2.5l mercury overheating

[LakeFever]

I was taught any voltage at all in the black/yellow means bad switch

[Smalltownbassin]

I don't know if the pics posted earlier with the heads off are from the present state of your engine or when your last one melted number 4... but the piston in that picture is DONE...if that is the present state of your motor I would not run it. The melted aluminum all over the top of your piston can cause predetonition by itself from all the peaks and valleys that get overly hot during combustion. Just my 0.02

Thats the "Frankenbeast" i built that smoked a hole first. Already got the parts to rebuild but need to address the issue first before it takes this other one out too.

[Smalltownbassin]

If the key switch is bad it will bleed voltage into the kill circuit black/yellow. Can't remember but I think anything more than a couple tenths of a volt is out of spec, when turned off.
Attachment 506916Attachment 506918Attachment 506920

Hahaha i screen shot that exact same 3rd pic the other day from a previous thread trying to troubleshoot this thing! But CDI has its own specs that differ from OEM. I barely have 1v on bias though. Im leaning on the SB's hard now. Almost a guarantee they are causing this. First thing is replacing the 20 year old key switch bad or not its $20 for peace of mind. Bout to check it now. Should i measure in the Kill or run state of the switch?

[Smalltownbassin]

I was taught any voltage at all in the black/yellow means bad switch

I Just tested it.

Kill position with key off = 0.000VDC
Run position with key off = 0.000VDC
Kill position with key on = 0.024VDC
Run position with key on = 0.009VDC

Not even a tenth but it is constant voltage potential that shouldnt be there.....right?

[Smalltownbassin]

Im about to hit the key hole with 180psi air to see if my voltages change after. If they do.....its pretty much a dead ringer after reading everything i could find in the archives. Reckon ill find out soon enough unless there are other potential causes i need to look at.

[Smalltownbassin]

25ft air hose wont reach so i cycled the key rapidly and checked again. Voltage went up to 0.034VDC. Cycled key a few more times. Voltage dropped to 0.013VDC. These were measured with key on and in run position where i was getting the highest reading. Seems shady to me for sure. Ive definitely got some unwanted pixies.

[LakeFever]

This is Forbes what he taught me about it. I changed my switch after he posted this. In fact i ended up with all new electrics aside from trigger. That motor still runs champ

The black/yellow is grounded to kill motor. If ya get any amount of ground whatsoever with key on, it won’t start. Shouldn’t have any voltage on it, being you at times read voltage, good chance the switch is getting dusty inside. Any voltage on the black and yellow wires will also kill the switchboxes, which could be explanation why a box took a dump in the first place.
Before you buy any parts, when it won’t start unhook black/ yellow wire/s From switch and see if it starts. If so there’s your problem, if not unhook em from switchboxes and see if it starts. If so problem is between boxes and switch. If not move on to another suspect, kill circuit not the problem.

[Jim Speros]

I was taught any voltage at all in the black/yellow means bad switch

As I recall Black/yellow has voltage present while running. Grounding it kills the engine.

[Smalltownbassin]

As I recall Black/yellow has voltage present while running. Grounding it kills the engine.

Should i be getting the readings i posted above with motor not running? I did all the voltage tests with both the key and kill switch in both positions.

[Jim Speros]

I just checked my old manual. Voltage is present in kill circuit at cranking spend. Testing with engine off is a different. The outboard key switch has 2 switches in it and the do go bad. One switch is positive. Red to purple when on. The other is Black to Black/yellow when off.

I have a short test harness. Makes it real easy to test harness while trouble shooting. Like the above post mentions unplug 10 pin at engine; check for spark followed by removing those from switch box.

[Jim Speros]

Should i be getting the readings i posted above with motor not running? I did all the voltage tests with both the key and kill switch in both positions.

I've never tested it that way. I presume there should be no voltage with engine off. I have witnessed switches doing funny things. A little moisture can make it act up. I have witnessed a boat cranking on its own unattended. I have found boats with keys mounted facing up have frequent issues as they collect and hold water easily. I also commonly spray crc in key switches as a seasonal maintenance thing.

Switches are generally inexpensive. Any suspect switch should be changed.

[OnPad]

Smalltown I just got close to the same readings as you did on two different boats. 0 in the off position. I was a little higher in run position with the kill thrown.

[Jim Speros]

I guess tiny fractions of volts are normal. I guess in that case an old fashion analog gauge would read zero

[OnPad]

The couple of triggers I've tested at cranking speed check out at 4 to 5 volts. My service manual states 4 to 6 volts. Not sure about the 10v that your getting?

Might try a bias and kill test, or mess with the timing light some more.

[Jim Speros]

I've read thru this thread again. SO Small town I do not see where you have said it sounded alarm. You say its overheating. What are you basing it on? Have you verified temp alarm sender. Many have commented on pressure and temps. My take is similar to most . 15 lbs no less on water pressure at 5000 or more. 3/4 to 7/8 on temp gauge is typical in warm water. No 4 can melt and be unrelated to water temp. One posted about an air leak. Good point. A cracked reed change or intake manifold. Warped intake or something maybe gasket doesn't line up properly.
Are the carbs on it original to engine? And set up per manual?
I hope you find something to narrow down your search.