intermittent spark

[catch2otwo]

So i have this 1979 yamaha qt50 scooter. It ran ok up until last week where it died. no spark. Replaced the plug, replaced with generic coil, and still no spark. tonight i threw in a used cdi box and bam spark, but pretty weak. Also after working on it for a while, i find out its intermittent spark. Its getting fuel, and air. compression is great.

Before i go buying a new coil, this time a yamaha one, would this be the part im looking to fix? if there is something else i should look at i would like to know. ive checked the stator and there is good power coming out of it going into and out of the cdi. battery is fully charged, kick start only.

maybe the cheapo 15 dollar generic coil was a bad idea?

[Kurlon]

Could be the pickup?

Have you cleaned all the grounds?

[Eric Baker]

My first thought is the ignition pickup not sending a consistent signal to the CDI.

[catch2otwo]

cleaned all the grounds, by ignition pickup, that would be the wire coming out of the motor from the stator? I had it hooked to a multimeter and it seemed pretty consistent.

[FTMS]

Pick up coil can either be a single wire or 2 wires depending on type. It is general test with a resistance reading.

[Eric Baker]

Usually the opposite side of the motor from the stator.

[Cerberus]

With intermittent spark you'll get intermittent test readings. Which is to say that you cannot trust your reading now to indicate what's going on when it's pitching a fit.

Weak or intermittent spark can be caused by a few things, but I'd start at the points/pickup coil/air gap/ground.

Hate to say it, but when plugged in the other cdi box and it fired up, that was the most pertinent diagnostic indication. Meaning you likely need a new cdi, but while you are there, a proper coil that the cdi is designed to power is a good idea.

[Cerberus]

Hang on a sec.. An friggin 1979 bike was running electronic ignition?
I would have bet money that was a points system....

[Stoneman]

It's simply the sparkplug wire. Change that out, fill up the cap with dielectric grease, pop it on the sparkplug and you're good. Cheap, fixed, good...

[catch2otwo]

Hang on a sec.. An friggin 1979 bike was running electronic ignition?
I would have bet money that was a points system....

no points here, its a cdi system, and while we're at it, shaft drive, and single sided swing arm!