Topic: spark at points nuttin at plug coils???

[nteek754]

Hey all  got a 75 750 good spark from points but nothing at the plug  put new plug in notta whats my next move coils??? connections are all intact 

[bryanj]

you shouldnt have a spark at the contacts so perhaps your condersers are duff. Try removing the plug caps and looking for spark at the lead end. It would be unusual for both coils to go down at the same time

[thomas evensen]

I have a small tiny spark on my points (CB 350 F). Is this bad? I have changed both the
points and condencers. The point clearance is also checked: between 0.3 and 0.4 mm.

greetings,
Thomas Evensen

[Einyodeler]

Small tiny spark good , big blinding spark bad

[Raul CB750K1]

are you grounding the spark body? It should touch the bike metal, otherwise there would be no spark. A small spark in the points is normal and nothing to worry about, even when condensers (capacitors) are good.

If you are having spark at the points, it means that the primary winding is getting energized, so you have 99.9 % chances that the secondary winding is getting juice. Just make sure the spark touches metal, like the engine head. Try not to touch the spark with the bare hand. Hold the spark plug by the rubber hood. Don't put it in front of the plug hole, or chances are that the gas coming into the cylinder will ignite.

Raul

[Warlock]

Greets,

 If the condensors are OK and there is still a large amount of arcing at the points, this would indicate excessive current passing through the contacts. If the coil primary windings are shorted or if the secondary windings are open (or high resistance), this could cause an overcurrent condition at the points with a resulting no spark condition. Check the resistance of the primary and secondary coil windings. Also check the resistor caps on the spark plugs. A cap with excessive resistance will allow secondary field voltage to bleed back into the primary winding since it would offer less resistance to current flow. When the secondary winding field collapses, the high voltage generated needs to go somewhere. It will find the path of least resistance....

[TwoTired]

Both the output leads of each coil must be connected as two plugs fire at the same time. If you are leaving one open to free air, you'll get no spark from the other connected one.

[Raul CB750K1]

Both the output leads of each coil must be connected as two plugs fire at the same time. If you are leaving one open to free air, you'll get no spark from the other connected one.

I can't agree with that. It's true that two plugs spark at the same time. But they do it in parallel, so if one is not grounded the other one should still give spark.

There is a simple way to test it. With the engine running in idle, remove a plug cap from cyl. 1 and another from cyl 3. That way the engine should run in two cylinders, but if your theory is true there should be no spark in any of the spark plugs.


Raul

[TwoTired]

Be sure to report the findings of your test, Raul.
 It will certainly be a blow to my understanding of physics and electricity, if you're right.

The SOHC4 coil output leads connect to a wire wound 15000 times around an iron core inside the coil.  There can be no current flow in this wire until a complete circuit is formed.  This takes place when the voltage in the coil rises to the point where it is sufficient to jump the spark gap.  The ioinized gas in the spark gap completes the circuit and the energy is drained from the coil.  If one plug is shorted (such as Carbon Fouled) the other may still spark.  However, if the gap between the coil output leads is too great, there will not be sufficient voltage for the arc and subsequent completion of the circuit.

Cheers,
 

[oldbiker]

Raul, for once I disagree with you. The two plugs are in series. The secondary of the coil is floating with the two ends connected to the two plugs and the circuit completed via the metal of the motor.

[bryanj]

Dont know why or how and cant explain it but you are all right! taking one plug cap off the other will fire but they are on opposite ends of the windings and here is one for you Back in the70's Suzuki used to recommend swapping the plugs over at intermediate service as one sparked from the centre to ground and the other sparked ground to centre and when there is a spark there is a tiny amount of metal transfer (hence the gap grows) I have no idea wether this idea is true or not and i defy anybody to say they can see which way the spark is going!

[Raul CB750K1]

You probably are right. If the secondary winding have to leads, let's call them A and B. Say that the voltage generated in it is 20.000 volts and that lead B is connected to ground. Then, the voltage in lead A - voltage in lead B is 20.000 volts (VAB=20.000)

Say that we put lead B connected to ground. Then, Va=20.000 volts. If we connect the A lead to the spark plug, we should have 20.000 volts in the inner of the plug above the spark plug body, so there would be spark. Both plugs should be connected to lead A.


Now if we connect lead A to one of the plugs and lead B to the other, if we measure between ground and lead A we will see +10.000 volts, and measuring between ground and lead B we will see -10.000 volts. That way, Va-Vb= +10.000 - (-10.000)=20.000 volts.

In one of the plugs the current flows from the live inner electrode to the plug body, and opposite in the other spark plug. That could be the reason why they recommended to swap the plug caps.


In that case, it would be true (and if you say so it is) that if you remove one of the plugs the other will not spark.

Regards

Raul