My '86 B2000 has began to backfire. It has done this a few months ago after I put premium(93 octane) gas in it, this happened once. But recently, I took the EGR valve off and rid it of a bunch of junk build up. After putting it back on, I adjusted the timing(not due to playing with the EGR valve but rather having access to one). Before adjusting the timing, it ran fine at operating temperature, but now backfiring terribly when deceleratingand I am worried I will blow some gaskets or something. We timed it by the TDC 6 degree mark on the right side of the block when facing the front of the truck. Though this mark was debatable. On my brother's 91 B2200, the 6 degree TDC was marked directly over top the pulley, but on mine its more to the right of the pulley. This was the only mark visible. From other posts, it sounds like timing issues and vacuum lines are predominant reasons for backfiring. My vacuum lines are fine and amwondering if if was the timing. Anybody have any idea of the 6 degree TDC mark on this engine. If it's where I've described, we timed it correct and what other things could cause this backfiring?

 

if your valves are not closing propely from build up then it will dump raw gas into the exust.this is one thing you can do to fix that problem.this will sound a bit silly but trust me it works.start your engine let it idle till its warm then have somone press the gas peddle until its at about 3000 rpms then take a water hose with a spray nossel and lightly spray water into your carb.dont let the engine reach the stalling point just enough till it bogs down.this will blow all the carbon out of the engine.repeat until no carbon comes out the tail pipe.if that doesnt work the only thing left is timing.