Engine's Performance with Spark Plugs Not Torqued

My experience of loose spark plugs is that it often results in a misfire. If they seal sufficiently then the engine will run normally. If plugs are really loose then I have seen melted plug electrodes due to the lean mixture and increased combustion temperature.

I have seen people do this in the past and didn’t have an issue other than plug leakage.  Change those plugs, make sure you have your thermal paste on all plug threads, do a compression test just to rule out any issue and then run the engine. Hopefully you should be fine since you said it was only two hours.

Thank you all for the replies.

Just to add on, we would also like to know that if there will be a difference in idle speed between (i) all eight fully torqued compared to (ii) two bottom spark plugs found hand tightened. i.e. if there will be a difference in idle speed between spark plugs being hand tightened and torqued.

 Also, is it possible for an engine which is tuned to around 2500 rpm idle sped at engine test cell run to experience rpm drop to 1600 rpm when installed at aircraft? If so, what can be the possible causes.