intermittant spark on a qt50 cdi? coil?

Joseph Lisi Iii /

I had gotten my yamaha qt 50 to run for about 30 minuits and it died and hasn't started since. The spark is now intermittant sometimes strong sometimes weak, sometimes nonexistant. I have removed the fly wheel and cleaned everything to no avail. Nothing looks bad in there, but could the cdi unit or coil be bad without looking bad. If so where can I get replacements for my 6v 1981 qt50.

Re: intermittant spark on a qt50 cdi? coil?

There's no practical way i know of to test a CDI unit .. But the chance of a coil (or a CDI unit) dying right when you are watching is probably one in a million. I'd suspect something else caused the spark to become weak all of a sudden.

Check and double check all the simple stuff. Replace whatever you can with parts known to be good... spark plug and spark plug wire .. even the CDI if you can find a spare.

Check all the wires and switches on the bike related to ignition.. kill switch or ignition switch .. Look for corroded or loose connections or broken or pinched insulation. Some bike ignition circuits even include things like a tail light bulb.

If it was OK 30 minutes ago and you still get a spark of some kind, i'd estimate the coil and CDI are basically ok.. but something changed and now spark energy is leaking away.

Re: intermittant spark on a qt50 cdi? coil?

FIRST! REPLACE YOUR PLUG! it will save you lots of time and frusteration! good luch. BTW: i have a qt50.

Re: intermittant spark on a qt50 cdi? coil?

Bill Switzer /

I had the same problem with one of my 80 QT50's. Turned out to be a bad ignition switch. In fact, I'm still looking for a 2 position switch for it if anyone has one. Mine still works but, sometimes I have to turn the key off/on, off/on, quickly, a few times before it will start. I suspect the contacts in the switch are worn.

Re: intermittant spark on a qt50 cdi? coil?

Same thing happened on my QT. It ran the first day that I got it and then the spark was intermittent. I tried everything and eventually traced it to a fault in the charge coil under the flywheel (its the white one). I got the replacement coil ($30) and the flywheel puller ($20) from

My flywheel was on so tight though that the puller actually pulled through the threads on the flywheel. So I went down to NAPA and borrowed one of their 5 ton pullers. It took a bit of doing to get it to fit, but once it did, that and my impact wrench ($20 @ Walmart) took it right off. I also used it to take the original flywheel bolt off.

Anyway... I'd replace the coil. Thats what everyone told me at the time. One thing mine did... if you let it sit for about a week, you could get a slightly more regular spark for about 2 minutes. It was never enough to maintain combustion... but you could check for that.

Hope that helps!


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