How stiff should my valve springs be on my ltr450?

I was curious if my valve springs are not tight enough i have a 06 ltr 450 and i had it apart adjusting my valve clearance cause i can not get it to run right. And i noticed i can push them down with my thumb i don't know how they should be but i thought that they would be really tight is this normal? I had it apart because i have had a issue with it for the past year now and the issue is when i have it idleing and if i give it all its got it wiill just start cutting out and backfireing sound like it would be out of time but its not its perfectly in time. I can just give it a little bit of throttle and it will rev up but once i get the throttle to a certain spot (right around half way) it starts to cut out a little and if i just keep going it will just kill the engine. I can not figure it out i actually think i give up on it i have probably 30 hours trying to figure it out going back and checking everything to make sure i did it right and all that and i still can not get it. But if anyone knows anything about any of my problems that would be greatly appreciated i am all ears right now cause i am clueless. An another thing is i bought a new tps sensor thinking that was it and i even set it to where it is supposed to be .58-.62 volts off the b/br wire and yellow wire and still don't work actually it won't even really run if it is set like that it will start and idle for about 3 seconds and then just die in order to get it to idle i have to put the tps sensor way up around .90 volts or so. That don't make sense to me so again if anyone knows what could be going on that would be awesome thanks for all your help in advance.