My '69 Datsun 1600 (OK, but I sold it 23 years ago and replaced it with my
first LBC) used to do exactly the same thing. It had Hitachi carbs which
were actually very close copies of SUs. It never bothered me much, either.
I always assumed that this condition was caused by the carbs not responding
adequately to the vacuum conditions of deceleration and supplying too rich
a mixture, leaving unburned fuel. A check of my Haynes Zenith-Stromberg
manual, however, indicates the opposite - it is caused by too LEAN a
mixture, which burns more slowly, and is still burning when the exhaust
valve opens. It goes on to say that the Z-S carb performs well in this
situation because reduced mass (of the piston) and reduced friction allow
it to respond immediately when the throttle is closed. So is this an SU
thing? Any of you more knowledgeable folks out there able to shed more
light?
Dan Julien
'72 TR6 78709U (undergoing body transplant)
Austin, Texas
djulien@mail.utexas.edu
----------
> From: Vincenti, Ross <Ross.Vincenti@transamerica.com>
> To: Triumph GroupReturn requested <triumphs@autox.team.net>
> Subject: Lift Throttle Backfiring
> Date: Monday, April 28, 1997 5:18 PM
>
> Greetings Scions.........
>
> Does anybody else experience this "problem"? (I put that in quotes
because I
> actually kind of like the effect I am about to describe). In my slightly
> tweaked Spitfire Mk I w/ 1147 motor, when I run it up fairly high on the
> tach (not to redline, just 3,500 or 4,000 or so), then lift off the
throttle
> such as coasting to a stop light or behind heavy traffic (with the car
still
> in gear mind you), I get a really nice terrific fat burbling sound with a
> couple of pretty good "pops" out the tailpipe (backfires). (Scares the
hell
> out of little old ladies and dogs, and gets smiles from college coeds so
I
> don't care to fix it frankly.)
>
> Question - what causes this? It sounds very much like some of the
vintage
> race cars I have watched at various races, but I've never understood what
> causes it. I don't think it is a "true" backfire where the charge gets
> blown back out of the carburetor (or am I wrong on this point?). Does
> anyone else experience this? Thoughts please,
>
> Thanks.
> Ross D. Vincenti
> 64 Spitfire 4
> 64 Porsche 356C Coupe
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