Mercury 850 no spark on one cylinder?

[Ricky_rich]

I’m having some problems with this motor. First the cables to the trigger was broken. So now I’ve replaced the trigger and put everything back together. But I have no spark on cylinder two?

Anyone have an idea how to find the fault? Motor is from 1977 from what I understand.

 

[racerone]

Some will argue.-----No money spent test for you.-----Charge the battery and load test the battery.----Take starter apart for inspection.

 

[Ricky_rich]

Some will argue.-----No money spent test for you.-----Charge the battery and load test the battery.----Take starter apart for inspection.

I have a good battery, and sparks on three cylinders but nothing on number two. Why should I take starter apart, it's spinning good?

 

[racerone]

Good luck with your project then.

 

[The Force power]

I have a good battery, and sparks on three cylinders but nothing on number two. Why should I take starter apart, it's spinning good?

That's so we can eliminate the Battery/starter (both need to be good to ensure the flywheel spins fast enough to produce enough electricity to produce spark.

Aside from that; have all plugs removed & grounded!
Crank the engine and check the voltage (AC) to the coil of #2
should be around 180 AC volts or more. if good it could be a bad coil (not likely)
if low/no voltage check the wire from the coil to switch-box & check power at that terminal on the switch-box.
if low/now voltage; it's either the switch-box (hope not) OR the Trigger-wires are not hooked up right (as you just replaced it)

I would check all wiring throughout the entire motor for broken/corroded/exposed wires (very common for that age)
Faulty wiring can cause misdiagnoses & send you on a wild goose chase

 

[Ricky_rich]

That's so we can eliminate the Battery/starter (both need to be good to ensure the flywheel spins fast enough to produce enough electricity to produce spark.

Aside from that; have all plugs removed & grounded!
Crank the engine and check the voltage (AC) to the coil of #2
should be around 180 AC volts or more. if good it could be a bad coil (not likely)
if low/no voltage check the wire from the coil to switch-box & check power at that terminal on the switch-box.
if low/now voltage; it's either the switch-box (hope not) OR the Trigger-wires are not hooked up right (as you just replaced it)

I would check all wiring throughout the entire motor for broken/corroded/exposed wires (very common for that age)
Faulty wiring can cause misdiagnoses & send you on a wild goose chase

I tried to measure the AC voltage on the terminals going from CDI to coil, but I can't get anything on my mulitimeter. I guess it's pulses of 180V, and maybe those can not be measured with a digital multimeter?
Anyway I have some parts from another motor so I did a swap of all coils and spark plug cables. And guess what, now I have sparks on all plugs. But when testing the sparks with a spark plug tester. I have strong sparks on 1, 2 and 3, about 3/8". But on number 4 it's barely the half compared to the other three?

 

[The Force power]

With the Engine running you should be able to get an AC reading (some may argue this) BUT a DVA -adapter is a must have to check stator out-puts @ cranking speed.
You said; swapped out all coils & cables and now #4 not good.
Take the coil that was previously on lets say # 1 (since that was good) and put it on #4
If no change swap a cable.

OR put the wire from #3 to Coil #4 to see if the problem follow to #3

 

[Ricky_rich]

When I tested the spark on cylinder 4 with a sparkplug I thought the spark looked the same as on other cylinders. So I gave it a try, and started up the engine. Got it running right away, and it run nice and even. So I think I'm happy for the moment. Might do a coil swap later just to see if I get a stronger spark.

A DVA -adapter sound like something worth buying.