2005 MFS15B2 Tohatsu weak spark

[393Clevor]

I just picked this up guy said it would not rev.. checking the plugs they were oily and carbon'ed up a bit one has weak spark nothing on the other.. Been looking around the web and i decided to ask here.. where do I start? one coil and only one side of it sparks that seems like a good place to start..

Thanks

 

[pvanv]

Whip out your Factory service manual, and start diagnosing... not testing. The siamese coil fires both plugs simultaneously, so if you have fire on one, you should have it on both. Check the plug wires and the plug caps. You can reverse plug wires to see if you just have a crappy plug.

 

[393Clevor]

Whip out your Factory service manual, and start diagnosing... not testing. The siamese coil fires both plugs simultaneously, so if you have fire on one, you should have it on both. Check the plug wires and the plug caps. You can reverse plug wires to see if you just have a crappy plug.

wish I had a factory service manual...

 

[km1125]

You can definitely check resistance on the coil and see if either of the secondary leads has any resistance to ground. That would show a short to ground and cause one of the plugs to fire and the other one to not fire. If you see any resistance to ground, it's bad. However, that test does not exonerate the coil if there's no resistance to ground as the short may only occur at higher voltages. Get new plugs, connect them to the coil but don't put them in the block, just wrap a copper wire around both threads, connecting them together. Crank the motor and see if both spark or just one.

 

[393Clevor]

I always thought ya had to test things to diagnose things.. maybe I'm wrong

 

[393Clevor]

You can definitely check resistance on the coil and see if either of the secondary leads has any resistance to ground. That would show a short to ground and cause one of the plugs to fire and the other one to not fire. If you see any resistance to ground, it's bad. However, that test does not exonerate the coil if there's no resistance to ground as the short may only occur at higher voltages. Get new plugs, connect them to the coil but don't put them in the block, just wrap a copper wire around both threads, connecting them together. Crank the motor and see if both spark or just one.

Thanks man.. I've been doing this stuff for a long time .. first time I see one coil that fires both holes at the same time.. not a marine tech but ASE back in the 90's when you could work on cars.. lol I'll test the leads see what we got..

 

[393Clevor]

Both leads are open, no shorts.. whats weird is I get a light spark with a screwdriver but with a new plug (I tried several not the recommended plug but a new ones just the same) I get nothing..

 

[393Clevor]

is there any safety switches I need to check/bypass maybe..

 

[393Clevor]

No resistance at all when I test the leads that feed the coil.. wish I had a book..

 

[393Clevor]

Ok so I got spark out of both sides but I have to have both plugs grounded.. very weak but there is spark.. kinda like the old Subaru's I worked on years ago .. If you pulled one plug wire off to check for a dead hole you lost both cylinders on that side of the motor..

 

[km1125]

With those double-ended coils you should not remove one wire without grounding it. The spark is supposed to be between the two high-tension leads and the path of the current flow goes from one lead, to one spark plug, jumps the gap, then though the head over to the other spark plug, jumps that gap, then back to the other high tension lead. It never actually wants to go to "ground" unless there is a problem with the coil.

Running the engine (or even cranking it) and making the coil fire without both the high tension leads connected or grounded will cause the coil to want to fire though it's internal insulation which will cause damage.

 

[393Clevor]

With those double-ended coils you should not remove one wire without grounding it. The spark is supposed to be between the two high-tension leads and the path of the current flow goes from one lead, to one spark plug, jumps the gap, then though the head over to the other spark plug, jumps that gap, then back to the other high tension lead. It never actually wants to go to "ground" unless there is a problem with the coil.

Running the engine (or even cranking it) and making the coil fire without both the high tension leads connected or grounded will cause the coil to want to fire though it's internal insulation which will cause damage.

yup just like the old Subaru's.. I think it was the legacy that first started using them.. had a few of us at the dealership scratching our heads at first.. had one that wouldn't shift proper ended up being a plug wire.. lol true story..

 

[MattFL]

With those double-ended coils you should not remove one wire without grounding it. The spark is supposed to be between the two high-tension leads and the path of the current flow goes from one lead, to one spark plug, jumps the gap, then though the head over to the other spark plug, jumps that gap, then back to the other high tension lead. It never actually wants to go to "ground" unless there is a problem with the coil.

Running the engine (or even cranking it) and making the coil fire without both the high tension leads connected or grounded will cause the coil to want to fire though it's internal insulation which will cause damage.

Interesting! This also means both plugs are in-series and you should replace both plugs with new ones, if either one is bad it could cause a break or excess resistance in the circuit.

 

[pvanv]

With those double-ended coils you should not remove one wire without grounding it. The spark is supposed to be between the two high-tension leads and the path of the current flow goes from one lead, to one spark plug, jumps the gap, then though the head over to the other spark plug, jumps that gap, then back to the other high tension lead. It never actually wants to go to "ground" unless there is a problem with the coil.

Running the engine (or even cranking it) and making the coil fire without both the high tension leads connected or grounded will cause the coil to want to fire though it's internal insulation which will cause damage.

Poppycock. It's just a siamese coil. That's all.

 

[393Clevor]

it's a runner... and a smoker.. puffs a good blast of smoke on a restart after it's sat a while.. valve seals.. I might just rebuild it.. 250.00 for like everything, pistons, rings, seals, etc. wont know till I tear into it.. I might just run it like,it is.. time will tell..

Thanks fellas

 

[Sea Rider]

If the problem was totally solved, don't need to tear the motor down, a good decarbon procedure with those products sprayed through the carb could be good...

Happy Boating

 

[km1125]

Poppycock. It's just a siamese coil. That's all.

Not sure what you mean by that.

Also, there are two types of "Siamese" coils... one where they share a ground and has two high voltage leads which are independent, and the normal 'waste-spark' type which is ungrounded and has two high voltage leads. The second type uses the spark plugs in series and both fire every time.

If you don't have a spark plug or an appropriate path for the current flow, where do you think it goes? If the coil doesn't have a spark plug or short for the current to flow though the voltage will continue to rise much higher than normal and can compromise the insulation at some point. No material is a perfect insulator especially at small distances.

 

[pvanv]

All these 2-cylinder Tohatsus (both the old 2-strokes and the newer 4-strokes have a dual output siamese coil, and use wasted spark. One trigger, one ground, two HT leads.