no spark

Just got my first Moped: a 1990 Tomos A3 and it's a basket case. Among its many problems is no spark. I've already checked the obvious stuff but the wiring on this bike is a mess from the previous owner(s).

Question: Is there a way to bypass the wiring harness just so I can try and start the motor?

Re: no spark

What color wires are coming out of the stator

Re: no spark

Hi Stephen,

I'll take a look tonight, but assuming the wiring has not been altered, the manual shows black yellow and blue (see attached diagram).

-yellow wire goes to the horn and then on to the combined LH horn/headlight switch.

-black wire goes to coil and then on to RH on/off engine start switch

-blue wire splits into to grey wires: one goes to tail light, the other to LH stop switch (is that a kill switch?)

Thanks, Jim

Screen Shot 2019-03-21 at 5.33.03 PM.png

Re: no spark

If that is correct ground the blue wire to the frame and run the black wire straight to the coil and it should fire if the points are working.

Re: no spark

Okay, the plot thickens: I followed Stephen's advice (thank you) and

- if I ground the blue wire to the frame and run the black wire straight to the coil I get no spark

- if ground the blue wire to the frame and run the black via an alligator clip directly to the spark plug bypassing the coil I get spark.

So does that mean the points are okay and the coil is bad? I find that suspicious because the coil is new, installed by the previous owner. Here is a photo of the coil with the black wire connected directly from the magneto. Is it possible that another wire needs to be connected to the coil?

coil.jpg

Re: no spark

cant tell by picture, but if the coil has two terminals then ground one side, black to other. if not make sure that the coil grounds are grounded good those shiny metal bracket spots.

Re: no spark

I removed the coil to clean the surfaces and make sure the bolts are clean given how that's how the coil is grounded. While I was at it I also measured the coil's resistance with a DMM and got the following readings:

-where the black wire connects and to ground (mounting holes) with meter set at 200Ohms (lowest setting) = 0.32

- at the tip of the spark plug wire and to ground with meter set to 20kOhms = 7.410

Curiously, if I remove the spark plug wire and measure at the threaded stud that inserts into the spark plug wire I get 2.145 with meter set at 20kOhms. Does that mean the wire offer a significant part of the resistance?

The Tomos manual does not give measurements for the coil. Wondering if anyone can tell me if these readings appear normal? Thanks!

Re: no spark

♣Slew Foot♣ /

That is an aftermarket ht coil.

Oem has a ground circuit. If you take the main ground junction and ground the base of the coil to it it should work. The ground was bypassing the coil and grounding to the frame before it could fire.

Re: no spark

Many thanks Slew Foot. So the blue wire (ground) directly from the magneto should be grounded to the base of the coil before being grounded to the frame. Is that right?

In the original schematic the blue wire goes to the tail light before returning to the coil.

Re: no spark

So... I grounded the base of the coil with a wire connected to the blue wiren = no spark

I then connected the blue wire directly to the bass of the coil = no spark

Any ideas?

Re: no spark

and another question (sorry): how can I have spark w/o the coil? If I connect the black wire coming out of the magneto to the bare spark plug and ground it, I get a visible spark when I kick start the engine.

Re: no spark

dont know if itll make a difference but get a non resistor cap. ngk 8030 or 8031 from oriellys. clean n gap points again.

Re: no spark

> jim magera Wrote:

> -------------------------------------------------------

> So... I grounded the base of the coil with a wire connected to the blue

> wiren = no spark

>

> I then connected the blue wire directly to the bass of the coil = no

> spark

>

> Any ideas?

Sounds like a bad/wrong coil

Re: no spark

♣Slew Foot♣ /

Sounds like an old Point Style A3 it has a c e v v flywheel you need an old-style high tension coil you need an old style condenser that is new start from there if you have an old Bosch HT coil it's good it will work they hardly ever go bad unless you can see a crack in it and it'll only be bad when it rains most part they're Bulletproof now you might have blown a coil and they're cheap to replace to test it you remove the coil from the circuit when it's isolated you take a battery and run the current through the coil it should magnetize enough to pick up a pen then you know you have a good coil the ignition coil you have to test differently you'll need a meter a probe a 9 volt battery A Match Pack and a little bit of reading in common sense

Re: no spark

Well guys, you were right - it was the wrong coil. Just installed a new coil and I have spark. Thanks for all the great advice.

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