I have an lq9 in a 72 chevelle it has been swapped for a while but as of late it has started backfiring and hesitating at low rpm’s when I try to give it gas but if I get it past 1700 rpm it smooths out

 

Is it intake back fire, or exhaust after fire?
What kind of fuel delivery? (carb, TBI, multiport)

 

It is multi port running a factory computer

 

And it backfires though the exhaust

 

That means fuel is burning in the exhaust

Running too lean can slow the burn rate enough that the fuel is still burning when exhaust valve opens. A manifold vacuum leak could be responsible for that at low RPM and then get masked as the engine speeds up.

There are other reasons it can happen but that's the one I can think of that can change over time.

 

Ok I’ve replaced the intake gaskets and checked the vacuum lines I am at a loss help please

 

A bad injector could cause your problem or a bad coil. Take a temp gun while at idle and check each primary exhaust tube. The one that is real low in temp is your bad cylinder. I had a similar problem and it ended up being a coil. Later i had a problem and it ended up being the injector.

 

Don't know if you have a IAC valve in your throttle body or not, but had one go bad ( stuck in one position) on a 2000 ls1 one time and it had the exact symptoms your describing. Had a pitter patter miss at low end like through the exhaust and lightly through the intake got much better with some RPM...... If you have a spare try swapping it and see if it helps, happy wrenching sir and good luck hunting that down.