Before we had Fuel injetion we had Carbs.

BACK IN THE DAY WHEN THE CARBURETOR WAS ALL THERE WAS

Just imagine working in a GM dealership back in the mid-sixties or seventies or before. The vehicle was delivered to the dealership, and all factory air/ fuel mixtures and timing settings were based on sea level, or at the altitude at place of manufacture. That’s the way it was for most vehicles. Let’s just say the elevation at the selling dealer is 5000 feet above sea level. The predelivery of the vehicle went kind of like this: the technician would set the front-end toe/ in / out. They would also aim / adjust the headlights, verify all fluid levels were topped off and find all the grease fittings (among other things).

When it came to the engine, the first adjustment was base timing. Advancing the base timing by 1 or 2 degrees for every 1000 feet above 2000 feet was a good starting point. That restored about 2 inches of intake manifold vacuum at idle. After that, the technician would go after air fuel ratio adjustments, which the vehicle needed to run good from cold to normal operating temperatures. The technician would have to lean down most or all of the carb settings.

Starting with a cold engine, the automatic choke came first. By turning the thermostatic choke control spring in the lean direction (counter clock wise), the choke would open sooner, thereby leaning out the mixture faster on a cold start. If the choke setting was too lean, it would cause the engine to hesitate and / or stall during the warm up period. Early choke thermostats were bi-metallic springs that expanded and contracted with temperature. When cold, the spring contracted and closed the choke plate.

This action also brought the carburetors “Fast-idle” cam into play. When the choke plate closes it brings the fast- idle cam up to its high step position, this lets the engine run at about 1,500 RPM during the warmup period. Fast idle also allowed oil to sling on to the cam shaft for better lube.
As the spring was heated by the heat in the exhaust crossover passages in the intake manifold, it would expand and release the pressure on the choke plate allowing it to open fully. Later carburetors had the choke thermostatic spring attached directly to the carburetor housing with a tube connected to the exhaust somewhere to bring heat to the spring. The latest carburetors used an electric choke. The power to the electric heater type choke came from the oil pressure sensor that way if you turned the key to the on position without starting the engine the choke would stay closed until the oil pressure came up. On Fords choke circuit power came from the stator terminal on the alternator about 7 volts on that system. Again, no power to the choke stat was allowed until the engine was running.

Choke stat adjustments meant drilling out the three rivets and grinding off the tang that prevented the choke stat cap from turning at all. On the divorced choke heater style, the linkage arm was bent to adjust the tension. The next critical choke adjustment was setting the choke pull-off device sometimes called choke vacuum break. I like the term choke pull-off because that is just what it does. With the engine cold, you would step on the throttle before cranking the engine and the choke plate would slam shut fully and restrict all airflow. Once the choke plate was closed the driver would release the throttle and start the engine.


Without the choke pull-off, the engine would flood on start-up or run so poorly that you would have to increase the throttle opening until the engine started to run on the amount of air you gave it by opening the throttle. It’s easy to tell if the choke pull-off is adjusted incorrectly. On cold start if the engine runs rough and black smokes the choke pull-off gap it to narrow (rich). If the choke pull off (plate) is adjusted to wide, the engine will start right up then stall sometimes 2 or 3 times caused from being to lean on cold start.The choke pull-off is supplied manifold vacuum when the engine fires up. To adjust the choke pull-off with the engine off on a cold engine, a technician would open the throttle to allow the choke plate to slam shut, then you would use a vacuum pump to pull the diaphragm back and then check for the specified choke plate opening. Usually the opening was checked with a round wire gauge like a drill bit. About 3/16ths of an inch was a normal spec. The term slam shut is just what the choke plate should do on a cold engine. If the choke plate lazily closes means you need to adjust the choke spring in the rich direction, or it means you need to clean the choke linkage or find out why it won't slam shut (could be bent linkage or bent choke plate)
Choke pull-off is how much air the engine gets with the choke closed on cold start. At 5000 feet elevation, you will need to increase the plate opening more by turning the Phillip’s head screw or bend the linkage attached to the choke pull off to allow more air on cold start. Please keep in mind all these adjustments are in the service manual for sea level not for 5000 feet elevation. You had to rely on your experience tuning cars at your elevation and all your senses the good Lord gave you. A good Ass-ometer (YOUR BUTT) was a must in those days.
The air fuel ratio screws had to be turned in to lean down the idle air fuel ratio. Also, the screws sometimes had limiter caps to prevent you from turning the air fuel mixture screws too far in or out. Breaking the limiter caps or drilling out the lead plugs that were sometimes installed at the factory to keep you from even getting to the screws was your only choice. If you were lucky, you would use a two-gas analyzer to get the adjustment spot on. If not so lucky, you would use a tach or a vacuum gage to get the adjustment spot on. If you didn’t have any of those tools at your disposal, you would rely on the experience you learned over time. The air fuel mixtures screws should only be adjusted when the engine is at normal operating temp, ignition base timing is set, idle speed is set, and the a/c is off (in that order).



With the engine running, the goal is for the idle air fuel ratio screws to be turned in lean as far as possible yet have a smooth idle, or the highest possible vacuum or rpm.
If you are buying a new or reman carb and the sales person tells you all adjustment are done just bolt it on, PLEASE DON’T BELIEVE THAT. Carbs are adjusted or tuned based on lots of stuff, stuff you need to know.
The next thing you might have to do to lean the mixture in the progression stages (off idle and higher) on some carburetors the mixture control was done by metering rods (or needles) that were positioned into the center of the main jets. The metering rods were controlled by springs that lifted the metering rods out of the jets (increasing fuel flow or allowing a richer mixture) a port inside the carb applied manifold vacuum to a plunger or piston that drew them down into the jets for leaner operation. To lean the midrange, installing a lighter spring under the metering rods was a common practice. The metering rods are held down (lean) by high manifold vacuum at idle. As you increase throttle opening, manifold vacuum starts dropping off, and the spring under the metering rods pushes the needles up causing the mixture to go richer. A lighter spring would push the needles up slower as the vacuum starts to drop off due to throttle opening. It’s a really neat balance between spring pressure and vacuum, I hope that makes sense. Before the carburetors were kicked to the curb because they could not keep the air fuel ratio in check for a long period of time. The feedback carb was used for a few years trying to by some time for the manufactures to come up with the next fuel system. The feedback carb used a duty cycle solenoid to control the metering rods to maintain 14.7 fuel ratio. This 14.7 fuel ratio is ideal for the catalytic converter to work efficiently. There was more to the feedback system, oxygen sensor, map sensor, TPS sensor, coolant sensor, engine control module.
Keep in mind, the manufacturer built the engine with either high compression or low compression, and they would recommend high octane fuel or low octane fuel based on the compression of the engine. High compression engines need a higher-octane fuel (slower burn time) to reduce spark knock (pinging) and cylinder detonation. Pinging and Detonation can cause severe engine damage and shorten the engines life. Pinging you can hear, but detonation you can’t hear. Detonation can be detected, but that is getting off the subject. Keep in mind when an engine operates at higher elevations, the need for high octane in that same engine is reduced, and the air fuel ratio needs to be leaned down.
Carburetors could lean down the air fuel mixture a small amount based on the atmospheric pressure pushing on the fuel in the float bowl, at high elevation you have less atmospheric pressure so just that alone would lean the engine down a small amount. Remember that float level adjustments are critical to mixture control.
The technician had to understand what adjustments the carburetor needed for it to allow the engine to run as cleanly as possible at 5000 feet elevation, and the GM Quadra Jet 4 Barrell carb would perform as good as any throttle body injection out there if setup correctly. But because carburetors could change adjustments due to were and tear on moving parts and deterioration of seals and gaskets, tailpipe emissions were hard to keep in check and fuel economy was not as good as it could be.
Fast forward to today, and high-pressure Direct injection is what the manufacturers were forced to develop. Even the high-tech fuel and ignition systems of today have their limitations and issues.
All engines, gas or diesel, have a sweet spot at a certain RPM and that forced the engineers to develop multi-speed transmissions to hit that spot, and also to satisfy the EPA emissions and meet Corporate average fuel economy (you get the idea).
The automatic transmission was referred to as vacuum controlled, meaning that the line pressure and shift speeds were controlled by a vacuum modulator. The vacuum modulator was a small can-looking thing sticking out the back or side of the transmission. That funny looking can has a vacuum line going to it coming from the intake manifold or carburetor. It needed to be manifold vacuum not ported vacuum (vacuum controlled). With the engine idling, you would have the same vacuum the engine could make based on elevation and other things. At idle, vacuum is high so the pressure is low in the transmission. When you accelerated, the throttle plates in the carb open up and the vacuum starts dropping and raising the line pressure in the transmission due to lower vacuum. Speaking of low vacuum, 5000 feet elevation, the transmission would shift later and firmer due to the lower engine vacuum. The fix was to install an aftermarket adjustable vacuum modulator. This modulator has an adjusting screw to allow you to relax some of the spring pressure inside the modulator, restoring shift feel and shift speed.
The vacuum modulator is built and works like the vacuum advance on a distributor. The kickdown cable has to be adjusted in a way that kickdown or passing gear does not happen before the throttle is put to the floor. If the kickdown shift accurses before full throttle the manifold vacuum is still a little high causing the pressure of the trans to be low. Please don’t confuse a part throttle 3/2 down shift with kickdown or passing gear, part throttle 3-2 down shift is happening under a light load and that's ok.

The late style transmissions and transaxles are referred to as computer-controlled units. Vacuum was a good way to determine load. Load is calculated by the map and mass-air flow sensors on the late style units, which control the electronic pressure control solenoid acting on the PR valve. These sensors are load sensing devises and so was the vacuum from the engine (high vacuum low load low vacuum high load)

In addition to being a great refresher course what does all this mean?

Today’s engineers and software tuners would have to start from scratch without this kind of information.

Nothing has changed when it comes to what an engine needs to run correctly. Intake, Compression, Spark, Power and Exhaust and the right amount of fuel based on engine specs, elevation, temp, and all at the right time. Today there is just newer technologies to keep things in check.



Paul Townsend
Paul’s Transmission & Repair
Baker City, Oregon 97814
541-523-6923

We were lucky here in Europe most cars had manual chokes it was really just some of the bigger (by our standards) Fords and Vauxhall that had automatic chokes. Although a few Jaguar, Rolls-Royce and Daimler had a separate electrically operated starting enrichment device controlled by a thermostat --- really a starting carburetor.

SU carbs so simple they were really a mechanical MAF but were prone to wear and every couple of years would need a strip down and clean and few float valve. However when emission controls controls came in renewing the needle and jet became a regular job. The Zenith-Stromberg CD carb was pretty similar but used a rubber diaphragm and was less prone to wear but changing the diaphragm was a biannual job.
Setting the mixture and synchronizing twin or triple carbs carbs especially SU types on MGs was never a job you could be 100% happy with result on. On the MG because of the port layout you would end up with uneven fuel air ratios on th inner and outer cylinders, this was why MG switched to a single Zennith-Stromberg CD carb for the US market.

The Fomoco carbs on the entry level European Fords used to wear out after two or three years use there would be so much wear in the butterfly spindle bearing getting a decent idle mixture and steady idle speed was impossible, they also caused a lot of cranking no starts due to flooding in cold weather. We converted quite a few of these to basic Zenith, Solex or Weber carbs, The more upmarket UK Fords used Weber designed twin choke carbs that were much superior.

The normal Weber carbs were virtually zero maintenance only job would be fixing the occasional blocked idle jet. Where a pair Weber DCOE carbs were fitted (Lotus most Alfas, some Chrysler UK) synchronizing the butterflys was fairly easy using the traditional length of heater hose as a stethoscope but getting the 4 idle mixture adjustment (almost) right ******* me. In those days the best method I found was using a device called a Gunsons Colortune, this was a glass spark plug. that let you look into the combustion chamber of the running engine ..................... a video paints 1,000 words.
It would be interesting to try one of these today with the ignition secondary hooked up to a scope and looking at the exhaust with a gas tester

Thank you for the great video and interesting info in your part of the world.