On Tue, 17 Mar 2015 17:56:47 -0400, wrote:

On Tue, 17 Mar 2015 09:39:20 -0700, Oren wrote:

On Tue, 17 Mar 2015 05:11:47 -0700 (PDT), TimR
wrote:

When I was growing up car backfires were common. As a kid I had friends who could make it happen at will.

Now that the carburetor is long gone, most younger folk have never heard one. Fuel injection eliminated this, I think; could be wrong.

I never understood exactly what happens. Usually a car would backfire when under heavy load, and the gas suddenly let up. So my guess is the carburetor had a full charge with nowhere to go. But what ignited it?

Or, possibly the lack of air made an overly rich mixture escape the exhaust valves, and the explosion was in the manifold?

Timing caused it. If the distributor was installed off by a tooth or
three. Timing could also slip if the nut loosened a tad.

Carbs caused it. Adjust dual four barrel carbs or three-deuces. Have
to get the cabs in tune. Flooded and it would catch fire so have a
shop rag handy to toss of the carb.

You could push in the clutch in, turn off the ignition, release the
clutch, turn on the ignition and cause a back fire. I was told it
could pop off the top of a piston but I never saw or heard of it
happening
Well, as a mechanic I can say definitely it could NOT damage a piston
- as the "explosion" was not in the engine.
Bad ignition was responsible for more exhaust bachfires than bad
carburetion. My Dad's '69 GMC pickup had a bad habit of slitting the
muffler every 6-8000 miles unless you changed the points first. Ends
up the factory had not installed the ground wire between the points
plate and the distributor housing, so the points burned badly - and
intermittent spark would allow the muffler to load up with gas when
the spark was off, and then light when it came back on.

An exhaust leak could also cause exhaustr backfire on decel or
over-run as the exhaust was rich with gas from the high vacuum, and
added air into a hot pipe would light it off, giving
bang-bang-bang-BANG!!! coming down a hill.

One day with 22 tons of farm equipment on the float behind the 292
GMC, I came down a long gentle hill to a narrow bridge across a creek
where guys were working on the road.. They were in no hurry to get out
of the way as I came towards them, so I shut off the ignition for a
second or two, then turned it back on. They jumped out of the way
real quick!!!. (had a drawn steel "cherry bomb" muffler on it that
withstood backfires quite well - after blowing a few Thrush cans off
of it.

It was easy to make a Model A Ford backfire. Soak a little gas on a
rag, slip muffler back a tad, backfire and light the rag. Fire up your
cigarette