'87 25hp Mercury runs on one cylinder

[Skipping Classes]

Hi,

I've got a 25hp 2-tact Mercury from approximately '87. It runs, rather rough, on one cylinder only. I've tried a number of things, and the most strange thing that it will only run on the lower cylinder connected to... either coil!?

Am I correct that the engine should not run if I connect the coil from top cylinder to bottom spark plug and the coil from bottom cylinder to the top spark plug?

Do I understand correctly that both spark plugs now fire at the same time - which is correct only for the bottom cylinder? Also, it seems that the stator is only producing 100V instead of 180V which confirms my suspicion they are firing at the same time, which is right for the bottom cylinder only.

Is this definitely the switch box? I wouldn't want to spend the big $$$ just to try, until I know for sure.

What else can I do? Can this be the trigger?

Also a strange thing - when I try to measure the voltage between BLK/WHT wire and the engine mass - the engine seem to rev up?

Any clues will be greatly appreciated!

Can somebody point me to the wiring diagram of the switch box?

Thanks!

SC

 

[Laddies]

Re: '87 25hp Mercury runs on one cylinder

 

[Skipping Classes]

Re: '87 25hp Mercury runs on one cylinder

This is awesome!

A) Is my thinking correct that with a GOOD switch box the engine WILL NOT run at all or on one cylinder if the green wires are swapped? (Because the sparks will fire at exactly the wrong point for both cylinders)

B) When I disconnect the switchbox and measure the resistance between GND and GRN/YLW (cylinder one) I get a short (bad?); when I measure the resistance between GND and GRN/WHT I get an OPEN (good according to the diagram?)

I should get both of the readings open? Is this correct?

C) Can anything else except of the switch box give me spark at the wrong time on both plugs and lower the stator output?

Many thanks in advance!

SC

 

[Laddies]

Re: '87 25hp Mercury runs on one cylinder

A bad trigger on a 2 cyl will not run at all- look at the drawing it's a single winding and with a open on it can not fire either cyl. A bad stator may run with the high speed winding bad as it will start on the low speed but cannot run at full speed as the high speed is not producing voltage to run on, if the low speed is out it will not start at all. If the box is bad depending on what fails it it, might not run at all but more commonly runs on 1 cyl, a coil can cause the engine to run on 1 cyl also. if a person does not have the proper test tools to check for 1 cyl problems, pull the plugs and see which cyl fails, then swap coil leads at the box, if the spark switches cyls it's the box and if it stays the same it's the coil. If you need the test for the system I can send it to you, let me know.

 

[Skipping Classes]

Re: '87 25hp Mercury runs on one cylinder

Thank you very much for the explanation! I haven't thought about switching the leads at the box, unfortunately, but I have done another test: I've switched the high-tension wires that lead from the coils to the sparking plugs. The problem stayed at the same cylinder, but both coils worked.

I took off the switch box and it measures a short between the condenser mass contact and the cylinder 1 coil lead. This sounds like a bad switch box to me.

I am now at home and planning to just get myself a new switch box. Or do you think I should try to swap the coil leads first and do the test you suggested before spending the money on the switch box?

Many thanks!

 

[Laddies]

Re: '87 25hp Mercury runs on one cylinder

Before spending the money take a minute and justify your finding by swapping wires.

 

[Skipping Classes]

Re: '87 25hp Mercury runs on one cylinder

So indeed, it was the switch box. Replaced and it works on both cylinders!