Changed my spark plugs today (9400 miles) left one look normal. The right one was white. After the install got a lot of backfiring and missing a lot. Turned it off. Waited a few minutes and started it back up still missing and backfiring some. Repeat process no backfiring some missing. Decided to take it for a short run (about 20 miles with 3 stops) Backfired once. No missing I could detect and ran just fine. I doubled check plug wires. Everything seemed fine. Any ideas to the backfiring and and the missing right after install?