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Re: Vacuum advance, pt. 2

To: (mgs)
Subject: Re: Vacuum advance, pt. 2
From: (Todd Mullins)
Date: Wed, 17 Jan 1996 10:42:29 -0600 (CST)
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    On the lovely Mississippi (USA) Coast

'74 MGB Tourer with Jiffy Pop exhaust

"A life lived in fear is a life half lived."

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