Hello and 156JTS tickover advice needed
I have a well liked 156 JTS which has run very nicely for 72K but now the engine runs lumpily at tickover (sounds over rich) and has a flat spot at around 3500RPM yet cruises fine at 70-80mph, quiet and smooth as usual. Power is down a bit but fuel consumption is as good as ever with 420-440 miles on a tank
I've had the cam belt changed and a lamba sensor replaced which was causing a motor control system failure message. Now there are no messages and three computer checks reveal no faults. I have substituted another MAF sensor -no difference. Plugs were changed at 60K.Throttle body cleaned by local mechanic -unchanged.
Could this be a connector problem? Where should I look next? Should the plugs be changed? Are there sensors which malfunction without giving error messages? I need a systematic plan of things to try and need to pick the brains of those who know this rather nice engine well.
Oh, and this my opening post, so Hello