Intake manifold combustion

["Warren R Anderson" <wranderson@xxxx>]

Generally, popping back through the carburetors is
>a matter of leanness, timing being off, or a bad intake valve allowing the
>normal combustion process to fire jets of flame back into the intake
>manifold.
>
>Since more than one set of carburetors have been used, it kind of
eliminates
>anything like float levels or such. I'm thinking more along the lines of
>leanness or such.

This post I agree with. Ignition failure will present as backfire through
exhaust. A mixture late burning in a cylinder (and for any number of or
combination of reasons) will continue to burn when the intake valve opens
and blow back into the intake manifold.

I remember a magazine road test on a brand new then torsion bar Chrysler in
1957 where Cahill (maybe) ran the thing down the track at 60 MPH and without
slowing cranked the wheel hard to make a 90. As I remember, in one direction
the engine flooded and the other direction it did not due to fuel slosh in
the float bowl. This was a factory fault in the carb design. Point is,
carburetors are imprecise things at best when dealing with vehicle dynamics
and just maybe this coupled with some other factor (timing advance problem,
weak ignition, bad coil, failing or wrong ballast resistor, etc) is the
culprit.

I haven't seen a post about this, so, a propane torch with the burner
removed and a small hose and then a short length of brake tubing substituted
makes a wonderful probe for locating intake/vacuum leaks. This tool is very
effective with just a tiny flow of propane but best used outside in good
ventilation.

Warren Anderson
Sedona,AZ