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Re: Vacuum advance, pt. 2

To: Todd Mullins <>
Subject: Re: Vacuum advance, pt. 2
From: "John M. Trindle" <>
Date: Thu, 18 Jan 1996 11:49:52 -0500 (EST)
On Wed, 17 Jan 1996, Todd Mullins wrote:

> 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.
This strongly suggests to me you have leaky valve guides on cylinders 1 
and/or 2.  You are not supposed to have a "considerable" buildup of 
deposits on todays gasolines.  The Spitfire did this because of oil 
deposits until I got the head reworked (that car responded extremely well 
to new plugs or careful removal of the deposits, you might consider that).

The fact that the front pair of plugs is different than the back pair is 
extremely suspicious.  There has to be an asymmetry somewhere... maybe in 
crankcase ventilation?  Excessive pressure could force oil vapor into the 
carb in certain arrangements I have had (and seen).
> 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 would DEFINITELY get a compression gauge (borrow one, even) and see if 
the front cylinders are different than the rear.  Burning a valve is a 
more gradual process than breaking rings, but I don't know how subtle it 

I would also try disconnecting the PCV system to see if there is excessive
pressure, and look in the shop manual for the procedure for testing the
gulp valve (mine disappeared eons ago, before I bought the car, so I
couldn't tell you). 

My car has been driven in snow, rain, and sun, raced, broken, up on jack
stands, and I got to lie in the cold, wet, sandy parking lot replacing the
exhaust.  All in the last 10 days <grin>. 

John M. Trindle | | Tidewater Sports Car Club
'73 MGB DSP     | '69 Spitfire E Stock | '88 RX-7 C Stock
Home Page:
"Cahn's Axiom:
   When all else fails, read the instructions."

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