Welcome to our resource for MG Car Information.



MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - 76 zs carb adjustment

hey guys i have a 76 mgb with the stock ZS carb. My 68 used to have the SU4 carbs so im lost. How do I adjust the ZS carb it is running to rich i dont know where the adjustment is. also what are the specs on how many turns out to adjust it

thanks guys


First, you need the special Zenith mixture adjustment tool, available from Moss (386-310) or VB. It's about $10, and you can't adjust the mixture without it.

The following is from the Moss catalog, courtesy of John Twist, but before you read it, here are a couple of links to articles on the water choke:

Also, one other thing - go ahead and replace the rubber diaphagm if you haven't already done so - better safe than sorry. If it splits, you'll foul your plugs and think you've blown an engine.

If you're not a member of the BBS, join so you can get into the archive. It's free, and there's a bunch of stuff out there if you search on Zenith.

Now, on to the tuning article:

Tuning the Zenith-Stromberg Carburetor - Thanks to John Twist of University Motors

Despite the assurances of the manufacturer that this single carb. could lessen emissions, the Zenith-
Stromberg often runs very rich. When this occurs, and if the air pump and catalytic converter are still in
place, the converter will glow cherry red hot. With regular maintenance, this carb. will perform properly,
offering relatively high mileage compared to the twin SUs.

To properly tune the Stromberg, one must have the Stromberg adjusting tool (Moss part #386-
310), a l0mm open end wrench, and small and medium screwdrivers.

Start the engine from cold, and adjust the spring-loaded idle screw for an engine speed of 1800
rpm maximum. Once the engine has fully heated and the speed dropped to its lowest rpm (choke all
the way off), adjust the locknut screw (hence the l0mm wrench) until the engine is idling at about
850—900 rpm. All further adjustments to idle speed will be made again with the spring loaded screw.
Now the mixture is adjusted. Lift the air piston slightly. If The idle speed rises as the piston is lifted, the mixture is too rich. Turn the allen wrench anti-clockwise to lean it out. If the idle speed slows or stumbles when the piston is lifted, then the mixture is too lean. Turn the allen wrench clockwise to richen it. With the proper idle mixture the idle speed will rise and steady, or rise and slowly fall off as the piston is lifted. As the correct mixture is reached, the idle speed will rise, and several adjustments may be necessary to maintain 850—900 rpm. Be certain to rev up the engine, to clear it out, between each adjustment.

The most common complaint is that no matter what is done, the mixture is still far too rich. There
are five major areas of concern:
1) The air cleaner is dirty. Leaks in the manifold or exhaust will soot up an air cleaner. This causes a
much higher vacuum in the venturi, and more gasoline is drawn into the air stream.
2) The ELC (Evaporative Loss Control) system is pressurized. This is very uncommon. Remove the
vapor line, the vent line, and the carburetor overflow line. This travels across the valve cover to the charcoal
adsorption canister. There should be NO CHANGE in the idling of the engine. If there is any change,
clean the ELC system, and check for blockages. It is more common for this system to develop a vacuum
which creates a lean running condition.
3) The automatic choke is the most common cause of rich running. These problems are:
a) The choke ass’y. has come loose from the carb. body. The vacuum from the carburetter then draws fuel from the float bowl around the valve which should be closed during warm running. Tightening the three slotted copper coloured screws eliminates this as a problem.
b) The choke lever and cam have stuck in a part-open position. A good tune-up requires removal of the choke assembly , thorough cleaning, tightening the pin nut, and lubrication.
c) The bi-metal spring in the heat mass is not correctly calibrated. Place the heat mass in a pan of
boiling water; the bi-metal spring will rotate clockwise to its fullest extent. The notch on the bi-metal spring should be in line with the mark on the aluminium housing. If it does not line up, scribe a new mark. The line on the heat mass, on the black plastic insulator, and the choke body should all be in line.
d) The small passageway from the carb throat (past the butterfly) to the top of the auto choke ass’y.
allows vacuum to pull off the spring loaded enrichment plunger. When this passageway fills with soot, the plunger remains seated, holding the choke ON. By cleaning this passageway with a small wire, the
vacuum can take its proper route.
e) The mating flange of the auto choke can become warped and allow fuel to bypass the choke valve.
Resurface this mating flange and thoroughly clean after the surface is smooth.
f) Occasionally the brass choke valve is not fully inserted into the housing, so that the choke pin
does not seat in (and therefore not closing off) the choke. This can be positively determined by removing
the automatic choke and covering the screw holes and fuel passageways with a piece of masking tape, then checking the mixture again.
4) If the rubber diaphragm is torn or perforated, then the engine will run very rich, and maximum
speed will be limited, spewing clouds of black, sooty, uncombusted exhaust. A torn diaphragm is easily
found by removing the top of the suction chamber (four phillips screws). While the diaphragm is being
inspected, pay attention to the following:
5) The tiny pin which retains the metering needle can break,allowing the needle to pop upwards. Adjusting cannot correct this. The needle’s shoulder should be seen on the bottom of the air piston. If not, and the needle has a lot of up and down movement, check the pin.

Leanness at road speed is often described as a hesitation, as if a wind was blowing against the car.
Causes can be retarded timing or a too lean mixture. There are several possibilities:
1) The ELC System is plugged, placing a vacuum in the float bowl. Remove the vent line from the carb. and note any change. A plugged charcoal adsorption canister, or a plugged vent line from the bottom
of the anti run on valve are the most common problems.
2) The float height is set far too low.
3) The air cleaner is not fitted to the carburetter. The Stromberg carburetted MGB will not run with
the air cleaner removed, nor with a filter which replaces the original assembly.

Sometimes the carb. will not idle at lower than 1000 or 1200 rpm. This is almost always a mechanical
problem, but the possibilities include:
1) The throttle cable is stiff or incorrectly adjusted. Leave loose until the carb is adjusted, then tighten.
2) The automatic choke cam is not returning to full off position. Levers may be bent or jammed.
3) The overrun valve in the throttle disc may “float”; the spring may have lost its force due to a carb. fire or other gross overheating. Replace or solder shut.

Yes the air filter being fitted to the carb is a big part of it suprisingly, dont over look it. People who switch to foam elements etc tend to have problems

This thread was discussed on 20/02/2007

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.