Vacuum/ Backfire

Whats a good vacuum for a AMC 360 with heads/cam/intake .030over, Im getting 15-17psi at idle with my truck avenger 670cfm, Im more so then normal when I let off in third gear with my T176. Any suggestions?

Vacuum will go up anytime you lift off the throttle and
will go pretty much to zero any time you open the throttle.
Idle vacuum is greatly affected by the cam and the overlap.
Whatever idle vacuum is, gauge should be solid and steady, no bouncing.
At idle 15-17 don't sound too bad.
Stock motors are like 18-21 I think.

Sounds like good vacuum for a mild cam. My CJ only pulls about 12" at idle.

Backfire out the carb can be a lean condition. If it's popping out the exhaust on decel, could be a little rich (raw fuel down the exhaust), or the diverter valve could be bad if you still have the AIR system (smog pump).

Bob
tufcj

Wait my text must of cut off at the end, what i was trying to say is that im backfiring out the exhaust when i let off the gas at like 3000rpm in third gear.

usually that comes from a lean condition or... an exhaust leak, Sucks cold air in when you let off and bang!

That's a very high vacuum condition then when it backfires.
Yeah an exhaust leak will do it. Make sure everythings tight
Carb, Intake, exhaust, has it got PVC ? Baffle ? Working ?
Or to much total advance,
once the vacuum advance all comes in.
Try it with the vac advance disconnected from the carb
and put a cap on that port on the carb.
Vacuum advance is added on top of initial and mechanical.

Got firing order right, right ?

-1-8-4-3-6-5-7-2-

I am having a similar problem on my Edelbrock EFI'd 360... it backfires sometimes when I let go off the gas at 2500 - up RPMs...

Is it because the mixture is too lean or too rich? I see conflicting answers on this thread.

To whatever extent the mixture is responsible for a backfire through the exhaust.. almost always a lean condition..
If you think about it a backfire is an uncontrolled firing of one of the cylinders at the wrong time..(I.E. whenthe exhaust valve is open..)

Holley is sending me a bigger accelerator pump nozzle, the stock one is like .28 and it needs to be atleast .37. Thats this bog hesitation problem im having, I think i might have an exhaust leak which is causing the backfire

to clear it up:
backfire on decel, manual trans/clutch not petal not pushed in or auto; big pop rich, small pop exhaust leak

spark knock/denetnation under throttle, lite load w/ good secondary ignition; lean

backfire thru carb; timing, too big accel pump, way rich

bog off idle; to small accel pump, vac leak, efi; clogged fuel filter, tps, injector timing off w/tps

but hey what do i know.