Homelite Line Strimmer No HLT26CDY S/N No 0912003243?

When I pull the cord the engine fires backwards and jams the pawl spring mechanism..The ignition unit can be lifted to change the gap but the slotted holes do not allow side movement in order to retard the ignition and prevent spark plug firing before TDC. Is 25 thou gap between the flywheel and the ignition unit correct.How do I retard the ignition? The unit is a Mightylite strimmer bought from B & Q The ignition unit is a Fenglong B-27 No's 850108003 FL26E 200803A I really would appreciate a solution...