550dude

I have a 1982 Seca 550 that will not start. I am currently trying to pull the timing plate off because it appears to be damaged. But as I turn a wrench on the integrated nut it also turns the crank. I put the bike in gear thinking that would stop it from rotating...no it did not. Cn someone help me on how to take the timing plate off of a 1982 Seca 550? Thank you.