I have a '67 Mustang w/ 289 - originaly a 2bbl/Auto now a 4bbl, 4bbl heads, headers and pertronix electronic ignition module (no more points). Ran great for a long time but now backfires at idle and during decceleration. I've replace the carb, coil and checked timing and carb adjustments - still backfires out the exhaust (sounds like my mufflers are gonna blow out!). It's a beutiful convertible and i'd like to get it running good again - any ideas that might help figure it out?
PS: anybody know what this car might be worth?
Could be a vacuum leak. Check all the hoses (luckily, you don't have many). If they're all good, get a can of WD40, and with the engine running, spray short bursts of WD40 around the intake manifold gasket and the base of the carb. If there is a leak, the engine will rev up a few notches when the WD hits the leaky area.
This is a problem with your fuel/air mix. This is defiantly a carb problem. It won't richen the mix on deceleration and is too lean at idle. Super clean the carb. Some ports are probably gummed up.
As for what it's worth, put it on eBay and see what it's offered.
Was this originally a calif car? If it has an air injection system (air pump) the problem is the anti-backfire valve also sometimes called a gulp valve. It needs to be replaced or the vacuum line going to the valve is leaking or off.