An airbox has 2 functions...
1. Somewhere to put a filter
2. It creates an area of neutral air. Means the engine draws an equal amount through each carby and also even pressure (no turbulence) within the throats of those carbs.
Engines will usually run the best when the mixture is lean, but not quite lean enough to cause overheating issues. Sounds like, by removing a restriction to the airbox, you have created gearter airflow, therefore leaning out the mixture the engine gets. If you have no heat issues, then leave it as it now is.
Do you realise how many holes there could be if people would just take the time to take the dirt out of them?
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