Point ignition wiring help

**Skunk** · 25th January 2009, 21:43

Thanks Speedpro. I still have your ramp too. hmmmnz came round today and diagnosed several things a very short time.
• the points were broken
• the coil works
• the plug cap was a resistor one which wasn't helping
He's given me links the all sorts of things and been a great help.
Thanks heaps hmmmnz.

**hmmmnz** · 25th January 2009, 22:11

i aim to please

love the buckets, they were awesome, good luck for next weekend,
gimme a yell anytime if you ever need a hand with anything

**speedpro** · 26th January 2009, 15:15

Unless you have a great BIG FAT ignition system that throws serious current down the HT lead a resistor plug cap is not going to make any difference. E=IxR so the voltage drop can be calculated. If you have a system that delivers say 20mA to the plug and you have a 5Kohm plug cap the voltage drop will be 100V. The coil may put out 10,000V so the 100V drop is only 1%. If that 1% makes a difference . . . . . .

**hmmmnz** · 26th January 2009, 18:25

that is indeed true, but resistive plug caps have a tendancy to blow the resistor,
1 less thing to worry about, the better i say

**speedpro** · 26th January 2009, 19:03

All the resistor plug caps I have dismantled have had a little carbon rod inside. The only problem I have ever seen is that for whatever reason the little carbon rod ceases to be a snug fit inside the cap with the result that when tested with a multimeter with the usual very low voltage on the ohm scale it shows an open circuit. If you were to poke 10,000V into it the arc would just jump over the tiny little gap which has the effect of making the plug gap .002" bigger. It's a commonly made mistake measuring a plug cap with a multimeter and tossing it because it reads open circuit.

**craisin** · 27th January 2009, 23:48

well done crew

**jonbuoy** · 28th January 2009, 04:39

I thought the idea of resistor plugs was to give a longer spark duration?

**Skunk** · 28th January 2009, 08:11

Originally Posted by jonbuoy

I thought the idea of resistor plugs was to give a longer spark duration?

No, they just stop radio wave interference.

**Skunk** · 28th January 2009, 20:54

Well, got everything together but have a random spark. Could be (as Speedpro says) the CDI 6 volt coil.

Originally Posted by Speedpro

The correct coil will have "AC" on it, for AC magneto. A coil for a points or transistor assisted system will have "DC" on it and a cdi coil will have "CDI" on it. They are not interchangeable, for best results, ideally.

Hmmm, don't have those marking on any of the coils... Anyone know what an RD400 has?

**speedpro** · 28th January 2009, 21:39

I'm not 100% sure but I think if it isn't marked it'll be DC by default. It'll have slightly different reluctance to an AC coil but should be good enough to run the motor. I haven't tried that so can't say for sure.

**Ixion** · 28th January 2009, 21:49

A CDI coil is unlikely to work on a magneto system. The CDI puts out several hundred volts to the coil, whereas the magneto puts out somewhere between about 6 to 30v

A DC coil (old fashioned car) should work OKish.

EDIT. When you say "random sparlk" do you mean that you have a spark on every revolution, but randomly as regards timing (ie sometimes at TDC sometimes at BDC and in between): or do you mean that there is a spark but not on every revolution ?

**Skunk** · 28th January 2009, 21:51

I might try old of the RD400 coils then. I don't think I have any car coils around (and it won't be 6 volt anyway).

Random as in not every revolution. Seems the faster the revs the less likely the spark.

**speedpro** · 29th January 2009, 18:22

Get a decent ignition you tight sod.

**Skunk** · 29th January 2009, 18:45

Originally Posted by speedpro

Get a decent ignition you tight sod.

Thanks for the offer of a KX80 ignition for free, Speedpro. I was having trouble finding an affordable one.

I have a TF ignition lined up for it but first I want it running with the standard stuff (before I bring variables into it).