['63GENIII]

The Christmas break / vacation gave me a few chances to work a couple of bugs out. Mainly an opportunity to adjust the valves which were a little noisy and to adjust the timing which kept throwing a "crank shaft sensor" code (PO1345). Id been reluctant to mess with the valves because it means removing the upper plenum, alternator and is just a big PITA in general. Ok so Im lazy at the moment.

So I got the upper plenum off, adjusted the valves, being very careful to get each valve on the base circle of the cam to get the initial resistance on the pushrod + 3/4 of a turn.

I then adjusted the timing which I think was supposed to be at 0 deg initial + or -- 2 deg (PCM takes care of all timing).

I put everything back together, fired it up and it ran great. Noticeably better than before. Less valve train noise for sure. Idle seemed much smoother as well. So I took it for a drive. After getting up to full operating temp for over 30-45 minutes, I notice a small but fairly consistent exhaust backfire in between the 1-2 shift as I let off the gas. Nothing between other gears or on hard deceleration. Just between the 1-2 shift medium throttle.

Now Ive searched and come up with a few possible causes which I'd be grateful for opinions on;

Ive read that it could be an exhaust leak letting air in and igniting any unburnt fuel. Only thing is, Im thinking it would have done this before the adjustments. Right?

Also there is the chance of timing (which I haven't checked with the scanner yet) being off. Again here though, the PCM is supposed to control timing.

Lastly (and the last thing I want to have to re do) is valve adjustment. Im hoping if it was this that the backfire would be more consistent instead of only between the 1-2 shift as in a backfire all of the time from a valve hanging open. The thought of pulling the upper plenum off again really... well its not fun.

The backfire is not really loud and I had to hear it a couple of times before I realized what it was. Not a loud POP but more of a muffled poof (if that describes it).

Any ideas? I think Im on the right track with the tuning so far. Its running better than the 396 ever did. Just a couple of bugs (which I think are fundamental) that need ironing out and think I need another set (or a few sets) of eyes.

Thanks again!