Merc 50 4 cyl switch box advice needed

[northernmerc]

I have a 1978 Merc 50 hp, 4 cyl. It only runs on two cylinders. Compression is excellent and fuel system is fine.

Only the two bottom cylinders are firing (determined with timing light hooked up to the plug wires one at a time). I switched one of the coils from the lower cylinders to one of the top cylinders and determined that the coils are OK. I also switched the leads from the stators at the switch box and determined that the stators are OK. After switching the stator leads, the same two cylinders still did not fire.

As far as I know, that means that the switch box is bad. Am I missing any other possibilities?

I assume that the switch box is not serviceable and that the only option is to replace it. Or is it possible to fix a non-functioning switch box?

Thanks in advance for your advice. I don't want to buy unnecessary parts and would like to narrow this down as much as possible.

 

[CharlieB]

Re: Merc 50 4 cyl switch box advice needed

Switch the trigger leads to see of the spark moves to the other two cyls.

DO NOT try to start the motor, just test for spark.

If the spark moves with the trigger lead swap then the problem is in the trigger, NOT the box.

 

[northernmerc]

Re: Merc 50 4 cyl switch box advice needed

I swapped the trigger leads on the switch box. The two top cylinders remained dead.

A quirky thing did happen after reconnecting the trigger leads to their correct spots. I rechecked for spark on the top two cylinders and found intermittent spark there (with the timing light). For ten seconds or so, each of those cylinders had a good spark. But the spark then again disappeared completely.

The problem developed about a week ago while out fishing (trolling at low speed). Although the plugs looked good, I thought they might have fouled, so I replaced them.Then when I tried the motor again on the water a few days later, it intermittently developed full power and then again lost power. It did this for five or ten minutes (surging and slowing) before losing all power on the top two cylinders.

Is it common or possible for switch boxes to behave this way, with an intermittent problem? If not, what else could be causing this? It's not the coils or the spark plug wires or spark plugs. I've moved those from the top cylinders to the bottom cylinders and it makes no difference.

 

[northernmerc]

Re: Merc 50 4 cyl switch box advice needed

Anyone?

 

[northernmerc]

Re: Merc 50 4 cyl switch box advice needed

It looks as if this board has lots of questions and few answers. With the number of postings every day, those who have experience in motor repair must feel overwhelmed and have gone fishing.

So if noone can offer advice on fixing this Merc, does anyone else have a late '70s or early 80's Merc 50 hp that they want to sell for parts? I checked with a repair shop and they told me that to have them do the repairs might cost more than the motor is worth. And to just start buying new parts and installing them until we eventually hit the problem makes no sense either. If you have a non-running Merc 50 that you want to sell, (preferably in Canada) please drop me a PM.

 

[CharlieB]

Re: Merc 50 4 cyl switch box advice needed

Switching the trigger leads proved that it is NOT the trigger at fault, nor is it the stator as the stator charges the box. Period. If the stator failed there would be either no low speed spark or a weak high speed spark, that is not your problem.

Your ignition module (box) is toast.

Since there is no return on electrical parts you may want to confirm this by DVA testing the triggers and the stator. When both of those check out OK then you can feel better about spending your money on a new ignition module.

Get the CDI Electronics module, high quality and usually far less than Mercury.

 

[northernmerc]

Re: Merc 50 4 cyl switch box advice needed

CharlieB, thanks for your reply and advice.

Since there is no return on electrical parts you may want to confirm this by DVA testing the triggers and the stator. When both of those check out OK then you can feel better about spending your money on a new ignition module.

Get the CDI Electronics module, high quality and usually far less than Mercury.

I'm no electronics whiz but I took the multimeter and checked the readings at the Stator and Trigger leads. The Stator readings appear to be in the recommended range (although it's hard to tell because this auto-ranging multimeter seems to move decimal places at will. There also is no continuity from the blue and red stator wires to the powerhead ground, which I think is as it should be.

The trigger brown and white/black reading is .799 and the white and violet reading is .811. Again, this is auto-ranging, so it may be moving the decimal by one place.

 

[CharlieB]

Re: Merc 50 4 cyl switch box advice needed

Ohms readings are sometimes misleading, a coil may ohm OK but put under load a broken winding can separate and fail.

The DVA Meter puts a measured load as it measures peak voltage, a much more reliable method of testing coil output.

You can order a DVA adaptor that will function with most VOM's, a wise investment to prevent unnecessary purchases of NON-returnable electrical ignition parts.

A relatively cheap analog VOM is easy to watch the needle sweep rather than try to catch a rapidly changing reading on a digital meter.

CDI sells a combo DVA VOM, I haven't checked lately but years ago it was @ 60, I use it almost daily. Almost as handy as sliced bread.