John M. Trindle writes:
> Both. Rich mixture tends to pop out the tailpipe, while lean pops back
> through the carbs. I call the latter "frontfire", although nobody else
> does <grin>.
Okay. This suggests to me that I have an overly rich mixture with the
throttle plates fully closed (I'm not getting any funny stuff coming
back through the carbs; that sounds scary!). But my plugs are a nice
even tan color, like they're supposed to be, not black and sooty, as a
rich mixture would produce. One odd thing: The front two plugs have
considerable buildup of deposits, but they're the "correct" light tan
color. The rear two plugs do not have any deposits.
> I'm not sure about the bit of it traveling all through the tailpipe
> before exploding, I think it happens when raw fuel hits the hot exhaust
> manifold. But I've been wrong before.
I figured the fuel travelled through the pipe, staying nice and hot,
until it encountered enough oxygen to have a party with. Of course,
assuming my air pump is injecting air (I haven't actually checked it), I
suppose the ignition could happen anywhere inside the pipe...
> Well, the reason those silly buttons were put on the throttle
> plates was to prevent overly rich mixture on trailing throttle.
This is also, presumably, the job of the gulp valve. Which I have.
> Aha! I
> must say though that I have replaced my plates with solid ones and have
> no backfiring problem (frontfire while the engine is cold, oh yes).
You must be running leaner than me, then, no?
Christopher Reichle suggested a burnt exhaust valve. I don't have a
compression gauge (maybe now is a good time to go get one...), but I set
the valve lash to spec at the same time as I was fiddling with the
dizzy. Two or three valves were tight, but I figured that was just
normal valve seat recession. Anyway, I figured a burnt exhaust valve
would be an obvious and sudden condition. Am I wrong?
I feel a little guilty making all this fuss over a little backfire while
some of you haven't driven your cars for a month or more, or have
projects on jackstands.
I don't feel guilty at all about driving the little bugger, though!
Todd.Mullins@nrlssc.navy.mil On the lovely Mississippi (USA) Coast
'74 MGB Tourer with Jiffy Pop exhaust
"A life lived in fear is a life half lived."