Mabye timing issue
 

I have a 87 N/A w/ a manual trans. When I speed up fast and shift at about 4000 RPM when i let off the gas and push the clutch in, my car backfires. It only does it at this at 4000 RPM and up. The car runs fine all the time and idles without missing or anything at all. The car use to never backfire and now its gotten worse over time. What do yall thinks the problem, Im thinking timing.

First thing where is Texas you at? Second I would check the timing it may be the distributor bolt is loose and moving when you rev it. Best way to check this is to grab the top of it and try to turn it if it turns even just a slight bit then try tightening it. Another thing to check is the cap and rotor if there is a crack and a carbon trail then it will back fire when it is spinning fast. This is unlikely but you never know. Also one other thing what kind of exhaust do you have is it aftermarket or the factory one, if aftermarket then there is the possibilty that there isnt enough backpressure and it is gonna do that.

Well it might be the exhaust I have factory everthing except block off plates on the EGR cooler. Both mufflers are old and on the second one back on the car next to the gas tank was really rusted and had alot of holes in it so I instead of listening to the exhaust leaking I cut off the muffler. So it has one muffler and about 1 1/2 feet of pipe. That might be the cause of it popping.

And I live about an hour from Dallas.

Since its happening at 4000 rpm and up it could have something to do with the ACIS system. The little flapper valve on the intake manifold that opens when you hit that rpm. You could take the vacuum line off of it so it doesn't work and hit the rpm's again and see if you still get the backfires. Also you can see if its even working by watching it as you run the throttle up manually. Ive never heard of it causing a backfire problem but its worth a look.
Check the dashpot system too by blocking off the little vacuum line, revving up past 2500 rpm and letting go of the throttle and see where the dashpot stops the rpm. It should be around 2000 rpm. It should also if working properly
show a slight hesitation for the throttle as it comes down. You can check that with the engine off. If your shifting quickly enough though the dashpot system wouldn't even come into play at high rpms.

Neither of these components is causing the backfire problem.

And....if the timing is set properly, it will not cause the backfire either, as it is completely controlled by the computer.