Merc 402 - Runs on 1 Cylinder Until Warm

I have a Merc 402 that seems to be running on only 1 cylinder until it warms up. This has happened the last 3 times out on the water.
Motor starts good and seems to run ok at low speed (it sounds kinda rough), but once you try to throttle down you only have half power (if that). Once every few minutes, you will hear the second cylinder fire (rpm picks up) and the power will come back momentarily; only to stop working seconds after. This repeats for perhaps the first 10 to 15 minutes, then it seems the second cylinder kicks in and you're pretty well good for the rest of the day. While running the motor continuously for the day, there have been two instances where you lose power while it's warm and running full throttle. This generally only last for maybe 30 seconds and doesn't come back.

All the connections are tight. Due to the intermittent nature of this problem, I'm thinking it has something to do with the ignition system. I'm not certain which cylinder isn't working - by the time I pulled the cover to pull the plug wire it started working again.

Model: Mercury 402 (2cyl, EL - Electric Start, Long-Shaft)
Serial Number: 7102322
Year: 1977

Compression: 120PSI for both Cyl #1&2
Spark: Getting spark from both plugs
Fuel: Seems good as I can run all day after warm up without trouble.

Resistance Tests
Trigger (white to brown): 819 Ohms
Stator (red to blue): 5100 Ohms
Coil #1: .2 Ohms
Coil #2: .2 Ohms

DVA Tests - Motor running on earmuffs (CDI PN: 511-9773 /w Fluke 85)
Trigger (White to GND) = .8V (changes from .6 to .8)
Trigger (Brown to GND) = 13V
Trigger (White to Brown) = 15V
***Not sure if I'm testing the trigger correctly?

Stator (Blue to GND) = 307V (drops to 245 after a few mins)
Stator (Red to GND) = 207V (drops to 183 after a few mins)
Powerpack out to Coil #1 = 208V
Powerpack out to Coil #2 = 198V


Any ideas what might be causing this? Anyone see a switchbox do anything like this?


Thanks,

Cambo

Re: Merc 402 - Runs on 1 Cylinder Until Warm

I had a thermal problem with a later model CDM module. Acted just like yours.....wouldn't fire cold and then when it warmed up it would. Your engine I think came out before that type of ignition. But maybe not. Since you said Coil 1 and 2, if you have 2 coils, or ignition modules with the output a high voltage wire that goes to a spark plug one very well may be bad, just like mine. Input voltage being ok is fine but you need to know if the output voltage is there. A battery operated timing light with the sensor just close to the spark plug lead will give you your answer. So no blink, no spark, no fire, dead duck. They are on sale here for about $100 for my engine.

Mark

Re: Merc 402 - Runs on 1 Cylinder Until Warm

I used to own a 402 and has a similar problem, it turned out to be a coil, Do a resistance reading before and after its warmed up.

Re: Merc 402 - Runs on 1 Cylinder Until Warm

Check your sparkwire end and cap for corrosion , also check wire-connections from switchbox to coil for corrosion.

Re: Merc 402 - Runs on 1 Cylinder Until Warm

Thanks for the feedback.

I just retested the coil resistance and I think I may have found the issue. I didn't originally test the resistance of the secondary winding on the coils (rookie mistake).

Coil #1: Primary = 0.2 Ohms, Secondary = 590 Ohms (this is a newer Sierra 18-5186 - about 4 years old)
Coil #2: Primary = 0.2 Ohms, Secondary = 2344 Ohms (this is original part)

The DVA adapter came with some specs that indicate that resistance should be 800-1100 Ohms. I'm not certain if 590 Ohms is typical for the new Sierra coils? Any chance there is a degraded connection issue that gets better when the coil warms/expands slightly (causing weak spark)?

I've replaced the coil with a spare (measures 1023 Ohms on the secondary winding). I get spark at both plugs. Motor fires up fine on the muffs, but I'll have to take her down to the lake to really test it.

Re: Merc 402 - Runs on 1 Cylinder Until Warm

I used to have a book on that motor but I gave it to the guy I sold the boat to, but when I got the motor I had to add a new wiring harness, CDI box and 2 new coils just to get it running. But after that I ran it for 10 plus years. Kept it till I sold the boat off and switched to a pontoon.

Re: Merc 402 - Runs on 1 Cylinder Until Warm

Sounds good to me. So the question is, how does the coil come up with more than twice the resistance? Only thing I can think of is termination corrosion where the coil wire is connected to one of the terminals. If the wire were to be the problem you would more than likely have an open circuit...broken wire. Usually it would be a short circuit or lower resistance reading due to overheating or vibration knocking the varnish off a coil wire that has aged.

Might take Iceman's advice and have a look in your caps for corrosion and another place you can have it is where the wire and the cap metal part make connection. Got to slide the boot off to see that.

Mark