hi there, i have a 50cc chinese quad, 4 stroke,2007, i am having real problems with it. every time i replace the cdi, it starts first time, runs great, when you leave it to cool down, it wont start, until i replace the cdi again, i have disconnected the alarm/immobiliser, all the rear kill switches, the front brake kill switch.ignition barrel, i have replaced the coil, every time i put a new cdi in it, it will go great. any ideas would b very welcome.

 

This would be a very unusual failure mode. Are you sure the CDIs are bad? Pull out the spark plug, reconnect it to the coil, and hold the threads up against the engine while cranking the starter motor (and all kill switches set to the "run" position). Do you get spark? It would really be diagnostic if you could do this with a good CDI, and also with a bad CDI.

I'm wondering if it is just coincidence that the quad starts with a new CDI, and the the problem lies elsewhere. If you still get spark with a bad CDI on a cold engine then that would bolster the coincidence idea. CDIs don't get that hot overall, and they certainly don't know whether the engine is cold or hot. If the CDI runs cool by nature, and it has no way of knowing how hot (or cool) the engine is, then what is the mechanism behind the failure? It makes no sense at the moment.

The most common cause of hard starting when cold is valves that are too tight, or choke/fuel issues.

How many CDIs have you gone through? Where did these CDIs come from? Are they all from the same source? Have you measured the inputs to the CDI (AC ignition power, trigger pulse, and kill switch) on a failed CDI? [I'm just gathering as much info as I can so I can think about it...].

I'm confused by the term "front brake kill switch". There is no such thing. I'm also confused by your statement about removing the "ignition barrel". If you remove the ignition switch (barrel) the quad cannot possibly run, nor will the starter motor even turn. Yet you imply that it made no difference . Again, I'm just gathering information and looking for clues...

 

It is possible to get CDI that will do that, I have had a few. My advice is to buy them from a different place. The ones that were less than good had a brown color connector. I went through 5 or 6 of them. Then I bought from someone else on ebay and got some that were green in color. I put one on and haven't had a problem since.

With everything you are talking about and the terms you used, I don't know if this will answer your question, However LynnEdwards can answer them, IF you can describe what you have done in words that we can understand.

 

just took the spark plug out and held it against the engine, all kill switches to run,tryed to start the engine, good spark with the new cdi, changed over cdi to a old one, there was no spark. i have had 4 new cdi, green ends. bought from diffrent places.i was having this problem before so i sent the quad up to edinburgh imports, who rewired the start button,to a heavy isolater switch, he replaced the cdi and got it going great, but as i brought it back down the road, the quad wouldnt start again. i bought a new cdi and it starts straight away.the front hyd brake lever has kill switch on it so u cant start the bike without the brake lever pulled in. hope this helps. ur advice is much appreciated.

 

This is a really strange problem. I can't think of anything on the quad that would wreck a good CDI, but at the same time allow it to run for a few minutes. All your symptoms point to a string of bad CDIs.

Armagh had a very similar story, as he already said, where he had several CDIs fail within minutes. But they were from the same source, and probably the same batch. I can see several scenarios where CDIs will work for a few minutes and then fail. Here are a few:

1) Inadaquate heatsinking of the rectifier diodes, SCR transistor, and a couple high wattage resistors in the CDI design.

2) Using capacitors (like the main storage capacitor which is the "C" in "CDI") which insufficient voltage rating, or insufficient ripple current rating.

3) Using the wrong epoxy potting compound, where differential thermal expansion coefficients between the epoxy, printed circuit board, and the components mounted on the circuit board jack each other apart when going through normal temperature excurisons.

I'm pretty sure that CDIs don't get run for any length of time when their sale price is just $9.00. I bet they just are plugged in, tested for spark for 2 seconds, and then shipped. It is also possible that there is one big source out there in China that has made a huge batch (or multiple batches) of defective units which haved filled warehouses across the globe. Eventually the source will get the word and clean their act up, but in the meantime everyone will be in denial. The distributors don't know, and the vendors don't know. They have a vested interest in not knowing, plus they deal mostly with customers who don't know either.

I'd keep trying CDIs from different sources till you find a good one.

The front brake switch should keep the starter from turning unless the brake is applied. This is a safety interlock very similar to your car where you can't turn the starter motor unless your transmission is in park or neutral. It is not a kill switch. Kill switches prevent spark from occuring. Kill switches do not prevent the starter from turning.