Mabye timing issue

[carfanatic89]

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.