Tuning a Zenith Stromberg Carburetor

I found this article on web regarding the tuning of the Zenith Stromberg carburetor. It was wriiten by John Twist of University Motors:
Preamble
is copywritten but you have MY permission to reprint it in any publication, as long
as my name John Twist, and that of University Motors, appears at the end of the text. Give
credit where credit is due!
THOSE DAMN ZENITH STROMBERGS
By John H. Twist of University Motors
Introduction
The Home model MGBs and Midgets were not plagued with this Triumph inspired
imitation SU, we are. The Home and most export model MGBs (except North America)
continued to use twin HIF4 carbs, the Midgets, HS4s on the Triumph 1500 engine, while
the Canadian, Federal, and California specification MGs were fitted with the Stromberg.
zenith stromberg tech tips http://www.bmcno.org/tech tips/zenith stromberg tech tips.htm
1 of 5 3/7/11 10:39 PM
Emission specifications were cited as reasons to move to one carb (more easily kept in tune than two carbs), but why not choose the HIF6? Despite all the assurances of the manufacturer that this single carb could lessen emissions, the Zenith often runs
dramatically rich. When the carburetor does run extremely rich, and if the emission control system (air pump and catalytic converter) is still in place, then the converter will glow cherry red hot. This carburetor is responsible for hundreds of underbonnet fires! Yet, with a little regular maintenance, this carb will perform correctly and safely, while offering relatively high mileage compared to the twin SUs.
Adjusting the Carburetor
To properly tune the Stromberg, one must have the Stromberg adjusting tool (a 1/8" allen wrench within a pinned tube), a 10mm long open end wrench, a small screwdriver, and a medium screwdriver. A tach/dwell is always most helpful.
The engine is started from cold, and the spring loaded idle screw is adjusted until the engine is running at 1800 rpms maximum. One the engine has fully heated and the engine dropped to its lowest rpm (choke all the way off), then the locknut screw (hence the 10mm wrench) is adjusted until the engine is idling at about 850-900 rpms. All further adjustments to idle speed will be made again with the spring loaded screw. Now the mixture is adjusted. Lift the air piston ever so slightly and judge the change in rpm. If the idle speed continues to rise as the piston is lifted, the mixture is too rich. Turn the allen wrench anti clockwise to lean it out. If, on the other hand, the idle speed slows or stumbles when the piston is slightly lifted, then the mixture is too lean. Turn the allen wrench clockwise to richen it. A proper idle mixture allows the rpm of the engine to rise and steady, or rise and slowly fall off as the piston is slightly lifted. As the correct mixture
is reached, the idle speed will rise, and several adjustments may be necessary to hold the idle at that 850-900 figure. Be certain to rev up the engine, to clear it out, between each adjustment. When adjusting the carb at home, tighten the air bleed screw (MGB) so that no air passes through this circuit.
Running Rich
The most common complaint is that no matter how many turns anti-clockwise the allen
wrench is screwed, the mixture is still far too rich. There are five major areas of concern:
1.The air cleaner is filthy and sooted. This is not uncommon, and any leaks in the manifold/exhaust, especially a cracked exhaust manifold, will soot up a new air cleaner in no time. This causes a much higher vacuum in the venture, and more gasoline is drawn into the air stream.
2.
The ELC system is pressurized. This is very uncommon, but easily checked. Remove
the vapour line, the vent line, the overflow line from the carburetor. This line travels across the valve cover on the MGB to the charcoal adsorption canister, or directly to the canister zenith stromberg tech tips http://www.bmcno.org/tech tips/zenith stromberg tech tips.htm 2 of 5 3/7/11 on the Midget. There should be NO CHANGE in the idling or operation of the engine. If there is any change, then the ELC system needs to be cleaned, and checked for blockages. It is far more common for this system to develop a vacuum which creates a lean running condition, in fact the primary reason for lean operation!
3.
The automatic choke is the most common cause of rich running. These problems are:
The choke assy has come loose from the body of the carb. The vacuum from the
carburetor then draws fuel from the float bowl around the valve which should is closed during warm running. Simply tightening the three slotted copper coloured screws eliminates this as a problem. The choke lever and cam have stuck in a part- open position. A good tune-up requires that the choke assembly be removed from the carb, that it be well cleaned in spray carb cleaner, the pin nut tightened, and the unit well lubricated. The bi-metal spring in the heat mass is not correctly calibrated. Simply place the heat mass in a pan of boiling water, and wait for the bi-metal spring to rotate clockwise to its fullest extent. The notch on the bi-metal spring should be in line with the mark on the aluminum housing. If it does not line up, then scribe a new mark with a hacksaw blade. The line on the heat mass, on the black plastic insulator, and the choke body should all be in line. The small passageway from the carb throat (past the butterfly) to the top of the auto choke
assy allows vacuum to pull off the spring loaded enrichment plunger. When this
passageway fills with soot (as it does after years of running), this plunger remains seated, holding the choke ON. By cleaning this passageway with a small wire, the vacuum can take its proper route.
The mating flange of the auto choke can become warped and allow petrol to by-pass the
choke valve. SURFACE this mating flange on a smooth block with fine grit paper and
thoroughly clean after the surface is smooth.
Only several times have we encountered a choke assembly whose brass choke valve was
not fully inserted into the housing, so that even when the choke pin was moved fully
downwards, it was not seated in (and therefore not closing off) the choke.; This condition 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.
Should the rubber diaphragm in the carburetor tear or perforate, then the engine will run very, very rich, and the maximum speed of the MG will be limited, until, at last, it can travel only ten or fifteen miles per hour, all the while spewing forth incredible 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.
Rarely does a needle disassemble itself, but it does happen. The metering needle is held in a small barrel against spring force by a very small pin. If this pin breaks, the needle pops upwards by 1/8" or so, and no amount of adjusting can correct for such a gross misplacement of the needle. The shoulder of the needle can always be seen on the underside of the air piston. If it has slipped up inside, and the metering needle has a lot of up and down (against the spring) movement, then the pin should be check and replaced (paperclip works wonderfully).
Running Lean
A leanness at road speed is often described as a hesitation, as if a wind was blowing
against the car. This can be caused by retarded timing or not enough gasoline in the
air/fuel mixture. There are several possibilities for a lean running condition:
1.The ELC System (Evaporative Loss Control) is plugged which can place a vacuum
above the gasoline in the float bowl. As noted above, remove the vent line from the carb and note any change. A plugged charcoal adsorption canister (from dirt or from a previous overflowing carburetor condition), 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 carburetor. as unusual as this seems, and for reasons that are unclear to this author, the Stromberg carburetted MGB will not run with the air cleaner removed (or a vanity air filter to replace the original). Oh, it will get to 2500 rpms or so, but then flattens out -- no good for any kind of driving.
Carb Will Not Idle Down
Sometimes the carb will not idle at lower than 1000 or 1200 rpms. This is almost always a mechanical problem, but the possibilities include:
1.Throttle cable is stiff or incorrectly adjusted. Leave this cable loose until the carb is completely adjusted, THEN tighten the two nuts with 7/16" wrenches.
2.The cam within the automatic choke is not returning to a "full off" position, sometimes caused by a melted plastic plunger on the bottomside of the unit. Melted? Remember that cherry red hot catalytic converter? Sometimes the levers are bent. They all work on nice, sharp, 90 degree angles.
3.The overrun valve is floating open at a very low manifold depression. The spring loaded valve on the throttle disc is designed to open only at extreme manifold vacuum -- deceleration. If there has been a fire within the carburetor, the spring may have lost its force. The valve is easily soldered shut obviating any further problem (and increasing throttle deceleration response). Heat the valve from the button side with a propane torch and flow solder into the valve from the spring side. The Midget overrun valve can be completely shut off by fully unscrewing the small slotted screw on the triangular shaped device on the right side of the carb.
Air Leaks at Idle
Any tuning or mixture adjustment is impossible if there are air leaks between the
carburetor and the cylinder head. This is a very common occurrence, unfortunately, but the leaks are easily identified. With a can of aerosol carburetor cleaner, spray into areas where leaks are found while the engine is idling. If there is an air leak, the rpms will rise or fall (depending on the size of the leak, the adjustment of the carb, and the type of aerosol spray). Areas the leaks most commonly occur are:
1.Between the cylinder head and the intake manifold, most commonly at the 1/2 intake
port.
2.The tubing and fittings associated with the smog pump (gulp valve, line from the valve to the 90 degree fitting, and the 90 degree fitting itself).
3.The EGR (Exhaust Gas Recirculation) Valve located right on top of the intake manifold between the brake master cylinder and brake booster.
Any leak MUST be corrected before tuning commences.
Other Common Problems
Oil in the dashpot is essential for proper operation! If there is no oil the engine will start with difficulty and will not accelerate quickly. Use ENGINE OIL, and do not worry, the dashpot (despite warnings to the contrary) cannot be overfilled. If the oil disappears too frequently, then the O ring must be changed. A need for oil more than once per fill-up would certainly be aggravating. The screws holding the heat mass to the choke assy are stripped and the heat mass will not keep a proper alignment. The holes in the choke housing can be tapped out to 10-32. The screws fixing the top of the suction chamber to the carb body are frozen tight. They
can be freed by using vise- grips for the first 1/10th turn (then use the screwdriver), or smack the end of the phillips (posi-drive) screwdriver with a hammer to seat the end of the screwdriver and shock the threads loose. Use new 10-32 screws when reassembling. The carburetor heater is cracked, broken, or not connected. There is no concern here. Everything works just fine without this piece of emission control.
A last warning. Do not spray carburetor cleaner down the throat of the carb. The aerosol will attack and expand the rubber diaphragm. Be safe -- carry an extra diaphragm! The Zenith Stromberg carburetor will work wonderfully well on your MGB or Midget -- if you keep it clean, oiled, and adjusted.Refer to your workshop manual for more details, and work with it BEFORE buying some foreign carb to replace it. Remember, if MGs were meant to have Webers, Kimber would have made a deal with Mussolini.
Please remember: This article was written by By John H. Twist of University Motors

5 of 5 3/7/11 10:39 PM

Poorly tuned car

Here in California we have our cars tested for emissions called smog. I just had my MGB tested for smog and it failed twice. The main cause is that it is running too rich (too much fuel) After the mechanic Bob from British European in San Francisco adjusted the carburator several times, the car still couldn't pass smog. He didn't have enough time to work on it so I will have to bring in again.
While driving the car back I did notice a big improvement in performance, the MGB ran like a bat out of hell. The carburator wasn't adjusted right and was just killing the performance.
So this brings me to say be sure you have your carb adjusted right it will make a massive difference in your driving experience.
I'm going to change the spark plugs and spark plug wires, along with an oil change before bring the car back for a third smog check. I'm going to check the fuel canister, if the line coming from the fuel tank and going to the charcoal canister gets clogged it can cause a backup of gas fumes Stay tuned for more updates

Cheap Chinese Replica Parts

Recently, I had purchased a turn signal switch from Victoria British Upon recieving the switch that looked like a pretty good, copy of the original (also labeled "Made in China") I was happy to have a the new part it was just new and in a 35 year old car anything new helps! After installing the switch and trying to use my brights (headlamps) the switch simply snapped in half! Yikes! Okay so I called Victoria British and they were nice enough to refund or exchange the part plus shipping. They didn't even want the "old part" back. I did not want another one. My plan was to purchased the switch from Moss Motors I did and recieved it,(it was made in Taiwan) installed it and it worked for about two weeks. The turn signals stopped working so I bought a new flasher, that didn't work so after some research, I found out that the Harzard switch could be the cause and it was. All the contacts inside the switch were dirty. The switch can be cleaned if it is disassembled. Which I did then everything worked. About two months later my both my headlights stopped working. I was thinking it could be the old headlight switch. I disassembled it and cleaned it and it worked for about another month.
Then my both my headlights stopped working again, I knew I had good grounds because the passing brights would illuminate. I suspected I needed a new headlight switch. I bought one from Victoria British received it, installed it and to no avail: no headights still.
Which brings me back from the start. I found my original turn signal switch just plugged it in and voila! my headlights worked!
I'm telling you these replicas from China or Taiwan are lousy repros, when you think that my original switch is 35 years old!
I'm going to call Moss Motors and let them know how crappy these reproduction parts are from Taiwan and China!

Fuses

With the few fuses the MGB has it is always a good idea to have spares on hand.
I had a fuse that looked perfectly fine but was burned out. It's always best check your fuses before thinking there is some kind of wiring problem...

I found this blurb on a MG repair site that is helpful:"Lucas is very good about color codes and what they do.
browns are power
blacks are grounds
whites involve ignition
purples are hot all the time
blue are head lights
reds are other lights
greens are accessories
When they use a colored stripe it is usually a modifier of the color for example, a green wire will operate an accessory and a green wire with a black stripe will be grounding operational wire for that accessory. Like a white wire is ignition wire and a white wire with a black stripe will be a grounding wire for the ignition system. This is general but it helps when trying to locate problems. "

New Fuel Pump Replacement Part 2

So I got the new fuel pump installed, I completely removed the old one along with the old fuel lines.
The great thing about this SU replacement pump is that is fits perfectly where the original was.
I have seen previous replacements where you had to drill holes and somewhere on the frame mount a rubber mount and even make a metal mount sometimes.

Fuel Pump Replacement

I am finally going to replace my fuel pump. A SU fuel pump replacement costs on average $199.00. Right now Victoria British has on sale and a like SU fuel pump replacement.
Victoria British website The fuel pump is only $99.99 .
The only modification I see so far are the fuel line hose and clamps. The original are braided with metal ends on each end of the hose.

Hope Everyone Has A Very Nice Thanksgiving!

Despite several mechanical problems I must say it really is a joy to drive my MGB! I get so many compliments on the car and it is a real dream just driving in the country roads! So remember you have a great car! You are an MGB DRIVER!

Turn Signal problems

Something to take note of: Should you have problems where your turn signals don't work at all, (assuming you know you have good turn signal bulbs and turn signal flasher) look to your hazard light switch!
Apparently, before any current goes to the turn signal flasher, it first runs through the hazard light switch. If you where to disconnect the hazard light switch, look at the connector going to the hazard switch, the bottom two connections can be "jumped" by inserting a small wire between the two. If your turn signals start working then you know it's the hazard switch. You can disassemble it and clean the contacts or just replace it. Hope that helps!

Broken Bracket motor mount

I had been hearing a knocking thinking was an engine knock when in fact it was the left side motor mount bracket that had cracked. This caused the air cleaner element to knock against the wheel well and the the exhaust manifold against the steering column.
I am going to tackle this task today, the bracket costs less than 10 dollars!
After viewing what is involved, this seems to be a task I will not be able to do.

I got the motor mount replaced at the
British Car Parts & Service-The Car Clinic

(916) 638-8290

2216 Cemo Cir Ste F
Gold River, CA 95670 near Rancho Cordova
Glenn the owner from New Zealand did a great job for under $200 dollars

Fuel Pump problems

This morning I had an issue with my car: it would not start, it would turn over run a second then turn off. I realized after a few tries I wasn't hearing the old "click, click" sound of the fuel pump. I was thinking it was shot. It had been replaced 10 years ago so it was a good assumption. I went ahead anyways and removed the mounting protective plate located inside the right side of the trunk. I checked the contacts and I used a test light with the ignition on to make sure it was getting current, that was ok. I then decided to research what is entailed to replace the fuel pump and found this site with pictures: here Which was great info. I read something somewhere else that talked about removing the gas cap then trying to start the car (something to do with vapor ventilation and the charcoal canister (which I need to replace the charcoal someday)
So just doing that removing the cover plate, reconnecting the wires onto the fuel pump, opening the gas cap. The car started up! So should I replace the fuel pump? I think I will just to be on the safe side and do it next payday!(I would recommend sanding the contact areas )

Driving at a snail's pace for 2 months!

The last two months have been horrible, my car was running at a snail's pace and lost all of it's power. A gradual loss of power over a period of about a month, until finally it would just crawl. I figured I would work on replacing the basics, spark plugs, cap and rotor, fuel filter. The plugs were fowled and seem like they were oily too. I also went ahead and adjusted the valves since it had been a while. That all didn't seem to work, running high octane gas didn't work either. The car did smoke a lot on the freeway at one time that I had to turn around and headed back home. I had lost about a quart or so oil and I figured that the engine was shot at that point.
I called several British car repair shops in the area. One in San Francisco: British European (415) 861-3030 asked for Bob (excellent mechanic!)and The Car Clinic in Rancho Cordova California Glynn (916) 638-8290(also a great mechanic) near Sacramento California.
The symptoms I was describing sounded just like worn valve guides, worn piston rings, or even a cracked piston. That being said it would have cost me a least $1500 to $3500 dollars to fix. Of course, describing car trouble over the phone without an inspection doesn't give you a an great picture on what the trouble really might be.
I didn't have the money to tow the car to either location, so I intended on selling it on craigslist sometime soon. SHOCKING I KNOW!
The car was used around town to get groceries and run errands.
I started to notice a stain on pavement below the engine after parking the car overnight. I checked to see if this was an oil leak or coolant and it was neither.
I finally figured out that the carb was the culprit and decided to give it a closer look.
The carb turned out to be wet and oily and needed attention. I did a little research on the internet and found an article on www.mossmotors.com where a Moss Motors LADY technician said that a poor running carb would fowl up sparkplugs and give an appearance of oil fowled sparkplugs. It sounded like a long shot so I did more research on rebuilding the Zenith Stromberg carb. One article suggested only to get the manufacture carb kit due to it's precision of parts due to trial an error with after market kits.
I found the kit at http://www.mossmotors.com and ordered it.
I also found some great videos on YouTube with John Twist of University Motors. He owns his own shop in Michigan. This guy is great he has got about maybe 90 videos or so on MG repair. He had a two part video on how to rebuild the Zenith Stromberg carb. He had a two part video on how to rebuild the Zenith Stromberg carb. and part twoAfter receiving my rebuild kit and watching John Twist's videos I went ahead and rebuilt the carb.
I was a little intimidated with some of it's complexity and was wondering if this would be all in vain.
After reinstalling the Zenith carb, I took the car for a spin and VOILA the car ran beautifully it was incredible, the acceleration was there! The power was back! The oil consumption stopped too! I guess a poor running carb doesn't aspirate the oil vapors enough and send it directly down the exhaust if that's possible?
I want to thank John Twist of University Motors for his YouTube videos and the LADY technician from the Moss Motors forums.

Helped by a nice old lady!

Well, I have a little story here: It was a hot 100 degree day I decided to take a spin in my MGB and didn't realize that my the positive cable post on MGB wasn't tightened. I was about a few miles from house in Roseville California then suddenly my car died. Sounded like I was out of gas or running out of gas.
Anyways, there I was up at a small hill where I started to push my car towards the downhill part by the steering wheel with one hand jumping in then jump starting going downhill which worked! While doing that I dragged my left big toe on the pavement banged my other toe on the pedals (black and blue ouch!) Made it downhill turned around and came back stalled again almost at the top.
An older lady with an old faded blue Nissan pickup came along side me and asked if I need a push? I said sure! She then backed up behind my car pushed up against my rubber bumper MGB and started to reeve up her engine and began pushing my car up the hill and into a business area where a bar was. I was able to park the car and she parked her truck. She asked if I needed a jump. I told her I could call my fiancee to do that. I thanked her and then she went into the bar.
Well after about a half an hour of call attempts to reach my fiancee, I decided to take that offer to jump start my car. I went in the bar which was dark inside and very few people were there. The lady was at the bar enjoying a drink maybe a brandy and before I could say something she offered to help me again. She was a rare breed not to many people would do what she did.
She got in her truck backed up near the back of my car and I was able to hook my jumper cables and got the MGB started.
From there I was able to leave and we parted ways, she told me laughingly "to be sure you tell your fiancee that a beautiful lady helped you to get home!"
Well all this trouble turned out to be the alternator not charging at all and I was running on battery power all along!

Common cracks near mirrors

This I find this to be typical in MGB's: in front of both mirrors bases, a crack develops and (due to the weight of the mirrors?) That happened to me, so I decided to to drill a small hole at the the end of the crack. (which will stop it from cracking)
Then I went to Home Depot and bought a small sheet of steel or aluminum maybe 12" X 12". I used tin snips to cut a about of steel or aluminum inch wide and 3 inches long. With a hammer, I gave the piece of metal a slight bend by finding a solid rounded curvy shape object (like a vise)and tapping the edges and center to give it form. The form being the surface of the door, starting from the window opening and down along the crack. With the crack being in the center. Once I have the shape, I then place it firmly on the door crack. It's a good idea to smooth out the edges with a file or bench grinder. Then, I will use pop rivets: I then drill 3 to 4 holes on each side of my repair piece. I pop rivet my repair piece and voila! Steel or aluminum rivets should work.
After that I will bondo this area. Bonding is an art itself. But basically you put your first layer down, wait 24hrs to dry, sand with may 60 grit sandpaper, then 100 grit sandpaper. You will see that you have to repeat the process several times; bondo, dry, sand. On the last application of bondo and sanding, you'll want to use a wet-sand paper that is about 300 grit or so. You can find great tutorials on bondoing on www.YouTube.com.

Here's a pic after the metal repair piece has been placed, riveted and bondo



Here is where the repair piece is:



Of course you could have it done professionally: welding or put on a new door skin but most of us can't afford that.

MGB back from the paint shop!

Well I finally did it! I got my MGB painted here are some pics. Maaco had a sale for 249.00 dollars so I went with that plus their prep special for another $150.00 bucks. Because the guy at the shop was saying that would look much better. Anyways after $409 dollars I got my car back and it doesn't look too bad. I have had this thing (MGB) for over 15 years and drove it all over San Francisco and even commuted with it from Sacramento to San Francisco for about eight months. It looks great to me!


I need to install the side markers...





I am ordering the plastic attaching nuts for the air vent through www.victoriabritish.com




New tail light set would look great from www.victoriabritish.com for about $45 bucks plus shipping.





The front bumper needs a good shine with ArmorAll

The over spray of paint needs to be removed, either scrapped off or sanded off...More details to come!

Next! The Engine and Engine compartment

I started with the valve cover and air filter element. Hand sanding with rough sand paper 60 grit then 100 grit medium grit. I then used Rustoleum Flat Light Gray Auto Primer and then let both items dry for about 4 hours. Then for the finishing touch this paint by Rustoleum called 500 hundred Degree Engine Enamel really looks super! Now I just need some more elbow grease to finish up the engine compartment. It is kind of overwhelming but if you think to do one item or an area at the time you will eventually finish up!


What a difference!

Rusty trunk! Urggh!

My old 1976 MGB is finally being taken care for! The bottom surface of the trunk is oily, rusty with paint peeling. I am using course sand paper about 60 grade, on the non-oily surfaces. Then I use a 100 grade sandpaper. It's not a pretty sight and I am posting some pictures of the trunk's condition.


This is near the gas nozzle:


This image is from the whole trunk view, with the exception of the bottom of the trunk, the trunk overall isn't too bad for a 30 year old car.


I find that Simple Green has a great product to remove surface grease from the trunk of my car. I mix Simple Green Max (which is non toxic) I apply it with a sponge to get the oil of the surface of the trunk and rinse it. On a hot day it dries fairly quickly.

Links to websites for parts for your MGB Roadster

http://www.victoriabritish.com and http://www.mossmotors.com

MGB Cars Today

Owning an MGB no matter what year is a great feeling! You own one of the funnest cars ever made! That's one of the reasons for this blog. To talk about your cars and show pictures, link to useful information and tips. Talk about auto parts and service links. There is still a great number of enthusiasts out there! So here we go hopefully this will be a fun blog for all!
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