No spark under compression....Revisited

[JDT]

Well it has reared it's ugly head again.
The mysteriuos "No Spark Under Compression" gremlin.

You would think that as many times as I have run into this I would have learned it's secret but it still baffles me every time.

So I had this hot little one lunger that I did a rebuild on after I rode it two months ago.
It ran like it had a bad seal and it also had a very slight noise when you rolled it over.
So after I fixed everything else I torn the engine down to nothing and found yes, a bad seal but also found a roasted bearing that I posted a picture of last month.

So with all new bearings, seals and gaskets I reinstalled the little 246 Japanesse motor back in the red and white chassis.
Compression tested at 130 as before the tear down and the brand new spark plug I put in it had excellent snappy spark while sitting on the head.
I did not re-ring this project as the cylinder bore looked like new and the engine had a beautiful port and polish job on it.
Only 1600 miles on this 1974 GP-F.

The fun begin when I went to start it for the first time. It popped on the second pull and fired on the third but only ran for about 10 seconds than died.
I pulled and pulled and all it would do is putter and backfire out both the carb and the exhaust.
So I grab the origianl spark plug and threaded it in. After yanking and cusssing for 20 minutes I gave up for the night.
Next day same crap. Pop, sputter, backfire but no positive signs of life.
I put the new plug back in.

All I can say is Yamaha makes one sturdy recoil because I worked the bejesus out of that thing for an hour.
Everytime I checked the spark it was great. I checked it in head with an external spark indcicator and all looked like it was a runner but no bark.
I stewed over everything for an hour. Rechecked timing, gaps, carb, Pulled the flywheel in pure desporation.

So I grab a different used plug off the bench and put that in. One pull and it was alive.
I thought I was dreaming so I shut it off and tried it again. One pull and it was running. Lights working and all.

So what was happening was a brand new spark plug was not firing under compression and neither was the used plug that powered the sled last month.
A brand new plug? Seen it before and even though I hope I wont I bet I see it again.

Why??

So on to the test drive. This little and I mean little 246cc single ROCKS!!
Very fast.
How fast? Somewhere out there on the trail is a guy still trying to figure out what is wrong with his Z440.
I have no idea who did the hop up on this tiny engine but is far above it's original 22 HP.

[nick80lf]

Todd,

You're not alone. I've been puzzled by this one before myself. I'd love to understand why this happens.

Nick