On my 1917 T Bucket I'm running a 302 Ford engine. When I'm driving everything is O.K. but when I let off the gas it backfires. Any ideas? The timing is at 8 before just as a stock Ford engine.
My first thought is that there is an exhaust leak. When you close the throttle, cold (cooler) air enters the system (negative pressure in the system. Superheated, it "exlodes". Others will correct my theory and also offer other ideas. But, most of the time when you get backfiring when the throttle closes, that's it.
On carbureted engines there's a gulp of extra fuel and air present on decel. That's one reason why modern fuel injection turns off injectors momentarily on decel. That's also why smog era air injection is shut off on decel. So you don't add extra air to an already explosive mix. My guess is flatheadguy is right or there's an external ignition source present like an exhaust valve that doesn't seal perfectly.
If it oops out exhaust you probably have a exhaust leak, out the carb could be valve hanging up or starting to burn.
my 49 ford with a chevy did that .carb screwed up had it rebuilt didnt backfire anymore. pissed me off .
a overly rich fuel mixture can cause it from retarded timing, causes a mixture that lights in the pipe from the next cylinder when it opens its exhaust valve . ford placed a dampner on there carbs to help slow down the throttle closing with the lower timing numbers ( gm did too on the q jets) My brothers POS O/t stang did this after he pulled all the pollution eq off it and slapped a holley on it , turned in more timing and it went away and the motor ran cooler too .
Thanks guys-- enough to get me started. I'm think the 600 carb is too much for a near stock 302 with just a .030 bore and Hi-Per heads and headers.
600 cfm is what they ran from the factory on alot of them . engine only takes as much air as it needs . the fuel mixture you have to fine tune ..
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