Spark Plugs gone bad?

Edward Buck

CB750 Enthusiast
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Great Malvern, Worcestershire
Hi, I don't know if anyone can help with this. I have rebuilt a CB750 K1 and it has new spark plugs NGK bought from a UK parts supplier.
The bike has run ok after the rebuild but only in the garage. Took it for a short test ride and it died on me very quickly. I suspected fuel starvation.
Did all the usual checks etc and spent a whole day trying to figure out what the issue was.
Finally as the spark wasn't great at the spark plugs, I fished out the old plugs that came with the bike. Tried them first against the cylinders and found a beautiful bright spark.
I fitted all four of the old plugs and the bike started first time and ran well????
I have rebuilt a few engines and never heard of a spark plug (4 of them) all going bad at the same time). When I had a closer look the spark seemed to be jumping not from the electrode to the bent bit (earth I suppose) on top but rather from the electrode to the side wall (threaded part) of the plug. It wasn't a great spark either.
Anyone ever had this happen??