Atwell Haines wrote:
> Good news, Bob, the backfire was gone without the air pump connected.
> Hooked it back up, and the backfire returned.
>
> Now lets try and reason the theory out.
Uh-Oh this could be trouble ;-)
> At 08:38 PM 4/26/98 -0700, Bob Sykes wrote:
> > I think there is vacuum switch
> >and an "idle detect mechanism" on the carb.
>
> Would that be the "by-pass valve" and "temperature compensator" on the ZS
> carb? The temp compensator is there to increase the idle when the car is
> running hot, isn't it (it's a bimetallic strip), so that wouldn't give us a
> backfire. But the bypass valve is 'sposed to hold the revs up on overrun
> isn't it, like an old-fashioned carb dashpot. Hmm.
Not quite. These are emissions control devices, but....
The temperature compensator (not present on your carb if it's stock
'78 model) is in fact, a bimetallic strip controlling an air intake
port to the carb. Its purpose is to lean the mixture slightly when
everything is nice and toasty warm under the bonnet. This device is
found on the earlier model ZS carbs.
The deceleration bypass valve (which you should have) is there to limit
excessive vacuum on the over-run. This could have something to do with
your problem. I haven't played around with adjusting one of these.
The manuals warn against tampering with them (not that it's stopped me
from getting into trouble before :-0 )
The "idle detect" widget I was referring to is simply a vacuum switch
attached externally to the carb throttle mechanism. I *think* it's
purpose is to switch on the vacuum to the diverter valve when the
throttle is closed. This is found on '75 (or so) model NA Spits, but
not our later cars. Due to a long line of POs, there were only a few
traces of this system left on my '75 Spit when I got it.
> I didn't touch it during the carb rebuild except to install a new gasket.
> I have also heard of some owners blanking it off when it fails (it holds
> the idle at 2500 rpm then, right?).
I'm not sure about this. It's probably best that you did not touch it
during the rebuild. The books say it should only be replaced as a
complete unit. I suppose I would be tempted to blank it off, if it
was suspect, rather than paying $$ for such a little part ;-)
[...]
> Club meeting tonite, I'm taking the Spit there and will report back.
>
> Comments on the above thoughts are welcome.
I would be interested in the collective wisdom also..
--
Bob Sykes
"You can't beat your brains for entertainment."
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