure if it actually makes 80hp...
top speed 55kph @ 5000rpm,
top speed duration 10min,
average trial speed 25kph,
trial time 45min,
total fuel consumption 12.5ltr,
fuel consumption per hour 16.5lph.

The boat goes easily on the plane at about 20kph @2600rpm and jumps to 33kph @2800rpm.
The sweet spot seems to be around 40kph @ 3800rpm.
I think I'm doing not too bad with the old lump hanging of the transom.

And this concludes my electrical problems with which I opened this thread.
Thank you for your helpful suggestions.

Cheers,
Frits.
 
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