Guys, I sometimes get a small pop/backfire when shifting. I running 4.56 gears and the odd time when shifting from 1st to 2nd or 2nd to 3rd at 3000/3500 rpm or so, right when I'm letting off the gas it happens.
It is running fine otherwise, just wondering could this be timing or carb related or?

You didn't say backfire through carb so I'm assuming you're referring to exhaust.

Letting up on gas indicates that your dumping raw gas into the exhaust and it's igniting in the pipes. This can happen if the vacuum advance does not retard fast enough when you let up on the throttle. Attaching a quart sized canister "T'd" into the distributer vacuum line can quicken the retard action - This worked for me after I blew a few mufflers apart. Leaks around the header flange does not help the situation either as it super heats and ignites any combustible gases in the exhaust when you close the throttle fast.

I used a vacuum reserve canister from under the dash of a 74 Volvo used to power the heater control servos - fits nicely behind the front wheel well of a Camaro.

If its vaccum advance, thats your problem...

Thanks guys, I am running vac advance and HEI both fairly new.

Vac advance in and of itself is not an issue.

Dean, do you know the specs for the timing? I'm running both with 10* vacuum advance with no issues, and 16* initial and 36* total at 3000 rpm mechanical.

Rick, I'm running 16 initial with 20* mech advance for total of 36*. At idle though my vac can adds ~20*, I was looking at getting the Crane unit so I can limit it some.

I have the new crane dizzy that has the dials to adjust curves. I think 20 is too much myself but that's easy to evaluate! Or shift faster so there's no rpm drop :).

Yeah I guess shifting faster would help and be fun at the same time!
I was talking about the Crane adjustable vacuum can to bring it down to ~15*. This way cruise vacuum would be around ~51*.
The headers are coming off anyway so I'll replace the gaskets to eliminate any leaks there.