ok so i have a 86 rx7 and i just did a manual convertion everything works just fine when i put it in gear and try to redline it i lose alot of power once i get to 4000rpm's and from wat it sounds form the exhoust it back fires all the way to redline but theres power from 1-3500 rpm's and theres no back fire at all but the weird thing is when its out of gear the motor redlines just fine with no back fire
idk wat to look at first can anyone help me?

 

That's about when your secondary injectors and 5/6th ports open up, so i'd check into them first by making sure the actuators aren't stuck and the injectors are clean. I don't think the exhaust puts out enough pressure to open them up out of gear.