Poor running.

Need some advice on the following.

A while back the bike felt as though it was under powered and running on one cylinder, so I swapped out the spark plugs for Iridium ones, in the process the front lead / plug screwed off, which I didn't think much of as it screwed back on okay, buttoned everything up and started her up, she felt much better but still felt under powered.

Is there anything else like faulty HT leads etc or is the plug issue as described the issue?

I do think I have tightened up the plugs sufficiently....

How long since the last oil/filter change?

If you're considering plug change to rid yourself of minor problems, I'd also look at oil/oil filter/air filter.

Which is a point, as mine needs doing...

TA baph, last time I think was about august at the last service.