ignition issues

brown · Apr 26, 2016

I have a customer bike that is really stumping me.Want to share the issues to see if anyone else is having the same.

I am on my 3rd CDI unit with a customer bike. Twice it have checked for spark and it has been good. However it would not start until I changed the cdi unit.

The last episode started like this. I checked the coil with the ohm meter . It was open. No spark. So I Replaced coil and tried to start motor. GREAT SPARK!

Howerver the motor will not state. Messed with the carb......no issues ......No start either. Checked for good compression. All good. Changed the CDI. Bam.....motor running fine.

Why did I have to change the CDI unit when I had good spark at the plug already without changing the cdi?

exokinetic · Apr 26, 2016

The CDI was working well enough to provide a visible spark outside the engine, at atmospheric pressure.

I am assuming you were holding the sparkplug against the head and visually checking for a spark?

Well when you install that bad boy into the head, and the piston compresses everything up in there, the increased pressure required more voltage to create the same spark.

Its a basic law of electricity, as ambient pressure increases (say inside a combustion chamber right before the spark event) the voltage required to generate a spark of a given distance (lets say 0.032" of spark plug gap?) also increases, exponentially.

So, yeah, it would totally generate a spark outside the engine, but shove it in that combustion chamber with "Checked for good compression. All good." and its TOO much compression for the "failing" CDI to handle, and, no more spark.

I will have you know I have also gone through this EXACT same headache with the Magneto, so don't leave it out of your troubleshooting process, lol.

crassius · Apr 26, 2016

we had CDIs that were going bad after a few minutes of running a year or so ago

you may have one of those

also at that time, many motors were coming out with the 480ohm magneto - I noticed that two vendors switched back to the 350ohm mags and CDIs stopped dying, but not sure if CDIs were also changed

somewhere on the board here, I show how to make a simple timing mark for these too

2door · Apr 26, 2016

Are you relying on the kit spark plug boot? I've seen them work then not work simply by moving the high voltage lead. The way they connect to the wire and that silly spring thing that sits on the spark plug is a joke that has probably been responsible for more 'engine won't start' complaints than any other one thing.

Tom

Venice Motor Bikes · Apr 27, 2016

brown said:
Why did I have to change the CDI unit when I had good spark at the plug already without changing the cdi?

These Chinese parts can can have all sorts of weird manufacturing problems!!
I've had this exact problem before; I think something inside the box that controls the timing of the spark shorted out. :/

It's good that you were able to figure it out!

I've also seen where the magnet was installed backwards from the factory & it had a spark but wouldn't fire... (That one took a little time to solve).

brown · Apr 28, 2016

2door said:
Are you relying on the kit spark plug boot? I've seen them work then not work simply by moving the high voltage lead. The way they connect to the wire and that silly spring thing that sits on the spark plug is a joke that has probably been responsible for more 'engine won't start' complaints than any other one thing.

Tom

No. I never ever use the kit boot. I have always have, up till today always used a NGK 5k ohm resistor spark plug ends.

Today I ordered a dozen non resistor NGK spark pug ends and plan on switching all my bikes over.

I also don't use the wire that comes with the kits. I always use a vintage braided type copper core wire or Belden stainless steel wire.

I decided to go with the non resistance NGK boots to see if they lower the coil ans cdi failure I have been having.

I wonder it the 5K ohms spark plug ends are too much resistance for this ignition system to handle?????

brown · Apr 28, 2016

Venice Motor Bikes said:
These Chinese parts can can have all sorts of weird manufacturing problems!!
I've had this exact problem before; I think something inside the box that controls the timing of the spark shorted out. :/

It's good that you were able to figure it out!

I've also seen where the magnet was installed backwards from the factory & it had a spark but wouldn't fire... (That one took a little time to solve).

I agree Norm. I have found out that sometimes it is best just to change the whole system. I was prepared to change the armature next just to make sure it wasn't the problem.

crassius · Apr 28, 2016

I prefer the zero boots - old 250ohm mags won't fire thru 5k at cranking speed

brown · Apr 28, 2016

crassius said:
I prefer the zero boots - old 250ohm mags won't fire thru 5k at cranking speed

Thanks for the information. It would solve some of my recent issues if indeed the 5K ohm boots are the problem because it is a very easy fix to solve.

I never thought about the 5K ohm resistance boots where a problem. I have been using them for several years. Well at least I think I have been. I haven't checked some of my older bikes yet. They may turn out to be non resistor ends.

2door · Apr 29, 2016

I've always questioned why some builders utilize any ignition parts that add resistance to the stock system. Non-metallic high voltage leads, resistor plugs or boots don't/can't add anything to the engine's performance. The stock ignition system by virtue of their design are already weak, comparatively. Adding resistance only reduces the voltage to the plug.

As for the kit supplied spark plug wire, even it is better than what you'll get from most auto parts stores today. They sell resistance wire almost exclusively and only a few still carry metallic conductor wire in bulk. Motorcycle or small engine repair shops still use it on a limited basis. So called 'high performance' ignition wire used with racing or high voltage ignition systems is marketed as low resistance wire but even it is higher in resistance than the kit wire which is a metallic core/conductor.

If you have access to a good ohm meter you can see the difference easily. Compare the resistance values between a resistance wire and one of equal length with a metal conductor. Same for a plug or boot.

Resistance means just that. It reduces the ability to conduct. And that can't help our feeble little ignition systems.

High priced, (overpriced) Iridium or other 'high performance spark plugs are also a waste. They are designed and made for high mileage and do not add anything to a simple single cylinder 2 cycle engine. That is unless you plan to ride your bike for a hundred thousand miles before replacing the plug.

Tom

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Venice Motor Bikes

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