what kind of car?

 

It's called dieseling... your timing is a little too advanced

 

Sorry for not telling about my car. 1973 240z with stock motor and duel weber DGVs

 

If your curb idle is set too high it can cause dieseling as well as what Ferrari said about timing being too advanced. On some of the later carbureted cars a faulty fuel cut solenoid will also cause dieseling.

 

Timing advanced or high idle or too rich. Or a combination of above. NK.

 

Time to decarbonize the head.

Some times carbon deposits ( caused by misadjusted mixture settings ) will become hot enough to maintain ignition of the fuel/air mixture acting like a glow plug, causing the pre-ignition or "dieseling" as Ferrari F has mentioned.

Run some Seafoam or the top-end cleaner of your choice through to clean all of the deposits off of the back of the vlvs and in the chambers, change your oil & plugs. Repost w/ results

My .02

Z,
Kevin