the lhs should have taken that engine back 4 months ago if that is how long you have had it.normally most lhs dont warrenty engines but they should have gave you another one within a week or so of your purchase.if i cant get an engine to fire after doing all the things these fellas suggested its back to the place where i bought it.i have never had an engine not fire that didnt have serious mechanical problems.i have purchased ringed motors that the ring was left off of or was broken.i own a lot of st engines and havent had any problems with any of them.i run 10% nitre in my st engines nothing higher and they do just fine.i guess you could but 20% seems a little high.