[Mk2] [Turbo] Misfire when boost comes in

[hamstermr2]

Hi all, recently did head gasket change on my rev 2 turbo, it was an engine out job, when all back together and timed up it ran like a dream but as soon as I started to give the beans it was misfiring when the boost came in, large pops and bangs, I rechecked the timing and it was at 7 degrees before tdc, was this the caus of the misfire or would it not make that much difference, I've re timed it to 10 degrees befor tdc, but unfortunately I can't test it as the knock sensor packed up on the last test run before re timing , so until the new one arrives I'm stuffed, all I need to know is would the timing being that much out caus the misfire I'm getting , just to put my mind at rest....