After a major missfire issue and subsequent rebuild of my bottom end (rings and big ends), I am nearing the magical 500 miles and looking to put some proper synthetic oil back in.

I still have a slight (lean fuelling issue) but a raise in system pressure has helped.

I am still finding with my narrowband AFR (wideband in the post) that when I decelerate or take my foot off the gas, the car pops and the AFR guage goes off the scale. Sometime it wont but big boost time it will.

Also from cold I get a slight missfire.

Now I have had , new plugs 0.8 gap, new leads, recon coil, new headgasket, new head bolts, cam belt etc. All timed up fine.

So confused I am or is it supposed to do that?