Gastester Digital

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This article is from British Car magazine dated 1993. The Gastester reviewed was the original MK II. This unit was replaced by the Gastester Digital but the concepts are the same. John Abbott

By Lorenzo Dunn

Failing the emissions test. Lack of power. Poor gas mileage. Does this sound like your car?

These maladies are often signs of a British car with improperly adjusted carburetors.

Carburetor woes are more common with a British car than With most other cars on the road simply because British cars are usually equipped with highly adjustable SU or Stromberg carburetors. These deceptively simple instruments are a source of rough running, poor gas mileage, and failed emissions tests second only to worn out Lucas ignition systems. The pity of it is that the innocent carb is rarely broken - the problem is usually just the way it's adjusted. Fortunately, with the aid of a simple CO meter like the Gunson's Gastester MK2, British cars are also among the easiest to get in tune.

Both SU and Stromberg carbs rely on an adjustable jet or needle to set the mixture. All carbs have an idle mixture setting but on SU and Stromberg carbs the idle adjustment also fine-tunes the mixture throughout the carb's entire range of operation. Most conventional carbs have fixed jets to control the mixture off idle, so a badly adjusted idle mixture is not a big problem; it makes no difference at other throttle openings. That's not the case with SU's or Strombergs - idle mixture influences all throttle openings.

The British car carburetor's adjustability is the proverbial two-edged sword. If you've just moved from the seacoast to the mountains or added a new free-flow exhaust system, the adjustability is wonderful. in many cases a few minutes spent adjusting the mixture will get you in tune whereas other carbs would require an expensive teardown and re-jetting. Talk to somebody who has spent hours elbow deep in jets and emulsion tubes while tuning a Weber and the ease of adjusting an SU doesn't seem half bad. However, if your car has been worked on by somebody who isn't familiar with British carbs or it's been the property of a less than experienced owner, the chances are that the carb is out of adjustment - maybe way out!

Engines need a very definite mixture of fuel to air to run at their best. A mixture of 14.7 parts air to I part gasoline is called the stoichiometric ratio - a
term that you may recall from a long-ago chemistry class. Near the stoichiometric ratio is an idle mixture for emission control. Engines need a slightly richer mixture at idle and a much richer mixture when starting or accelerating. They can run on a leaner mixture when running tinder a light load.

This difference in mixture for varying conditions is called mixture spread and in a SU or Stromberg, it is taken care of by the carb needle profile, so you don't need to worry about it. Your task is to set the idle mixture correctly and let the carb take care of mixture throughout the rest of the range.

In days gone by, it took the powers of Merlin to read the signs and get the idle mixture set correctly. Adjusting the mixture was a test of a mechanic's faith and ability to ken the unknown. You'd set mixture by following a mysterious ritual in the manual: "Lift pin 1/32 of an inch or so. if the engine picks up, the mixture is rich." Frankly, I had better luck using an Ouija board to commune with the spirits of combustion!

The more pragmatic approach was to start by adjusting the carb rich, then leaning the mixture out until the engine speed increased and then just began to drop. Less arcane, but still only good enough to get mixture in the right ballpark.

These days, 'in the ballpark' isn't good enough. Your engine can seem to run quite well with the mixture set incorrectly (especially if the mixture is too rich). But the emissions test station (and, over time, your mixture set incorrectly wallet) will detect the difference.
A car that sounds fine may still be spewing pollutants (and getting significantly worse mileage than it could be getting). At the other extreme, a lean (or 'weak', in British English) mixture can cause loss of power and (if bad enough) poor idling or even engine damage.

Years ago I was taking a summer school auto shop class while trying to coax a very worn-out Mini 850 into daily commute duty. I brought it into the shop and told the instructor that I wanted to rebuild the carb. The car was difficult to start, getting bad mileage (hard to do for a Mini) and generally running miserably. He told me the biggest problem with my carb was the location ... right on top of the engine where it was all too easy to fiddle with! After a tune-up and a mixture adjustment the car was running like a champ again and producing its full 34 HP of pavement rippling power.

I found out the hard way that the same adjustability that let the BMC competition garage tune Timo Makinen's Mini Cooper for precisely the conditions of the '67 Monte made it easy for me to comes with a complete, well-written instruction booklet. When used correctly, it can enable a novice other links (correct ignition settings, engine condition, float level, etc.) are in place you are likely to have a very frustrating time tong to get the engine running perfectly. Adjusting mixture should be the last step in a tune-up, not the first.


There are a few things you need to do before using the Gastester. Setting the mixture is a link in the chain of proper engine operation; unless the other links (correct ignition settings, engine condition, float level, etc.) are in place you are likely to have a very frustrating time tong to get the engine running perfectly. Adjusting mixture should be the last step in a tune-up, not the first.

The first thing to do is to make sure your car's ignition system is in excellent shape. in general you should start your first mixture fine-tuning session after doing a complete ignition tune-up; new plugs, points, distributor cap, rotor and plug wires. This is especially true if you've just bought the car and don't know the history of these components. I've seen ignition problems sideline British cars way more often than carburetion problems. Ignition tune-up procedures should be covered in your shop manual.

Second, take a look at the overall condition of your car's engine. if it's burning a little oil don't worry about it. That won't make a big difference. However, if you have a cylinder that is down on compression from bad rings or a burned valve, then you need to take care of that before expecting miracles from a carb tune. Third, check to see that your carb and intake system are in generally good shape. Any leaking gaskets, broken vacuum hoses, or loose intake manifold studs will make mixture adjustment difficult or impossible. Make sure that your carb doesn't have a leaking float valve. If there's fuel on the top of the float chamber and a smell of gas under the hood, the float valve is probably leaking and you have a dangerous condition to correct.

It's also worthwhile to take the float chamber top off and make sure the float level is correct if you're in doubt. Check your shop manual for details on how to do this. On older SU carbs it is pretty easy to do; you don't have to take the whole carb apart. On SUs and Strombergs it's much more involved, but these carbs tend to have fewer float level problems anyhow. Unless the float level is correct, setting the mixture adjustment at idle may not make it correct at other throttle openings.

The one last thing to consider is whether your carb has the right jet, needle, spring, etc. for your engine. Why wouldn't it? Well, one SU or Stromberg carb looks much like another and it isn't unusual to find a car where the correct parts have been replaced by whatever was handy at the time. if the needle, etc. is wrong, you can get the idle mixture set correctly and find that mixture is wrong at other engine speeds.

If your car has a modified engine it may need a different jet/needle combination than the original one. Getting that set up is more involved than a simple mixture adjustment; get a tuning manual for your car if you're running something other than stock. Having said all that, don't get too worried - if your car has run well in the past you probably have a jet and needle combination that works. If you tune the ignition, adjust the mixture at idle, and the car still doesn't run well, it's worth checking to see if this is the problem.

Get familiar with mixture adjustment on your carburetor. Do it with the engine cold so you won't have to feel out unfamiliar carb parts on a hot engine! Check your shop manual or one of the manuals specifically for your type of carburetor. There are quite a few different mechanisms for adjusting mixture and a special tool is usually required (or at least makes the job easier). These tools are available from most British parts suppliers and don't cost too much.

Tuning Single Carbs

When you are (finally!) ready to tune the carb, get the car well warmed up. Letting it idle in the driveway won't do it. Go for a good drive until the temperature gauge is up to normal. Make sure the choke is fully pushed in. If you're not sure about the choke adjustment check the carb linkage to make sure the choke isn't sticking before setting the mixture.

At last you're ready to connect the Gastester, let it warm up and calibrate it. Connecting the Gastester is simple. You hook up the power leads to the battery and turn the machine on. On my Mini, the battery is in the boot so the power leads have only a short way to go. if your car has the battery under the hood you may need a set of jumper cables or some other extension to the leads. Gunson's could have made the leads a bit longer to make this easier.

Warm-up and calibration is where the low-cost nature of the Gastester reveals itself. You need to do calibration by letting the Gastester warm up with the probe sitting in clear air (outside the exhaust pipe) for eight minutes and then use the calibration knob to bring the meter needle back to its calibration point. The instruction book covers this in more detail; it isn't a big deal aside from the wait.

In my experience it isn't a bad idea to repeat this process every half hour or so during use to ensure that the calibration is still correct. As the Gastester is used it may drift and require adjustment. This would be an inconvenience in a busy mechanic's shop, but it is something that the enthusiast mechanic can easily five with.

Once the Gastester is calibrated, put the probe in the exhaust pipe and proceed to set the mixture. You want to get the probe about eight inches into the exhaust pipe and you want to make sure that the hose is routed back to the meter without any kinks in it. Position the meter where you can see it while poking about under the hood (or enlist an assistant). Let's look at a single carb car first, then we'll delve into the more complex situation of multiple carbs.

As you richen the mixture, the Gastester will indicate a higher percentage of CO. As you lean the mixture out, the Gastester will show a lower CO percentage. As somebody said, "It's just that simple!" It does take about fifteen seconds for the meter to react and a minute for the reading to fully stabilize, so make your adjustments slowly and a little bit at a time. Let the meter catch up before making more adjustments.

If you've done mixture adjustments without a meter, this is when you'll start to smile. It's great to see the result of your adjustment instead of struggling to hear a change in the way the motor sounds. You know what's happening instead of guessing. You may be surprised (I was) how a several percent change in CO% makes no difference in how the engine runs at idle.

Your state may have particular CO% limits for emissions tests or your car's manual or under-hood sticker may show a CO% setting. If not, aim for a CO reading in the middle of the green zone on the meter - between about 2% and 5%. In general, the lower percentage end of the range will give you better economy, the higher end more power.

Incidentally, you may see very rich mixtures used on race cars. Don't try it for the street. Race engines run rich to get the advantage of the cooling effect of extra fuel vaporizing. Racers change their oil after every race, rebuild engines frequently and don't pay much attention to fuel economy. Unless you want to do the same, avoid ultra-rich mixtures.

If you're after economy or trying to pass a smog test, you may be tempted to go for a very low CO% setting. It's false economy because a mixture that is too lean can cause engine damage. And it won't help you at emissions test time because too weak a mixture won't burn efficiently and the unburned gasoline will show up as a high HC (hydro-carbons) reading - something that doesn't show up on the CO meter. Aim for the midrange and you should be OK

You may find that you need to raise the idle speed a bit while setting the mixture (especially if your carbs are a bit worn). That's fine as long as you're not above 1500 RPM or so. just use the idle speed adjustment screw to temporarily set the idle higher. Don't pull out the choke knob to get a fast idle - you run the risk of going too far and having the choke richen the mixture.

With one carb all you need to do is set the mixture, adjust the idle back to what it should be, pack up your gear and go for a country drive. Maybe this is why big single carbs are the hot setup for Minis these days! With multiple carbs things are a bit more complicated.

Tuning Multiple Carbs

The CO percentage the meter measures at the tailpipe depends on the settings of all of the carbs, not just the one you are adjusting. Unless your car has a pretty unusual intake and exhaust system there will be mixing between the carbs before the intake charge enters the cylinders and mixing in the exhaust system before the tailpipe. The CO meter will see a mixture that's the average of the mixture and airflow through all the carbs.
It's possible to have one carb set too rich, the other too lean and still have a mixture that, on the average, looks correct to the CO meter. This actually happened to me with an old twin-carb Volvo 122S. A slipshod mechanic at the smog test station simply leaned one carb out all the way to get the car to pass the test. This isn't good for power, mileage, engine fife, or emission control.

To prevent this, what you need to do is ensure that all the carbs are set to the same mixture adjustment when you start and adjust them all equally when you are setting mixture with the meter. This is easier said than done, but it isn't really all that difficult. More on this later. The other thing that you must do (and this is good just for general performance as well as setting mixture) is make sure that the carbs are balanced (or synchronized) so that each carb has the same amount of air flowing through it. Your shop manual will spell out the procedure for doing this. Basically, what you are doing is disconnecting the throttle linkage between the carbs, setting each carb's throttle plate to the same position so the same amount of air will flow through each one and then reconnecting the linkage before moving anything!

The difficult part of this is ensuring that each throttle plate (or butterfly) is open the same amount and therefore the same amount of air is passing through each carb. There are airflow meters that can help this process or you can rely on the age-old rubber hose method (one arcane bit of British car lore that is actually easier than it sounds).

With that preparation out of the way, you need to make sure that the carbs are all at the same mixture setting. To do this, adjust each carb all the way to its full lean position (with the engine off). Count how many turns of the adjusting screw or 'flats' of the adjusting nut gets you to full lean.

Next, richen the mixture on each carb back to roughly the original setting. You do this by adjusting each carb by an equal amount. Be careful when doing this - count quarter turns of the adjusting screw or 'flats' on the adjusting nut. You want to have each carb set to the exact same mixture setting. It won't be the right setting yet, but each carb will be wrong by the same amount! There's another way to do this on SU carbs, recommended only for the terminally precise. Take the suction chamber (die top part of the carb) off and actually measure the position of the adjustable jet and ensure that it is the same for all carbs. What you want to measure is the distance between the jet and the top of the carburetor bridge.

This operation requires a deft touch - you need to be able to pull the suction chamber off straight to avoid bending the needle and you have to be careful not to drop the piston (which is spring loaded inside the chamber). If you've rebuilt your carb a few times this will be a simple enough operation and not as difficult as it sounds, but you do need to know the innards of your SU pretty well. Spend a night curled up with a carburetor manual (some fun ... ) before you attempt this method!

After you have synchronized the carbs and set them all to the same mixture setting, you are ready to tune. Start your car and adjust the mixture as you would for a single carb. However, make the same adjustment on each carb in turn. in other words, if the car is running rich, lean the mixture by the same small amount (like one flat or one quarter turn) on each carb. Wait for the meter to stabilize and then continue the adjustment. By doing this you are keeping the mixture setting the same between the carbs, adjusting the multiple carbs as if the mixture adjustment screws were linked together.

When you're through with this, your car should be ready to give peak performance as well as good economy and low emissions. Check the mixture from time to time with your CO meter. You'll be surprised at how little it changes. if the CO percentage does change drastically, suspect the ignition system, float level, or an intake manifold leak before you start playing with the mixture adjustment. It's actually very rare that a correctly adjusted carb needs to be fiddled with; they usually need adjustment because somebody else has messed them up, some other engine parameter has changed, or after a rebuild.

All in all, I was impressed with the Gunson's Gastester MkII. it is a basic, no-nonsense machine that makes mixture adjustment a straightforward process instead of a test of patience. The Gastester MkII is without frills, but it is simple to use and gets the job done. it's a worthwhile investment for the enthusiast mechanic. It could pay for itself quite quickly in avoided retests at the smog station, saved gas, and fewer trips to the mechanic.