Multimeter to test magneto/coil

[Fletch]

I've done some searching and I'm a complete novice when it comes to anything electrical. I bought a multimeter but have never used one before (watched some youtube instructional videos).

My engine has been dying from what I thought was fuel starvation going up long monster hills. In the past after it dies I can start it up again. This last time nothing.

I'm not getting spark. Replaced plug, bypassed kill switch, tried new cdi, visually examined coil and magneto (no stray wires and not wet). So I'm to the point where I think it is probably the magneto from what I have read. There's a ton of threads on this, but I need the "for dummies" version!

To test the magneto or coil, what do I need the multimeter set on, what wires do I connect to the positive and negative diodes and where specifically? I read about checking resistance and it says" blue magneto to black magneto should read 200 ohms +/- 10%." I feel like an idiot asking this, but is that with the magneto turning, and is that with the diodes touching the blue and black wires on the outside of the engine case, or inside the case?

I really hate not knowing anything about electrical, and could use very specific instructions to make sure the magneto/coil is good/bad.

Thanks!

 

[shell shock]

hey, dont feel like an idiot. whether the engine is running or not should not really effect resistance. before you change anymore parts, scrap all the factory wiring. it is c**p. get a good automotive cable for your CDI. also, take your magneto to someone who knows how to solder, might just be a garbage solder job. it "COULD" also be the whole magneto assembly.. including the magnet. im not sure if this post will be of any assistance, but i hope it helps set you in the right direction

cheers
SS

 

[Fletch]

I fixed it. It was the megneto. I replaced it with one from an old engine I have and immediate spark. Back up and running (for now).

 

[shell shock]

alas, good to hear! pesky magneto!

 

[Fletch]

Yep! Haha I had my fingers crossed man. I cut the white wire off completely from the magneto because I have no use for it and it is just a potential problem if it touches anything metal. I rewired the blue wire with my terrible soldering skills. It's ugly but it works!

I have a question though. I don't know if you know the answer, but how exactly does a magneto go bad? It wasn't the magnet but how does a magnet go bad? I'm assuming it was the coil, but I don't know how the thing fails. Does it get overloaded? If your plug gap is too high does it have to "work harder" to get the spark across the gap? Can the coil and magnet be okay, but the metal part go bad somehow?

I guess that's more than one question Someone asked me how they go bad and I had no idea what to tell them.

 

[Deleted member 12676]

Sometimes people replace the magneto coil, thinking it was bad, but all the while it was just a bad ground connection. The new one works for a while and then "goes bad" also because the owner is not cleaning the metal that is used for a ground path.
But they can truly go bad by the very thin wire they are made of just burning thru where it was already too thin (maybe it was nicked, etc). They aren't high quality.
A magnet can lose magnetic force by being dropped.

 

[Fletch]

That's good to know, thanks. The area my ground wire was wrapped around was definitely not clean. Although I tested for spark with another cdi as well with only the blue and black (ground) wires attached. I'm assuming you mean the ground wire that you run between the blue wires and not the one screwed into the magneto.

Nice site BTW. Lots of good information. I have an 18mm carb coming tomorrow and just read your bit about not going to large on the carb. It comes with a 90 main jet, and I ordered an 85 and 80. Another member said he had luck on a 49cc HT with an 80 main. In another thread it looked like he had jumped up to 85. Are the jet numbers equal between different brand carbs?

 

[aaronwilde]

Sometimes people replace the magneto coil, thinking it was bad, but all the while it was just a bad ground connection. The new one works for a while and then "goes bad" also because the owner is not cleaning the metal that is used for a ground path.
But they can truly go bad by the very thin wire they are made of just burning thru where it was already too thin (maybe it was nicked, etc). They aren't high quality.
A magnet can lose magnetic force by being dropped.

I've had to replace my magneto coil 2 times in a month scince i got it. I thought that water got in and ruined it.. still haven't got an answer as to wether or not liquid can ruin a magneto coil. My 3rd magneto just stopped working now too. I was wondering what this "cleaning the metal that is used for a ground path" thing is about? What do you mean "bad ground"?

 

[Deleted member 12676]

Electrical current flows in a circuit loop which on motorcycles the "ground" part of it is the engine metal and often the frame. The current that flows to the spark plug goes from the ground side of the high voltage coil, thru the engine metal, and to the spark plug. That is one half of the loop. The other half is the hv coils secondary winding, the spark plug wire, and the center of the spark plug.
If the spark plug is not tight then it has a bad "ground" connection which creates resistance to electrical current flow. If the stator coils ground wire terminal is not screwed tight against cleaned metal then it may have a bad connection which introduces resistance or makes a completely open circuit which causes ignition system failure.

 

[aaronwilde]

thanks for clearing that up. makes perfect sense . i noticed on the head of my engine where the spark plug is screwed in is very black from oil. im going to go clean it right now and the washers and bottum screw in end of the spark plug is a bit dirty with oil too. would that affect the circuit loop?