Bob -
You should also check to make sure the vacuum advance
is working on your car. You are supposed to have
maximum vacuum advance on the ignition under these
circumstances... so if you have a vacuum leak or your
vacuum diaphram is old or broken... it may contribute
to these symptoms.
If the spark can't advance far enough on closed
throttle, the fuel mixture has trouble having enough
time to completely combust in the chamber before
exiting to the exhaust system... where it exits the
tail pipe and meets cold fresh oxygen and pop
combusts. Remember on closed throttle you are not
getting full compression in the cylinder... that's why
the advance is supposed to be maximum forward under
these conditions.
Another cause may be related to a weak spark - either
bad points/condensor or bad coil... or possibly bad
ignition or engine ground or bad spark plug wires.
Cheers,
Alan
'53 BN1 '64 BJ8
--- Bob Spidell <bspidell@pacbell.net> wrote:
> Listers,
>
>
> My BJ8 has started making fairly loud popping noises
> through the exhaust
> recently. This occurs mainly when coasting downhill
> in gear, doing a little
> engine braking, but also happens sometimes after a
> downshift on the flat
> (always under closed throttle). I supporse it's not
> actually a backfire, a
> singular, random "bang," but a series of moderately
> loud popping noises in
> the exhaust.
>
> I didn't change a thing--it was running
> perfectly--this started about a
> month ago. I tried both leaning and enrichening the
> mixture with little or
> no apparent effect.
>
> The only thing I can think of is the new, summer CA
> gas formulations.
> I believe we're getting up to 20% ethanol mixture
> now, with no more MTBE.
> I'm wondering if this gas is causing the backfiring
> due to differing vapor pressure
> or something. Anybody know what else could cause
> this?
>
>
> bs
> ********************************************
> Bob Spidell San Jose, CA
> bspidell@pacbell.net
> '67 Austin-Healey 3000 '56 Austin-Healey
> 100M
> ********************************************
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