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RE: Vacuum advance, pt. 2

To: mgs <>
Subject: RE: Vacuum advance, pt. 2
Date: Tue, 16 Jan 1996 17:59:00 -0800 (PST)

From: mgs-owner
To: mgs
Subject: Vacuum advance, pt. 2
Date: Tuesday,January 16,1996 3:46PM

Hi, all.  I'm the one who asked about dizzy vacuum advance units.

While corresponding with one list member about this, a rather brilliant
idea hit me.  I realized that I could test the operation of the vacuum
can by simply removing it from the distributor, installing the
distributor sans vacuum, and running the engine up and down with the
vacuum can in my hand.  I did this, and (even though I wasn't able to
move it by sucking) it works fine.  It looks as though it doesn't
contribute more than 5 to 7 degrees of advance, but it snaps in and out
like it should.  I also used this method to verify the operation of my
gulp valve.

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
adjusted properly, though I don't have any of those fancy tools or clear
spark plugs.  I use the method outlined in the Workshop Manual: lean
best idle plus a quarter-turn rich.  But since I'm now sure that my gulp
valve is operative, I can't think of anything else that might cause
backfiring (I have solid throttle plates).  Any gurus?  Denise?
A burnt exhaust valve could do it. When you go to compress the fuel air 
mixture it leaks out into the exhaust and causes a back fire. Check your 
compression, they should all be close to the same value and put a vacuum 
guage on the vacuum to look for a bouncing needle to check for a bad valve

ObNoseRubbing:  Today I took the 'B to pick up some brake parts for the
Swedish steed.  The nearest dealer is in New Orleans, a 50 mile drive
from work.  The midday sky was bright blue and full of cheerful, fluffy
clouds.  The sun was intense, bringing the temperature well into the
70s, although Lake Pontchartrain was still harboring some chilly
breezes.  Of course I had the hood down...


Todd Mullins    On the lovely Mississippi (USA) Coast

'74 MGB Tourer
'84 Volvo 245GL (old Volvos had SU carbs, remember?)

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

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