Weak Spark on 2,4,6 cylinders

[ejthomp]

This is a 1986 Mercury 90 hp Inline 6 cylinder. It starts and idles fine, but misses at higher rpms and under load. Spark test revealed weak spark on numbers 2, 4, & 6. First thought was CDI so we swapped the two but the weak spark did not change to 1, 3, 5 so we've ruled out a bad CDI.

Compression is 125,126 across all cylinders and I just can't imagine having three bad coils all at once.

This leaves what? Trigger?

I'm an experienced DIYer, but not 100% familiar with this motor or how this system works. I just bought this motor....(with this known problem).

Thanks for your ideas.

 

[j_martin]

Re: Weak Spark on 2,4,6 cylinders

Common parts for those 3 cylinders are switchbox and stator coil.

Try swapping the solid blue and red stator wires with the striped ones. Should move the bug to the other 3. If so, stator is bad.

The stator has two tapped charge coils in it. One goes to each switchbox, which selects the tap it wants depending on speed.

The trigger has 3 coils in it. Each fires a cylinder opposite in the timing sequence, so a bad trigger coil usually kills 2 cylinders at a time.

Each switchbox has 3 circuits in it. Each stores up power from the charge coils, fires the plug it's connect to when the trigger it's connected to says to, modified by the bias circuit.

The jumper between them is the bias circuit. It's jumpered to even it out. Sometimes we remove that jumper so if a bias circuit problem is the culprit, it'll be isolated by not sharing the bug.

hope it helps
john

 

[ejthomp]

Re: Weak Spark on 2,4,6 cylinders

Thank you so much! I've been reading other posts and leaning toward the stator too. I will go out and switch those wires to confirm. Better wait a little bit though (6:30am).... don't think the neighbors want to hear this quite yet!

 

[Faztbullet]

Re: Weak Spark on 2,4,6 cylinders

Sign of weak stator voltage.. simple ignition system. Here it is in basic's:
Flywheel magnets spin around stator and inside trigger,voltage created by stator sent to pack and stored in capacitor, trigger magnets create voltage(timed) and sent to SCR in pack, the SCR dumps capacitor to coil. This is a very very simple version.

 

[ejthomp]

Re: Weak Spark on 2,4,6 cylinders

Replaced the stator today.... problem solved..... baby screams now!

It was a very simple swap....aside from having to buy the flywheel puller....and then the 1-1/4" box wrench....but everyone needs more tools, right?

My total cost of repair..about $200..$150 for part and $50 for tools. The boat seller gave me a $300 allowance on the purchase price so looks like I came out okay.

Thank you all for the guidance.

This is an older motor, but sure seems strong. It has 700 hours on it. Just had carbs rebuilt, new water pump and new spark plugs installed. Should be all ready for fishing season.

Since this is a new motor to me and you all are the experts, is there anything else you think I should do to the motor now or things I should look for/expect?

Thanks again.

 

[emckelvy]

Re: Weak Spark on 2,4,6 cylinders

You might want to do a Link 'n Synch to follow the stator replacment:

http://forums.iboats.com/showthread.php?t=168855

And the ol' gal would probably benefit from a decarbonization with some Seafoam, as well:

http://forums.iboats.com/showthread.php?t=158076

BTW you probably saved $500 or more by doing the job yourself. Congrats!

The other thing that you can do to keep your Tower of Power happy is to make sure it's propped to achieve WOT (Wide Open Throttle) rpm in the mfr's recommended range. 5300-5500 with a light load is just about ideal.

This will ensure the engine is not "lugged" by trying to drive a prop with too much pitch (death to an Inline), and when you're cruising at reduced boat speed, engine rpm will be in the "sweet spot" for best speed vs. fuel efficiency.

Cheers.......ed