I own a honda prelude 2.2i vtec but when i engage the vtec it backfires but under normal drivin it's superb

i've recently had a diagnostics all fine did not show up anything my emissions are bang on the car has only done 83k only 1000k sine i've owned it oil is topped up with 5w-30 and i us 93 octane fuel but i belive i may have a small hole in exhaust would this affect it (money to tight at the mo for honda repairs) plz hlp car not so nice to drive when have to avoid engaging vtec