I just changed out my timing belt on my 1987 B2000. I would swear that I put it in right but after putting it all back together and starting it, it idles fast and won't drop down. Is it the timing belt? Is it off a tooth?
You can check the timing belt readily by taking the top timing cover off.
Go under the truck, use a mirror and wrench to turn engine so lower pulley is at TDC #1.
Of course, the pulley will already be on the gear, so will look like this (use a mirror), the woodruff key needs to be at the top (forgive picture being a tad off, I stuck the camera up to take the photo)
Then the camshaft mark can be confirmed to be at TDC #1
Questions: did you remove the distributor when you did the timing belt change? If so, did you check ignition timing again?
If that's OK, I suspect might not be timing belt related...