Hoping someone can answer this question. Does a coil fail completely or does it gradually stop working. This is why i ask, on a recent tour my bike failed to start, i was about 3k into a tour. We pulled the sparkplug and the spark was weak, i tried my new spark plug and it was weak also. Someone suggested it might be an ebay fake, so another member of the group had a 10k plus old plug which we tried and got a better spark. we fitted this and it got me all the way home another 2k without mishap. Starting the winter stripdown now i am back home and looking for advice on best way to proceed. bike is a xt660z 09 my

Just to be clear..two of your plugs gave a weak spark,one old and one new.

A 10k 'old' plug gave a better spark and you had/have no further start/breakdown issues?...

Coils can fail completly or breakdown over time...

Was the failure on a cold/warm engine?..

Were the weather conditions dramatically different on the failure dates?..ie cold and potential battery /amps issue...

Hi Nikroc. Yes thats right, the bike was cold, ticked over for about 30 seconds then died. On pulling the plug the spark was weak, i tried a new plug that i carry and it was also weak and thats when i got the offer of a used plug from another member of the group. It was in Morocco and i had been riding for a couple of weeks with no problems. Today I have been working on the bike and fitted a new iridium plug from a Yamaha dealer. the spark looks fine. i fitted it and the bike started right away and ticked over fine. The used plug when I pulled it had a nice colour and still has a good spark. After fitting the plug I took the coil off and tested it with a multimeter and the primary resistance is 4.4 ohms the secondary was 18.1k which was taken with the cap in place. I have fitted it back on and bike seems to be running fine.

Problem solved!!