Merc 402 - running rough after installing new CDI stator

[cambo]

Had some intermittent problems last year while boating. I could be out fishing for hours perhaps going wide open, then all of a sudden the motor would cut to half power. It would continue like this for a few mins then go right back up to full power. Due to the intermittent nature of the problem, I guessed it was ignition related.

I hadn't done it ever, so I rebuilt the fuel pump and carburetor. But the motor was still very difficult to get started. It did finally start after installing new plugs. It seemed to run fine, but after stopping it, I couldn't get it to start again. Checked the spark and it seemed a little weak.

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

Started testing the ignition system.
.#####################################
Compression
CYL1: 120PSI
CYL2: 120PSI

Ignition
Trigger Resistance:
White to Brown = 830 Ohms

Stator Resistance (Merc OEM)
(LS) Blue/Ground = 5600 Ohms
(HS) Red/Ground = 60 Ohms

CDI DVA - Stator and Switchbox output to coils
(LS) Blue/Ground = 114.4V
(HS) Red/Ground = 35V
Switchbox to COIL #1 = 80V
Switchbox to COIL #2 = 82V
.#####################################

DVA reading from Stator LS (Blue/Ground) should have been 180+ Volts. I took the flywheel off to inspect the stator. Found there was a crack and something bubbled out of it. Likely overheated.


I installed a new stator CDI 174-5255. When I turned it over, it started but was running really rough. For a short while it would run ok then back to rough (rpm seems to affect how rough it was - lower was so so, mid range really bad, high rpm it seemed not too bad).
Took some new readings with the new stator.

Trigger Resistance:
White to Brown = 830 Ohms

Stator Resistance (CDI 174-5255)
(LS) Blue/Ground = 2840 Ohms
(HS) Red/Ground = 52.3 Ohms

CDI DVA - Stator and output to coils
(LS) Blue/Ground = 177 to 181 V (this dropped to 120 when it was running really bad)
(HS) Red/Ground = 135 to 142 V
Yellow 1(Rectifier lead 1) = 0.34V
Yellow 2(Rectifier lead 2) = 28V

Switchbox to COIL #1 = 146V (dropped to 108-112 when really rough)
Switchbox to COIL #2 = 145V (dropped to 108-112 when really rough)

Trigger CDI DVA
Brown/Ground = 1.7V
White/Ground = 1.7V

Coil Resistance (Sierra 18-5186)
COIL #1 - Primary = 0.3 Ohms, Secondary = 594 Ohms
COIL #2 - Primary = 0.3 Ohms, Secondary = 246.6 Ohms



I'm at a loss as to what might be going wrong. Voltages are low according to the documentation. When my old stator was working (notes from July 2013), it put out 307V & 207V - output from the switchbox to coils was 208V & 198V.

- disconnected the rectifier - no difference
- swapped out switchbox - no difference


Video of motor running.
https://www.youtube.com/watch?v=b2TEDej4iZg


Could this new CDI stator be bad? Flywheel? Output to coils is supposed to be at least 180Volts. Anyone know what the resistance specs should be on those Sierra 18-5186 coils?


Thanks,

Cambo

 

[cambo]

No Title

Problem was a hairline crack in the flywheel trigger magnet causing the misses. Also found bad rectifier, hidden cracked insulation on engine harness (tachometer and rectifier leads), and a bad ignition switch.

 

[Texasmark]

Thoughtful of you to answer your question as you did. We learn from others. Somewhere down the line someone will benefit from your taking the time to share this experience. You do know that anything you post on here is www visible and anyone posing a question on a search engine like Google, for example, will have access to it if asking a similar question.

 

[cambo]

I've got a lot of great information from these forums over the years. Figured maybe someone will run across this one day and it might save them a little time. This was a real head scratcher. I had inspected the flywheel magnets and looked at the trigger magnet, but didn't notice the crack until looking really close.

 

[Texasmark]

I've got a lot of great information from these forums over the years. Figured maybe someone will run across this one day and it might save them a little time. This was a real head scratcher. I had inspected the flywheel magnets and looked at the trigger magnet, but didn't notice the crack until looking really close.

Cracks are hard to spot. That's why engine blocks need a "Magnaflux" treatment when one rebuilds one to help in preventing call backs, even if you are working on your own engine. I know a guy that overhauled a Ford Power stroke and didn't flux. I was watching him when he did the initial run up after the o'haul and could not believe that he had to find another block and tear it all down and build it back up. Sad day.

Mark

 

[RandyJ]

Thanks for posting your solution to the problem. I've got a 1972 Merc 402 doing similar things... ran great when I brought it out of the cob webs. Now it starts easy but weak fire on one cylinder.... similar DVA readings. Not sure which component is the problem or if it is stator, switchbox, or trigger.... I'll post a thread about it to see what kind of answers I get.