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Re: Vacuum advance, pt. 2

To: Todd Mullins <>
Subject: Re: Vacuum advance, pt. 2
From: "John M. Trindle" <>
Date: Tue, 16 Jan 1996 18:39:00 -0500 (EST)
On Tue, 16 Jan 1996, Todd Mullins wrote:

> New question:  Backfiring.  I don't quite understand it.  My exhaust
> makes a raspy, popping sound upon overrun.  I had assumed that it was
> unburnt fuel travelling through the pipe and exploding when it
> encountered the outside air.  Others have told me that backfiring is due
> to a lean mixture.  Which is right?  I believe my carbs (dual HIF4s) are

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>.  

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.

> valve is operative, I can't think of anything else that might cause
> backfiring (I have solid throttle plates).  Any gurus?  Denise?
Well, the reason those silly buttons were put on the throttle 
plates was to prevent overly rich mixture on trailing throttle.  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).
John M. Trindle | | Tidewater Sports Car Club
'73 MGB DSP     | '69 Spitfire E Stock | '88 RX-7 C Stock

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