So I've been looking on the internet trying to figure out a problem I've been having but haven't figured anything out.

First off I just built a 357, with a 488/510 lift cam, and a Holley 750.
I just got exhaust put on, and ever since, it has been backfiring through the exhaust only when I shut it off. I turned up the fuel on the carb and it got a little better, bit it still stumbles out the exhaust some when I shut it off. We've adjusted the timing several different times and it does not affect it. The idle is set at 1,000 rpm. Is it running to rich or to lean? Or could it be something else?

 

I'm guessing you have a 327... not a "357"....

3 things contribute to backfiring.... 1)Timing is off possibly by 1 or 2 teeth.. with the distributor... (to far advanced/retarded...) and or firing order is off by 1 terminal... But generally.. when it backfires out of the exhaust the timing is in a retarded setting... If it was to far advanced... it would backfire thru the carb.

2) The idle is still to high... which is also caused by the timing... "Low idle system" on the carb is inoperable due to improper timing..

and or improper choke settings (Runing to rich)...
Depending on the climate you live in....

Should be 1 setting for winter start up and a different setting for summer ..
... also without actually being there in person to make sure everything is in the correct postion/timing/carb settings....
These are just a few suggestions.
I'm sure someone else will chime in with more suggestions/ideas..

 

For starters, that 750 cfm carb is way too big for that engine. That engine will never have that type of flow. Adjusting the screws only effects the idle circuit. It would not effect the performance of the engine once you're off the idle circuit. Raw fuel being dumped into a hot exhaust as the engine is being shut off will definitely cause an explosion in the exhaust system. A warped flange can also cause it. Unfortunately, I'm leaning more towards a burnt exhaust valve or a bad valve seat.

 

X2

 

Sorry it has taken so long to get back I've been unbelievably busy. However after adjusting the carburetor it was definitely runninng to rich and I've fixed the backfire when it turns off. I bought the carb with intentions of upgrading the motor in the near future but with a new mortgage it looks like that's going to be out of the budget for a while. Could I turn down the flow on the 750 as a cheap fix? Or would I have to get a 600 cfm carburetor or rejet the 750?

And no it's a 350 bored .040 over not a 327. Nothing is stock in it except the crank.

 

just drop the jet size down about 4 points if the carb was new. and reduce the power valve a few steps. a quad is 750 cfms. the primary jets and power valve. determine how rich or lean the carb runs and if a eng. backfires one time the powervalve is junk.

 

Ok so stock is 73s so I think I'm gonna drop it to 70s. What would I drop the powervalve to? I've never really messed with carburetors before

 

check how much your eng. is pulling in vac. in gear with a vac. gauge and go down 2 numbers stamped on the side of the power valve. the cam determans the power valve numbers. but to low the number the richer the idle and part throttle richness. i allways run a oxh senser and gauge to know how rich my fue mix is. if there is a power valve in the rear bowle plug it and go up 3 jet num. to be on safe side. you're supost to go up 6 but i never go that high because i'm never at full throttle that much.run 14 deg. timing with dis. vac. unhooked if running hose and they rec. running dis. vac. all the time to keep the eng. and exh. cooler. i don't do this because my car stumbles when pulling off the line because your dis. loses vac. at that point and the timing retards causing the eng. to stumble for a sec.