YFZ450 spark issue need ideas

[driftquadsquad163]

I just picked up my buddy's YFZ450, which had been sitting around for three years after it quit on him mid-ride. My son and I have tackled a couple of rebuilds and done some electrical fixes before, so we thought, why not give this one a shot?

We checked the timing and valves, cleaned the carb, and put in a fresh battery. It actually fired up for a few seconds, which felt like a victory, but then it just died. Now, when I hit the starter, it only sparks once. And it's a weird thing, if I toggle the kill switch off and back on, I get that one spark again.

I've even tried swapping in a good plug, but no luck. The plug tip feels a bit loose in the boot, too, which is kind of concerning.

Has anyone run into this? I'd really appreciate any advice before I start just throwing parts at it. Feeling a bit stuck here

 

[onionring89]

I've even tried swapping in a good plug, but no luck. The plug tip feels a bit loose in the boot, too, which is kind of concerning.

really check all your main ground points from the battery, engine, and frame. Corrosion can hide. A bad ground can give you just enough juice to spark once, but not maintain it

 

[brenty2444]

That single spark after toggling the kill switch could point to a grounding or ignition issue, I think. Have you checked voltage at the coil when cranking? Also, that loose plug boot might be worth replacing just to rule it out.

 

[Toph]

Your CDI box is probably toast. That single spark when cycling the kill switch is classic failed CDI behavior, not just wiring. Swapping ignition components one by one could waste time, start with replacing the CDI

 

[Thelittlefriend]

You might as well check the plug boot. If it's loose it may not give a proper spark. With that said, it probably won't spark once and fail to work after that. Maybe check the kill switch in case it's related to having or not having a spark?