Chicagoland MG Club:Photos
MGB Zenith-Stromberg Carburetor Rebuild
May 3, 2003 - Downers Grove, IL

The victory picture, running well at day's end. But it took some doing to get to that point.
Click for larger images. - Larger pics average 44KB.

We are here. Come on in. Pull the air cleaner, clamp off the water hoses for the hot water choke, disconnect fuel line, fiddle with four mounting bolts, and remove carburetor.

Well, almost remove carb. Don't forget to disconnect the throttle cable. Front view of the carb showing the hot water choke actuator still attached. Outlet end of carb showing the resistance heater element coiled around the throat. Engine minus carb.

The nitty gritty of disassembly. Give yourself plenty of table space. Oh, and don't forget to drain out the fuel before disassembly (yeah, sure). With the vacuum diaphram and air piston removed, nose down and tail up showing the outlet end with throttle plate still in place. Checking the side play of old and new throttle shafts, it looks like the bushiings are in good condition. New shaft lying next to the old shaft. If you get it the wrong way around, there is a subtle difference in position of the throttle plate mounting holes, not quite centered on the shaft, as the throttle body is a bit thicker on one side.

Now upside down, time to remove the float chamber cover, which is on the bottom. This is why we hold tech sessions. Nearly everyone here has never seen the inside of a Zenith-Stromberg carburetor. Keep that shop manual handy! Bottom cover removed with floats exposed. Then float arm flipped over to expose the Grose Jet (fuel entry control valve), upper right of cavity area in photo.

While Bob Simon is cleaning up a few parts, we get a short break outside to take a look at the top of the front end of the exhaust manifold where the heat riser pipe for the air cleaner attaches. Because of a large difference in temperature between the hot exhaust manifold and cold winter air flowing past at this point, this is a common place to get a crack in the manifold. When that happens some tainted exhaust gas can pass into the air intake, sometimes cogging up the air cleaner to the point where the engine will not run. Happy to report that this one looks okay.

Now about that bottom plug they tell you not to remove unless you are replacing it with a new one, .... here's why. It is a plastic part that deteriorates and gets brittle with age and heat cycling, so it is VERY likely to break when removed (and it did). Quite likely this is (was) the original factory part, now more than two decades old. Order this part separately if it is not included in the carburetor rebuild kit. This is the bit that can drip petrol near the hot exhaust pipe if the o-ring seal leaks.

New Grose Jet installed and float arm flipped back over to working position. Use a length of 0.125" diameter rod as a gauge between the carb body and the float arm to check/set the float height at the valve shut-off point. Install new gasket and bottom cover, and snug up all screws. Now back right side up showing inlet end of air throat.

A common modification is to solder the poppet valve shut on the throttle plate. That was already done here, but we did remove the small spring on the flip side that looked like it was about to fall off. After some curiosity about why the throttle wasn't closing all the way, we had to remind Cowboy to back off the idle screw. Second photo above shows the throttle plate completely closed. Before tightening the screws, nudge the plate gently to get it seated against the throttle body throat all the way around. When all goes well you will have a near perfect seal with no visible air gap anywhere, and then you tighten the screws. Original screws here had a split tip so you could swedge them open at the end to keep them from coming unscrewed. If one of these screws should fall into the air intake, it could end up inside the engine beating dimples in a piston and possibly bending a valve or two. If you are re-using the old screws, or if you are using plain machine screws here, be sure to apply a drop of thread adhesive to keep them permanently in place. Attach the last throttle arm to complete the throttle shaft assembly.

Then we installed a new fuel metering needle. This is a biased needle with the built in wiggle and a light spring to hold it slightly off center against one side of the main fuel jet. This is designed to negate the need for critical alignment of the needle to the jet. This concept was introduced along with the temperature compensating carburetor, where the main fuel jet automatically moves a bit vertically with temperature changes to reduce drift of the fuel mixture. The earlier MGB 1972-1974 using SU HIF-4 carburetors also used this same principle. A set screw in the side of the air piston engages the key slot in the side of the brass collar on the needle. This screw does not come tight on the brass collar, but serves as an alignment key while allowing the collar to move vertically for adjustment of fuel mixture (primarily in the interest of setting the idle mixture). Rubbing of the needle on the side of the bore in the main jet causes some wear on these parts over time, which in turn causes the fuel mixture to run rich. Standard adjustment will compensate for this wear for a while, but eventually the worn needle and jet will need to be replaced.

Here we have the rubber diaphram (shown first upside down) that is installed with the main air float (piston). This part effectively negates the requirement for precision machining that was used on the earlier MGB SU carburetors when fitting the air float to the vacuum chamber. Notice the keying tabs on the rubber part. One tab at the bottom edge of the rubber part will fit into the notch seen here in the throttle body. The other tab at the top lip of the rubber part (about45 degrees clockwise) fits into a similar notch in the moving air piston. This provides for correct orientation of the air piston in the throat of the carburetor, as required by asymetrical venturi ports in the bottom of the air float.

With the diaphram and air float in place, the top cover and damper plunger are installed. More fiddeling with four hex nuts in confined quarters to reinstall the carburetor on the car. Curse the expensive wrenches and use one of those nice cheap thin ones. Reattach throttle cable and fuel line, and reconnect the hoses to the hot water choke. With everything connected, reinstall the air cleaner and get it connected to the heat riser pipe. A little tune up adjustment follows, and the proof is in the pudding. With the CO sniffer in the tail pipe, and a small final tweak of the fuel mixture, it not only runs better but will also pass the clean air test. See victory picture at top of page, and hand out the graduation certificates.

Meanwhile, as these tech sessions go, there are some interesting sidebars. We take a short pause in mid afternoon to sniff and tweak a 1980 MGB LE to bring it into compliance for the clean air act. Setting ignition timing at TDC and leaning out the mixture a bit brings the CO reading down from 10.6 (YIKES) to under 3.0 where it will happily pass the test.

Then we were on to check a 1978 MGB which not only has all of the original emissions equipment, but also had a brand new catalytic converter. One sniff here, and it was already reading 2.5 on the CO scale, so no need to touch a thing, just take it in to collect the official clean air certificate.

One notable visitor for the day was our old friend M. L. Hillard, who was back home temporarily from work assignment in China. His MGB bears Illinois license plate "MGB" of all things. What do you have to do to get one of those, for crying out loud??? Chrome bumpers are always welcome at tech day, even if it doesn't have a Z-S carburetor.

©2003 Chicagoland MG Club, All rights reserved.