85 3/4ton M code block. So in an effort to help pass an IM tomorrow I did a few things to the truck. Plugs (delco r44t) air cleaner, PCV valve, oil change. the usual ****. Idling @950, timing @4* Hooked my smog pump back up..had to change the diverter valve. When the diverter valve took a crap earlier this year it backfired bad so I just pulled the vacuum line from it and it stopped backfiring. Well, now its backfiring again. AIR pump is still hooked up and it does it with/without the diverter valve vacuum line hooked up. I noticed with the air pump hooked back up I get much more of a rumble tone in the exhaust. Possibly the choke sticking? I gave been having some issues with the choke. Possible carbon buildup? Old plugs didnt look bad, actually looked slightly lean..wondering if Ive got a lean backfire however, I set the mixture with a vacuum gauge.