1972 Mercury 402 firing good on one cylinder, intermittent on #2.

[RandyJ]

2 cylinder 40 hp, 1972
Serial Number 3406967

I brought this motor out of some 10 years of storage a couple of months ago. At first it ran super good after replacing the rectifier. I bought and installed a new wiring harness 84-822253T20 from motor to control box and all was well except that I had to rig up a kill button. With the kill button wired to the switch box it would not start

Got up one morning and tried to start it... wouldn't hit a lick. The day before it ran beautifully, like new or BETTER. I purchased an internal wiring harness#414-2770 and still nothing. It definitely needed replacing so no big loss, worth the expense. Apparently I will have to study and go over the switch wiring as the motor will not start at all if I hook up the kill wire to the switch box. The 84-822253T20 wiring harness is not an exact match to the original but should work with no problem.... wires had to be soldered to the key switch.

I found that the rectifier was toast so installed a new (used) rectifier. Now it is getting good strong fire on #1 cylinder. For a while I couldn't get any fire on #2. Today I tinkered with it and ran every test I could come up with going by the CDI Electronics/troubleshooting manual. Obviously something isn't doing what it is supposed to do. I'm just totally confused and somewhat at a loss. I can pretty well tell that the coils are good. I switched coils and seemed to get better fire on #1 and #2 but #2 is still weak or intermittent. I was able to put the boat in the water and get the motor running. At first it seemed to run fine, very easy starting, but obviously only running on one cylinder with an occasional hit on #2. Not sure but seems the stator is performing okay. I just can't positively eliminate the stator, trigger, or switchbox. Hopefully someone will help me with my confusion here. I ran various tests before I got it running.

Once I ran it a while I got different results. ... I'm assuming the tinkering I did with the motor cleaned up some corrosion on terminals/ground wires and changed the numbers a bit. I pulled the spark plugs to get the higher rpms with starter to do these tests.

tests results on wires to switch box...#338-4733
blue-white 0 ohms
red-blue 5540 ohms
brown-white 845 ohms

2nd tests on switch box... DVA
blue-white 85 volts... should be about 180+ volts
blue-red 265 volts.... should be @ 25+ volts
brown - white .5 volts... exact desired voltage

3rd test DVA
blue-white 85-125 volts
blue red 269 volts
brown-white 1.98 volts

4th test DVA
Blue-white Connected 66-108 volts Disconnected 28-33 volts
Red-Blue 187-213 524 volts
Brown-white 2.5 6.7


Green- neg. ground @ 190 volts both sides of Switch box.

I was not getting steady readings... the DVA readings were jumping around but I managed to get a range or steady enough reading I could compare to the numbers listed in the CDI manual.

My only conclusion is that there may be a wire with insulation cracked or rubbed through that is making contact with the engine....

 

[Bill kubiak]

When you say you switched coils, did you do that by installing different coils or did you just swap the 2 coils around to see if the problem follows the coil? Coils when they sit unused for years have a habit of leaking goo. I had issues in the past with old coils crapping out when they warmed up.

 

[gm280]

It is really hard to trouble shoot an engine not being there and testing things for yourself. And seeing readings of voltage is equally hard. But if you are getting hit and miss fire on the spark, you have to verify the input to the spark coil is the reason and not the coil itself. I would certainly remove every connection and clean to shiny metal first thing and repeat things to see what is still not working properly. Then if you still have verified intermittent at the input to the spark coil, then you have a problem that is in the ignition system that needs attention or replaced. But verify things are bad first before throwing parts at it. And sadly that old of an engine could be hard to find replacement parts... JMHO!

 

[RandyJ]

When you say you switched coils, did you do that by installing different coils or did you just swap the 2 coils around to see if the problem follows the coil? Coils when they sit unused for years have a habit of leaking goo. I had issues in the past with old coils crapping out when they warmed up.

swapped them around.

 

[RandyJ]

It is really hard to trouble shoot an engine not being there and testing things for yourself. And seeing readings of voltage is equally hard. But if you are getting hit and miss fire on the spark, you have to verify the input to the spark coil is the reason and not the coil itself. I would certainly remove every connection and clean to shiny metal first thing and repeat things to see what is still not working properly. Then if you still have verified intermittent at the input to the spark coil, then you have a problem that is in the ignition system that needs attention or replaced. But verify things are bad first before throwing parts at it. And sadly that old of an engine could be hard to find replacement parts... JMHO!

I definitely am not one to throw parts at anything. The more I search to find the cause of a problem the more I learn not to buy a lot of unnecessary and often times expensive parts... too, if you just replace an electronic part rather than fixing the problem you have a fair likelihood that the new part will be destroyed if the problem is not fixed... As for the parts being available, seems there is every part this motor could use easily obtainable via internet.

 

[Bill kubiak]

Lots of parts available for almost anything, about 7 or 8 years ago I rebuilt a 1960 something Merc 10 Super Hurricane with the automatic shift and I found every part I need for it, including some NOS pawls in the auto shift. Got it running good and sold it.