Backfire on deceleration

[Bob13]

Hello;

Took the new Roadster out for the first ride, sunny day in Tomahawk, Wisconsin. Finished the build over the winter. No problems except for backfire on the start of acceleration and lots of backfiring off throttle, deceleration. Here is the skinny:
302 H.O. stock cam and heads. Zero miles on engine.
Timing is 14 at idle (800) and advances to 38 (3000)
Timing order checked for a H.O.
Carb is a Holley 80457-10 600 CFM and vacuum secondaries

All cylinders are firing and it idles great. Could it be float level or power valve?

Any help would be appreciated. Thanks

[Jeff Kleiner]

Backfiring on closed throttle deceleration is almost always due to an exhaust leak, either at the header to head or header to sidepipe.

Jeff

[NAZ]

By all means, look for exhaust leaks as Jeff suggested. Exhaust leaks close to the head can result in pulling in air on overlap and leaning the mixture.

For a backfire to occur in the exhaust there has to be unburned fuel. The unburned fuel is ignited in the hot exhaust, typically in the header or collector. On short open exhaust, many times you'll see a flame exit the exhaust along with the backfire. Unlike EFI, a carb can provide unwanted rich or lean mixtures at various engine loading and this is something we have to tune out. If the mixture on decel is too rich or too lean to burn in the combustion chamber (intermittent missing) raw fuel will be pushed into the header on the exhaust stroke.

If you have the typical short vent tubes in that Holley carb, these can puke raw fuel into the carb on hard decel and cause a misfire and can even puke enough to kill the engine under hard braking. And the whistles used by Holley to prevent this are not 100% effective. If you suspect this, use a short piece of fuel hose to extend the vents higher into the air cleaner.

BTW, a fouled plug will cause the same backfiring in the exhaust but usually more pronounced under hard acceleration. It's a classic sign of a bad plug.

[BEAR-AvHistory]

+3 on the leak. Had a bad weld from FFR, they replaced the unit. On a first run most likely a loose header flange or header to side pipe joint. Start a cold engine & run your hand near the seams & joints.