backfire?

[jsaylor]

I have a 1973 340 4sp. Whenever I reach about 4000rpm It backfires.
Things I have already done:

tried two carbs "thermaquads"
changed the distributor
tried several different timing locations
plugs
wires
dist cap
rotor
HELP!!! thanks so much for any suggestions

[rumblefish360]

Ignition box or lean/vaccum leak that apears at that rpm and higher due to vaccum sucking in a gasket or enuff air from a leak allready there.
Coil?

[dewme5]

is backfiring all it does? any hesitation right before. any stumbling anywhere else.

if all else fails, and always the best place to start, is the mechanical state of the engine. Do a leak down test. I'm a little suspicious of a bent intake valve.

air/fuel/spark = fire Problem is, you are getting fire in your intake. Which means it's coming from inside the combustion chamber. So... either your spark is coming in to soon (which youve checked everything except the ignition box it sounds like) or, it's slipping out of the chamber, which could be a bent valve.

And last but not least, spark scatter. Either in your distrubitor cap, or jumping across plug wires. make sure your plug wires aren't bundled together anywhere. if they cross, do so at a 90 degree angle. prevent any 2 wires from laying against each other.



All the description, and I'm assuming we're talking about a back fire thru the carb.

if it's thru the exhaust... well, then it might be a bent exhaust valve ( leak down check), or a plug not firing. Check your gap.. read your plugs(Shut the engine off when it backfires, not after idling.) and, check for any errenous spark, which is best done at night, in a very unlit area. just pop the hood with the engine running, and look for the spark jump. if there is any, it will be pretty easy to see.


rumble... yeah, I try to poke in here as much as possible.. which isn't often enough.

[jsaylor]

Yes it does stumble and hesitate, and it seems to be backfiring out the exhast. I also failed to mention with the engine at tdc
in order to time the engine properly the vacuum advance on the distributor has to be rotated fully counterclockwise towards the manifold. Instead of pointing toward the passanger seat it points toward the drivers headlite. thanks again for you help

[MoparMarcIdaho]

Since you already freshened the ignition and we are pretty sure its good now,valvetrain is next.Look for bent pushrods,push down on the valves by hand and check for weak valvesprings.Might also be a flat cam lobe,get in there and poke around you will find it.

[jsaylor]

I removed the valve covers and noticed one of the exhaust valves not moving nearly as much as the others and seemed to be very slow. Would a flat cam cause a loss of power and backfireing at high rpm's thanks again

[Lee Hawk]

if you have one valve with less actoin than the others then you more than likely have a worn cam lobe .

[MoparMarcIdaho]

you found it,now you have to fix it.