Tuning Multiple SU Carbs

A long time ago I published this article on a web page, and it seems high time that I bring it into the 21st Century...

Tuning 101:
Getting your SU's set properly.
...or...
How to get your car running the way you want, without frustration.

Note: This page is under continual construction/improvement as I learn or think of new stuff...

The biggest hassle with getting an MG (or other British car) running properly is multiple SU carbs. Not that there's anything wrong with the carbs themselves, but there's this incredible temptation to "fiddle" with them, and the range of possible adjustment is large and varied enough that you can really get things out of whack in a hurry. This tends to result in owners going prematurely bald from tearing their hair out trying to get the car running properly.

I've been there, done that, and fortunately my scalp healed up nicely. However, in the interest of keeping other MG owners from having to wear baseball caps ALL the time to hide the missing patches, I humbly present a methodical process for making your MG run it's best.


DISCLAIMER:  This is just one person's experience.  If you prefer to do things differently, fine.  If you've had success by some other method, good for you (but stay away from my car).  Also, I can't possibly cover everything that might be wrong in one article (or even six).  If you've got a problem that this doesn't deal with, contact me and we'll see if I can help you figure it out.

I will assume that your carbs are in reasonable repair (not in need of new seals, not leaking gasoline, not dropped from a height of 150 feet onto a granite outcropping). We will be checking for some of the more insidious possibilities as we go along.
 
 

Tools required: Screwdriver (flat blade) Wrench to fit valve cover hold-downs Wrench to fit valve adjusting lock nuts Wrench to fit Distributor clamp Spray Carb Cleaner or Starting Fluid Feeler gauges Timing light (if you need/prefer to do dynamic timing) White-Out (for dynamic timing) 12V test lamp (if you need/prefer to do static ignition timing) SU adjustment tools

Tools that are nice to have, if you like them: Dwell/Tach (I use this regularly) Gunson ColorTune (I have one and use it sometimes) Gunson Clik-Adjust (I have one, use it regularly, and love it) Uni-Syn or other Carb Synchronizer (I do without this. Had one, didn't like it, sold it. The fella I sold it to likes it, so different strokes...) Vacuum gauge (one of those tools that you'll rarely need, but when you need it, you REALLY need it) Compression Tester (ditto) Point file (for the truly meticulous)  Pediatric ear syringe (handy for emptying float bowls)

Parts you may need (only if you find they are messed up): Valve Cover Gasket Spark Plugs Spark Plug Wires Distributor Cap and Rotor Points and Condenser

First Principles:

Vacuum controls everything else, therefore, get the vacuum right before pursuing anything else.

The factory had a pretty good idea what they were doing, so no matter how radical you want to get, start with the factory settings¹.

Ignition and Carbeuration affect each other. If one is way off, the other can be set up to compensate, but at a cost in efficiency or power.

Patience is a virtue. So is being methodical. If physicians worked on us the way we tend to work on our cars, there'd be a lot more dead people.

Okay, now to start getting into the meat of things.

Step 1: The "Cheat Sheet"
The first thing you do is sit down with the manual and write down, in big friendly letters and numbers (so you can read them in poor light, at a funny angle, with sweat running in your eyes) the following statistics.

• Valve Lash (and whether it's set HOT or COLD) {If you use a Clik-Adjust, you'll want to record the number of clicks, once you calculate that.}
• The most efficient order in which to set the valves (under the valve adjustment section)
• Points gap
• Dwell (if available)
• Spark Plug Gap and Type
• Timing (and whether it's set STATIC or DYNAMIC, and if Dynamic, whether it's set with the Vacuum to the distributor CONNECTED or DISCONNECTED)
  

Trust me, you won't remember them long enough, particularly if you work on multiple cars. I actually have one prepared for each of our cars, laminated so that it can survive multiple tune ups and not become an oily, smeared mess. I use one of those "magnet-with-a-clip" jobbies to hang it under the bonnet while I'm working.

Step 2: Adjust the Valves
Some people skip this step, at their peril. Remember our first First Principle? (Vacuum controls everything else...). The vacuum is created by the airflow into the cylinders from the intake manifold and the flow of gases out of the cylinder through the exhaust manifold. The valves control that flow of gases. So the first thing to get done is to adjust the valves.
Valve lash controls three things, one of which is in direct conflict with the other two.

• Valve lash is the final arbiter of valve timing (smaller lash opens earlier and closes later, wider lash steals power).
• Valve lash is the final arbiter of valve lift (smaller lash opens the valves further, wider lash steals power).
• Valve lash provides the only opportunity for the valves to dissipate heat into the cylinder head, where it can be carried away by the cooling system (smaller lash makes burned valves more likely, wider lash keeps them cooler).
  

The valve lash specified by the factory is a compromise between the first two and the last one. Personally, I don't like doing valve jobs, so the fact that the factory is conservative is fine with me.
Okay, let's get to it:
Preliminary: Are your valves supposed to be set HOT? Then start the car and get it to operating temperature.

1. Remove the spark plugs (this makes it easy to turn the engine over). Set them where you'll be able to remember which cylinders they came from.
    
2. Remove valve cover. If the previous mechanic had their act together, and the valves were last adjusted sometime this decade, it should come off easily. You only need a new valve cover gasket if the old one gets damaged in this step.
    
3. Look at the table of valve adjustments that you copied down in the preliminary step. It will tell you what valve you want to see all the way open, to adjust the one that's all the way closed. Turn the engine over and watch the valves.
    
   • Some people bump it over with the starter. I prefer to use a hand crank (if you've got one) or put the car in 4th and pull it towards you. Much easier to get the right position.
   • Another trick is to put the box-end wrench for the adjusting locknut on the rocker that's OPENING. It's easier to see the movement of the shiny wrench than the dark rocker arm.
     
4. Loosen the locknut on the CLOSED valve & slip the feeler gauge in between the valve stem and the rocker. Turn in the screw until you just feel drag on the feeler gauge. Hold it there while you tighten the nut. Repeat (3) and (4) for the other 7 valves.
    
   • Check your work after you tighten the locknut.  Use a feeler 0.001" larger and one 0.001" smaller than the desired setting.  The smaller one should have no appreciable drag, while the larger one should have a lot of drag, if it fits in at all.
   • If your checks seem to always come up off, make sure you're holding the screw steady while tightening the locknut.  You actually have to apply some counter-clockwise force to the screw as you tighten the nut down, or it'll close.  But be careful, apply too much, and it'll open.
   • You don't have to crank down on the locknut with all your might.  In fact, you shouldn't.  Just "snug" will do.
     
5. Replace the Valve Cover (with a new gasket if needed). If you have to replace the gasket:
    
   • Clean both the valve cover where the gasket goes and the top of the cylinder head scrupulously, and don't let any of the crud fall down the pushrod openings.
   • Use a gasket sealer between the gasket and the valve cover, and a wipe of grease between the gasket and head, and you'll be able to use that gasket for years.
     
6. Take a look at the plugs before you put them back in. Take notes, because this is the best look you're going to get at what goes on inside the combustion chamber, unless you want to spring for a Gunson Color-Tune.
    
   • The electrodes should be nice and square, the gap should be right, and the deposits should be a light tan and dry.
   • White (or no) deposits mean you're too lean.
   • Dark and dry means you're running rich, dark and wet (smelling of gasoline) means you're REALLY rich, and you may have some problem in the carb. We'll get there later.
   • Black and oily and you're burning oil.
     
   Compare the different cylinders, it may show you something useful about what is out of whack with which carb.
   If you've got access to a compression tester, this would be the time to use it. All of the cylinders should be close to the same reading. If one's way off, put a teaspoon of oil down that bore and test again. If that fixes it, you are looking at a ring job, the sooner the better. If it's still way off, re-check the valve lash for that cylinder. If it's still way off, you're looking at a valve job, the sooner the better. You can finish the tune-up, but understand that with a compression loss in one cylinder, there's only so good you're going to get.
    
   
7. Put the plugs back in, or replace them with new ones if it seems reasonable to do so.
   

NOTE: I consider a Gunson Clik-Adjust to be one of the best "specialty" tools I've ever purchased. I can adjust the valves on either of our MGs in about 15 minutes with it (from Cover off to Cover on) and know that they're right on the money. Setting one up is well described in the instructions that come with it.  If you're curious as to what it is and what it does, go to http://www.mgcars.org.uk/MG_Elec-Tech/Clikadjust_0.html.  A few people have also asked where to get one.  Other places may have it, but I got mine at Little British Car Co.

Step 3: Vacuum Leaks
Okay, now you've got the baseline vacuum as stable as it's going to get. Start it up and let it come to operating temperature. With the car running, spray your carb cleaner or starting fluid on the carbs where they meet the manifold, the manifold where it meets the head, the throttle shafts at the carbs, and all vacuum fittings. If the car seems to run differently when the spray hits, you've got a vacuum leak there. Fix it. If it's the throttle shafts on the carbs, you may have to put up with it for a while, or shell out $$$ for rebuilt carbs.
Do not seek perfection until you have the vacuum leaks resolved, for down that road lies madness....

Step 4: Timing
If you're absolutely certain that your ignition system is perfect, and the timing is spot on and stable, you may skip this. If you can't get the car running right in the next step, however, come back here and do this stuff. Remember the third and fourth First Principles. Get the ignition timing right before going to the carbs. Timing can be easily checked and verified, and is pretty mechanical. "Right" (as defined by the factory) is easily identified. So get the ignition "right" before going to the arcane mysteries of carbeuration, and you won't be sitting there wondering if you should have checked the ignition. Once you have a baseline on ignition and carbs, you can "tweak" from there, just make sure you know how to get back to where you were. Take notes, and read the end of the article on The Notebook.

1. Remove the distributor cap and rotor.  Got Points? If not, lucky you. Skip to (4)
2. Take a close look at the points with a strong light. The contact surfaces of the points should be parallel and flat. Worn points will have a slight pit in one, and a slight "peak" in the other. If it's slight, use an ignition file to square the point with the "peak" and make sure the points meet squarely when closed. If the points are badly pitted or burned, replace them and the condenser.
3. Turn the engine so that the points are fully open. Check the point gap with your feeler gauge. If it's off, check your manual for how to adjust it.  This is a pain in the posterior, and many people put in electronic ignitions for this reason. I have one car with, and one without. On the one without, I have a second distributor with fresh points and condenser, so if they're bad, I just swap the distributor and replace the bad points at my leisure. This is probably a good time to put a few drops of oil on the felt pad in the end of the distributor shaft (if there is one), and some light grease to the points rubbing block.
    
   • If you have a Dwell Meter, this will allow you to set the points with more precision.  Look at the instructions that come with the meter, or hang in there for a future article.
     
4. Take a close look at the rotor. The contact should be reasonably clean and smooth. If not, clean it up or replace it.
5. Same with the distributor cap. Check the contacts on the inside, and look for matte black "tracks". See any? Replace the cap.
6. If you're replacing the cap, I usually replace the wires as well. If one's gone bad, the other is soon to follow.
7. Set the timing.
    
   • STATIC TIMING Turn the engine (in the normal direction of rotation ONLY) until the timing marks line up.  (Turning it the other way will introduce error into the timing from slop in the timing gears).  Disconnect the contact wire from the coil (the contact wire is the one that goes to the points, + on a Positive Ground car, - on a Negative Ground car). Clip your 12v test lamp between the wire you just disconnected and the opposite terminal  on the battery (connect to + if it's a Negative Ground car,  - if it's a Positive Ground car). Loosen the clamp that holds the distributor steady. Is the light on? Turn the distributor body clockwise until it just turns off. If the light is off, turn the distributor body anti-clockwise until it just comes on, then clockwise until it just turns off. If you don't have a test lamp, you can eyeball it. You want it at the point where the block on the points is just being touched by the cam. Tighten the clamp, but take it easy. It only needs to be firm enough to keep the distributor body from moving. Put the rotor and cap back on.
   • RUNNING TIMING Put the rotor and cap back on. Find the mark on the timing cover where the timing is supposed to show up (your manual should have a diagram). Put a dab of white-out on the spot. Turn the engine over until you see the TDC mark on the crankshaft pulley (your manual should have a diagram). Put a dab of white-out on the spot. Start it up. No start? Check to make sure that you put the points together properly and have the plug wires in the right places (see notes below). If you're supposed to disconnect the distributor vacuum, do it, and plug the line (golf tees work well). Clip your timing light to the #1 plug lead, and shine the light on the timing marks. Do the dots of white-out line up? ("Pretty close" is good enough, and it may jump around a little.) Great. If they don't, loosen the distributor clamp and rotate the distributor until they do. The car may start running pretty badly, but don't sweat it. Tighten the clamp, but take it easy. It only needs to be firm enough to keep the distributor body from moving. Don't forget to reconnect the vacuum line, if you disconnected it.
     
   A few tips on Ignition:
   
   
8. A car that runs fine for a while, then dies, but comes back when it's cooled a bit usually has a bad coil or a bad electronic ignition control or sensor. Both tend to "degrade" to a point where they run OK when cool, but overheat and fail. Don't wait to replace them, because sooner or later, they won't come back.
    
9. Replacing plug wires or cap (or both)? One plug lead at a time, please. It's way too easy to get them mixed up, and you'll go NUTS if you get it wrong, and feel foolish when you discover what the problem is.
    
10. If you can't quite see to work on the points, take off the distributor cap (with the plug wires still on it), and pull the distributor by unbolting the mounting plate from the engine. It will only go back in one way, and the mounting plate will keep the timing in roughly the correct position. Replacing points and condenser is MUCH easier with good light and access on a workbench than in the dark, confined, recesses of the engine compartment. As I noted earlier, I keep a spare distributor with fresh points and condenser sitting on the bench, gap set and ready to drop in.
     
11. If you're replacing points, pay close attention to the order of terminals, washers, insulators and spacers on the shaft. Get it wrong and the car won't run, and the reason won't be obvious without disassembling it again.
     
12. While you've got the distributor open, grab the end of the distributor shaft (where the rotor goes) and wiggle it back and forth (not twisting, but straight back and forth). There should not be perceptible "wiggle" in the shaft. If there is, you're going to have trouble getting the point gap consistent and the timing right, because the wobble in the shaft will screw up the points opening and closing
    

Step 5: Carbeuration
Finally! We're to the main event. Really, though, without checking all of the preliminary stuff, you don't know for sure that what you do to the carbs will be the best you can get, or just compensating for some problem elsewhere.
The goal here is to get each of the carbs independently metering the correct amount of fuel, the same way, into the same volume of airflow, under the same conditions. There are essentially three steps to this process

First: Get the carbs flowing the same amount of fuel.
Second: Get them flowing the same amount of air.
Third: Get them both flowing the correct amount of fuel for the amount of air.

Take them one at a time, and you'll be golden. However, let's take a moment to check a couple of sneaky problems that can crop up.

SNEAKY PROBLEM 1: Sticking Piston Take off the air filter on each carb, remove the damper caps (the little knobs on the top of the carbs), and use a finger to push the piston in each throttle bore all the way up. Let go. It should drop back down smoothly, landing with a click at the bottom of the travel. Motion not smooth? No click? You've got an alignment problem within the carb, or some bit of grit or goo restricting the piston's movement. Fixing this is almost the topic of another article, so for the moment, consult some living breathing human who knows his way around SUs for advice (my e-mail address is at the bottom of the article, and I'd be glad to help.)

SNEAKY PROBLEM 2: Fuel Flow A car that seems to run fine at idle, and even at speed, but starts to have a “power fade”, that can be improved momentarily by pulling the choke, has a fuel delivery problem. Check for blocked fuel lines, block suction line (from the tank to the pump) or a blocked fuel filter. (You DO have a fuel filter in there, right?) If you haven't had this symptom, you're probably OK.

Okay, now that we've elimiated a couple of “sneakies”, let's proceed with the basics.

FIRST: SAME AMOUNT OF FUEL
Assuming that the carbs have the same needle, the amount of fuel each flows is dependent on the level of fuel in the float bowl. The more fuel in the float bowl, the higher the fuel level in the jet. The closer the fuel level is to the tip of the jet, the more fuel it will meter at the same needle setting. The level of fuel in the float bowls is set by the float and float bowl needle. When the float rises, the flow of fuel is shut off. (If you're thinking this is like a toilet tank, you're right. That's how my Dad taught me about this) I'm not going to write up all the possible procedures for this. (The article is already too dang long!) So, pull out your manual and read up on how to set the position of the float arm (for HS carbs) or the needle height (for HIF carbs).
The manual will give you a measurement between some part of the mechanism that closes the needle and some reference point (like the mounting flange). In all reality, as long as you're within reason, it's more important that they be the SAME, than that they be precisely at 29/64" (or whatever).
While you've got the float bowls uncapped, look inside for accumulations of gunk.   Clean out any you find.  A pediatric ear syringe is handy for getting the gas out of the float bowls and even suctioning out loose crud.  Just don't use it on your kid afterwards.  This might also be an opportune moment to replace/install a fuel filter.
The other thing that sets the amount of fuel is the height of the jets, so:

• HS CARBS:  Turn the jet adjusting nut on each carb counterclockwise (looking at it from above) until it stops, then clockwise 2 full turns (12 flats).  The funny little wrench in the SU tool kit is for this, but you can usually get it with your fingertips.
   
• HIF CARBS:  Turn the adjusting screw counterclockwise until it stops, then clockwise 2 full turns NOTE: I was recently corrected on this
  

SECOND: SAME AMOUNT OF AIR

1. Disconnect the throttle linkage between the carbs. You need them acting independently for now. You don't need to remove anything, there's a part of the linkage that can be disengaged while remaining in place.
2. Get out your SU tool kit. You'll find 3 aluminum tubes (one mashed into an oval in one end) and 3 oddly bent pieces of wire (1 with a loop in the middle, and 2 with a loop on one end). If you only have 2 carbs, ignore one of the round tubes and the wire with the loop in the middle. Take the remaining two pieces of wire and look at the loops. One of the loops is fatter than the other. The fatter loop goes into the mashed (oval) tube end. The other loop goes into the other tube. If it's hard, you're doing it wrong.
3. Take off the damper caps, and fit the end of the tubes opposite the wires into the top of the carbs. Turn the tubes so that the tips of the wires are close to each other, and "tweak" them so that the tips are at the same height.
   • While you're at it, lift up gently on each tube, about an inch or so, and let go.  It should drop smoothly, ending with a soft "click".  If it doesn't, you've got a basic problem with the carb, which will involve some disassembly of the carb to fix.  The work is beyond the scope of this article, but there are books available on how to do the work, if you're game to try it.
4. Back the fast idle screws out until they are well clear of the fast idle cam. Back the idle speed screws on each carb (check your manual to find them) out until they just touch the tab they press on, then in about 1-1/2 turns each. This should put the throttle butterflies in pretty much the same position.
5. Start it up. It will be idling fast.
6. Take the RPMs up to about 1500 - 2000 RPM by adjusting the idle speed screws the same amount, 1/4 turn at a time.
7. Observe the ends of the wires, are they still at the same height? (They'll be vibrating a bit, but should be buzzing at the same height.) If they aren't at the same height, turn in the idle screw on the carb with the lower wire until they are. The carbs are now flowing the same amount of air.
    
   If you want to use a carb synchronizer, either instead of the tube and wire arrangement or as a check on your work, go ahead. The instructions that come with them are pretty clear. If you have a really good sense of pitch, you can use a simple rubber hose. Stick one end in your ear and the other end into the mouth of the carb so that you can hear the pitch of the hiss of air flowing through the carb. Repeat with the other carb. If they're right, they'll be the same pitch. The problem I have with either the Uni-Syn or the hose trick is that you have to run the engine with the air filters off. Tuning the car with them off, and then putting them back on, which changes the airflow and therefore the vacuum (back to First Principles) seems inefficient to me. There's also the issue of being a little fumble-fingered and dropping small parts in the vicinity of a pair of open carb throats.   But as always, your preferences may be different.
   

THIRD: CORRECT AMOUNT OF FUEL
NOTE:
HIF Carbs:  Your mixture is adjusted with a screw sticking out the side of the float bowl.  Clockwise is RICHER, counterclockwise is LEANER. NOTE: I was recently corrected on this

HS Carbs:  Your mixture is adjusted with a spring-loaded nut on the bottom of the carb, that the jet presses up against.  As you look down onto the top of the carb, clockwise is RICHER, counterclockwise is LEANER

1. Back the idle speed down by turning the idle screw on each carb back out about 1/4 a turn at a time on each, until you're at around 1000 RPM. (You must back them out the same amount, or you will loose the synchronization).
2. You may want to check your timing if it was set dynamically with the vacuum connected. If the airflow was really off, the distributor advance may have moved on you. (If it's only a tiny bit, don't sweat it.)
3. You're probably running a bit rich. Test it by pressing up on the "piston lifting pin", or lifting on the tubes that are still sticking up out of the tops of the carbs.  Check each, one at a time.  NOTE: You're only looking to lift the piston about 1/16th of an inch!
    
   • If the RPMs rise and stay high until you drop the piston, it is too rich. Adjust the mixture on BOTH carbs one flat (HS) or 1/4 turn (HIF) leaner, then repeat Step 3.
   • If the RPMs fall and the car sounds as though it is going to stall, it is too lean. Adjust the mixture on BOTH carbs one flat (HS) or 1/4 turn (HIF) richer, then repeat Step 3.
   • If the RPMs rise briefly and then settle back down to something like the original RPM level, you're in good shape.
     
   
   You don't need a tachometer for this, by ear is plenty good enough. I test each carb, and adjust each by the same amount to begin with (i.e. test them both, adjust both in the same direction, repeat), until one passes and the other fails, then I just adjust the carb that's failing. There should not be more than 1-2 flats difference between the carbs (and I call 2 flats pretty extreme).
   Some people adjust one until it passes, then adjust the other. The problem with this is that the airflow *is* interconnected between the carbs, through the "balance pipe", and you can easily end up with a car that idles fine, but stumbles and dies on acceleration. That happens because at idle, enough air will flow through the balance pipe to allow a very rich carb to compensate for a very lean one. At idle, each cylinder draws *mostly* from it's associated carb, and some from the other carb(s) through the balance pipe. However, the airflow though the balance pipe is limited, so when you open the throttles, each cylinder draws almost exclusively from it's associated carb. In our "rich carb carries the lean carb at idle" scenario, when you jump on the throttle, one pair of cylinders is drawing almost exclusively from a too-lean carb, while the other draws almost exclusively from one that's too rich. You won't like the result.
   If you want to use a ColorTune (which is a nifty little glass-topped spark plug), this would be a good time to do so. It lets you see the combustion as it happens, and allows you to fine-tune the mix. I rarely change a setting more than 1/2 flat based on the ColorTune, but it *is* a pretty cool light show. It also helps build confidence that you know what you're doing when you finish mixture adjustments, pop it in, and see that nice Bunsen burner blue.
   

Step 6: Time to wrap It up and keep it going

1. Back the idle speed down to around 800-900 RPM, by backing off each idle screw 1/4 turn at a time. I sometimes try to take it lower, just to show off. A car that's really well adjusted may idle smoothly as low as 500 RPM, but that's not enough to have the Generator/Alternator and Water Pump do their thing, so take it back up to 800-900.
2. Reconnect the throttle linkage between the carbs.
3. Check to make sure that the vacuum line to the distributor is connected, and that the distributor clamp is snug.
4. If you took the air filters off, put them back.
5. Turn in the fast idle screws until they're just about to touch the fast idle cam.
6. Take the tubes out of the tops of the carbs, take the wires out of the tubes, wipe the oil off the tubes, and put it all away.
7. Top up the oil in the carb dampers (just to about 1/4" below the top of the inner tube), and reinstall the damper caps.
8. Make sure you've got everything else back where it belongs, and put your tools away
   

Keeping it that way - The Notebook:
You now have a basic set-up that should be eminently driveable and pleasant, but you may want a bit more. That's what the notebook is for. You now know how to set timing and fuel-air mix. Once the carbs are balanced, you shouldn't need to mess with balancing them again for a long time, IF you take careful note of any changes you make.

Drive the car, note how it runs, acceleration, idle smoothness, and whether it seems to "run out of breath" at high RPMs. Feel free to make adjustments, just make sure that you:

• Adjust both carbs the same way and the same amount,
• Don't advance the timing a ridiculous amount,
• Change only one thing at a time, and
• Take notes on the changes and the result.
  

Be methodical. A bit of anal-retentiveness here will pay off at "playtime" later.

A note on factory specifications:
Despite the fact that I keep harping on factory specifications, the factory specs are not the be-all and end-all of car tuning. The factory specs were settings that gave reasonable performance on every new engine. Whether those same settings give the best performance on any given engine when it was new, (or on your engine now that it's been run in a bit, and perhaps modified) is anybody's guess.

That having been said, you have to start somewhere, and the factory specs are as good a place to start as any, and better than most. Don't be afraid to make minor changes within reason, but remember that every performance-enhancing "tweak" has a trade-off. Ultimate fuel economy and longevity can carry a cost of decreased performance, and vice-versa. You know what "optimum" is for you.  However, as you seek "optimum" be methodical and scientific.  Make ONE incremental change, drive the car, note the difference, think about what you want to achieve before making another change.  Take notes.  Lots of them.  Cars whose owners have good detailed notebooks of what's been done, when and why run better than cars whose owners don't keep notes.

Of course it is possible to get an engine so out of whack that both performance and fuel economy suffer, which is why you start with the factory specs, OK?
Now, the question may be "Which factory specs?".  Lots of things may have been done to your car by a previous owner/mechanic.  Different carbs, changed carb needles, different distributor, emissions equipment removed, different engine.  All I can give you in these circumstances is some general advice:

• Go by the factory specs for the engine, not the car.  If your 78 MGB has a 75 engine in it (based on the serial number), use the '75 tuning specs.
• If the emissions controls have been removed or disconnected, reconnect them, put them back, or use the tuning specs for the most recent vintage engine that is similarly equipped.
• Some things just aren't possible.  If you or a previous owner put in a "hot" camshaft, and the idle is "lumpy", you can minimize it, but not eliminate it.  The things that make a performance cam make more power at high RPMs mean that he engine will have a "lumpy" idle.  Deal with it.
  

Books and literature:
The Haynes Weber Carburetor Manual : Zenith Stromberg-SU Carburetor Manual (Haynes Techbook Series)
                     by A. K. Legg, Don Peers, Robert Maddox, John H. Haynes
Coveres the principles of carburation and the specifics on how they apply to the specific carburetors covered.  Useful for thinking through how things are happening, and for teardown and rebuild work.
Of course, the manual for your car.  I use both the Haynes and Bentley manuals.  The Clymer and Chiltons manuals are for use only in desperation, though the tables of tune-up specifications are handy and well laid out.

And Finally...
My thanks to the following folks for their input and feedback:
Doug Ingram
Mike Gigante ( both from the "Spridgets" list)
Mike Janacek  (from the MGs list)
(What is this with the "Mike"s?)
Blake J. Urban (aka Bullwinkle)
This article Copyright 2001- 2024 - Chris Kotting
Permissionto reproduce is freely granted upon request