16 May 1997, Damian Penney <dpenney@fortpt.com> wrote:
>
> how do you adjust the mixture correctly on a pair of twin ZS carbs. When
> I used to have the choke out it would idle at ~2000rpm but performance
> would be great, so I guessed I needed a richer mixture. I played about
> with the mixture adjustment screws to little avail, I couldn't seem to
> get it to the point where revs would rise when I lifted the piston,
> every time I tried the engine coughed, spluttered and almost died.
Hello Damian,
Are you adjusting the screws on the side of the ZS carbs just above the
Temperature Compensator Housing? They are the Idle TRIMMING Screws, not
the Idle MIXTURE Screws. You can crank on them all day without great
effect on the engine's idle quality. To quote from the Haynes manual on
ZS carbs:
>> The Idle Trimming Screw provides for very fine adjustment to the
idling mixture ratio to compensate for the difference between a new engine
that is "tight" and later when it is run-in. This is achieved by
regulating an air bleed which by-passes the air valve, thus weakening the
mixture ratio. <<
>> A new engine may be run with a comparatively weak mixture, when the
idle trimming screw will be fully unscrewed, the maximum bleed being
controlled by the size of the drilling (an extension of the passage to the
temperature compensator). As the engine "frees off" during the running-in
period the screw may be progressively screwed in until it is seated and the
air bleed is blanked off, giving a richer mixture. <<
The best adjustment for the Idle Trimming Screw is to turn it all the way
in until it is gently, but fully seated. No further adjustment is
required.
*******************
The idle mixture is controlled by the depth to which the metering needle
seats into the jet. On older ZS carbs, the needle was fixed and the jet
assembly was adjustable up and down. The jet adjusting screw is located
in the middle of a protrusion on the bottom of the float chamber cover...
under the carb... facing down... the height of convenience and easy
access. :-( Screw the jet in for a leaner mixture and out for a richer
mixture.
On newer ZS carbs, the jet is fixed and the mixture needle is adjustable
with a special tool (B 20379Z). The tool is basically a glorified allen
wrench. An outer tubular sleeve has a locating peg on the bottom end that
engages the air valve guide to keep it from turning, while a long allen
wrench extends through the sleeve to engage the adjustable jet.
Remove the damper cap and rod assembly from the top of the carb. Insert
the tool and rotate the outer sleeve until you feel the locating peg engage
the air valve guide. Slide the allen wrench in and rotate it until it
engages the allen drive hex on top of the needle. Hold the sleeve and
air valve guide fixed while turning the allen wrench and jet to adjust the
idle mixture. In to lean the mixture and out to enrichen the mixture.
If you turn the mixture jet in too far, it will screw itself right out of
the bottom of it's threads in the air valve (the large piston that moves up
and down to restrict the main carb bore). It will not drop out of the air
valve, but reversing direction will not re-engage the threads and the jet
cannot be backed up. In that case, stick a finger into the carb bore
under the air valve. Gently push up on the protruding needle (finger nail
works well) while turning it counter-clockwise with the allen wrench...
and holding the tool-sleeve with your third hand.
This method does require a special tool (in-expensive), but it's otherwise
much more convenient and accessible than gaining access to the bottom of
the float chamber cover on the adjustable-jet style carbs.
******************
Not sure which style carb you have? If your carbs have a rather large
screw on the bottom of the float chamber, the mixture jet is adjustable.
If there is a plastic plug blanking off that area, the mixture needle is
adjustable.
Constant depression carbs (Zenith Strombergs, SU's, et al) are not
intuitive... at least if you cut your teeth on "normal" carbs. Before you
go turning every screw in sight, I strongly recommend you get a hold of a
manual and read it until you understand what's going on. They aren't
terribly complicated, but they are different.
Good luck, Damian
Regards,
Tim Engel
Lotusphile lurking the Triumph List
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