With all due respect to Bob Holton (who posted a separate response to
Frank's post and who may have more years of experience with Z-S carbs then
me, for all I know), a Z-S carburetted MGB engine will start but will not
run "just fine" without the air cleaner on. If it does, then it is not tuned
correctly! As the Moss book says, these are very sensitive carburetors
(note, I didn't say efficient!) and they are set up to comply with emissions
requirements with the air cleaner in place. As such, they are not delivering
a proper mixture down that convoluted intake manifold when the air cleaner
is off.
John Twist's advice about testing the mixture strength by raising the piston
a little bit is very applicable to SUs, but not to Z-S carbs. With a Z-S,
all you can really do is adjust the mixture for the fastest, smoothest idle
(as always, after making sure the ignition system is in perfect working
order and that none of the emissions valves are malfunctioning). Our 4-gas
analyzer shows that very small adjustments, via both the needle adjustment
and the trimmer screw on the side, make a noticeable difference in the
exhaust gases. This, of course, also tells how efficient the engine burn is.
So, for optimum engine performance, it's best to have the final adjustments
made - with the air cleaner in place - by someone with a 4-gas analyzer.
Lawrie
British Sportscar Center
----- Original Message -----
From: "frank krajewski" <frankk@intap.net>
To: "MG List" <mgs@autox.team.net>
Sent: Thursday, April 04, 2002 12:31 PM
Subject: ZS Confusion
> I am down to fine tuning the motor in my 1980 B, which I totally rebuilt
> over the winter. Thanks to Lawrie I now know which way to turn the
> special ZS carb adjusting tool for leaning and enriching the mixture. I
> am a little confused however when I attempt to follow the instructions
> provided by John Twist, whom we all agree is one of the gurus (along
> with our own Lawrie) of MG repair, on page 85 of the MGB 16 Moss
> Catalogue. At the top of Twist's guidlines is the sentence, ""Lift the
> piston slightly. If the idle speed rises...etc". That is great and it
> assumes the engine is running or how else can the the idle speed vary at
> all? To lift the piston in the ZS carb entails removal of the air
> cleaner to allow access to the piston. Now my confusion, at the bottom
> of those same guidelines (Item #3 under "Running Lean") there is a line,
> "The Stromberg carburretted MGB will not run with the air cleaner
> removed...." OK, what did I miss? Let the flaming begin....
> Frank "Swamp Yankee" Krajewski
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