Greg Lemon wrote:
> My question is this, do you think the plug, on the verge of going bad and
> offering up its electrode to my motor, was causing the miss and the backfire,
> or was the new fuel pump dumping in too much fuel and the backfire caused the
> plug to shatter at the electrode?
>
Greg --
Was it a true backfire (through the carbs) or technically an afterfire
(through the exhaust)? The former is often caused by leanness or timing
too advanced, while the latter is often associated with richness, but
either *can* be caused by either. The really loud afterfires are
basically unburned fuel-air mixture exploding in the exhaust system, and
more subtle ones are from the flame front in a cylinder traveling so
slowly that combustion is still occurring when the exhaust valve opens.
That slow flame propagation can be from leanness, richness or bad
ignition, and if it's slow enough, you can get a big backfire through
the carbs when combustion is still going on when the intake valve
opens. Other root causes include intake or exhaust system leaks, or bad
or sticking valves. I don't recall spark plug damage from backfires
being a common problem, but then there are lots of things I don't recall
these days...
As part of the rebuild, you'll want to check the fuel system, anyway
(pump pressure and float valves), and if they're O.K., and your
manifolds are leak-free and the valves are new, you shouldn't have much
to worry about.
--
John Miller, who has had plugs break for no apparent reason at all.
|