Tuesday, March 13, 2007
A carburetor is a fuel-metering device. It meters out a pound of fuel for every fifteen pounds of air, or thereabouts. The 'thereabouts' comes about because 'Air' is a generic term. Hot air is different from cold air and wet air is different from dry air and air that is full of dusty chicken feathers, pollen, smoke and stray bullets is different from nice, clean, bunny-soft oh-so-perfect air.
Metering the mixture of gasoline to air is not a trivial task. Fifteen pounds of air is a chunk about six feet on a side. A pound of gasoline is a chunk about three and a half INCHES on a side.
Buncha air. Itty-bitty gasoline. Super-easy to get the mixture off by a tad. I'll have more to say about that in a minute.
When air is squeezed through a venturi (lookitup --- and no, it ain't in your spell-checker, you gotta crack a book :-) it's velocity increases AND it's pressure drops. How much depends on where you are. If you're in Mexico City, almost half a mile higher than Denver, the nominal air pressure is different than if you're drag-racing in Long Beach, California, with an average elevation of six inches or so :-) (Bogotá is even worse – over 10,000 feet above sea level. Brawley, California is better; it's more than a hundred feet BELOW sea level. Nice thick air in Brawley.) So what's your elevation? Or, more importantly, what's your air density? Wanna find out? Go ask the fuel in the float bowl. The float bowl is open to the atmosphere; whatever the air pressure (and therefore its density) happens to be, the Float Bowl Knows.
Know how a siphon works? (You'd better, if you wanna understand your carb.) Go flush your toilet. Or use your Okie Credit Card to siphon water out of a bucket. (No, you can't use it to siphon gasoline no more. Read the find print. Modern-day gasoline is toxic; you shouldn't even get it on your skin, let alone in your mouth.)
Suck air through your venturi, the drop in pressure will cause some gasoline to be siphoned out of the float bowl. How much? That depends on the magnitude of the pressure drop and the size of the opening – the 'jet' -- the gasoline flows through.
The 'jet' term is a big, big mistake. It got started because of a poor translation of the term 'siphon fountain' back in the early days of internal combustion engines, which just happened to coincide with the introduction of the siphon toilet. Both toilet and carburetor shared space in the technical journals of that era. When the French term for 'siphon fountain' appeared in English-language technical journals it was translated as 'jet.'
No jet. It's just a hole. Siphon action sucks gasoline through the hole to a level higher than the level of the fuel in the float bowl, premixes it with some air in the aerator tube then sprays it into the throat of the carburetor.
Doesn't really matter. Call it what you want. You've probably got the wrong size anyway. Why? Because of all those Instant Experts. "You prolly need to re-jet yer carb," they say pontifically when they haven't got a clue why your ride is running so bad. Re-jetting the carb sounds like the epitome hi-tek kewl so it's gotta be good, right? Ummm… mebbe. But what works in Brawley will cause you to run rich in Bogotá. I'd rather go by the manual and all those cryptic Service Bulletins that lists the jets according to engine size, the type of carb AND THE ALTITUDE. For a given geographic region running a stock carb on a stock engine, the fuel metering system is virtually bulletproof; nothing to adjust and, with regard to the jet & emulsion tube, nothing to go wrong, assuming both are clean. For that reason, after making sure the RIGHT jet is installed, I eliminate all other possible problems before even considering a change in jet size. And one of those possible problems is the fuel itself. Modern, clean- burning 'gasoline' has about 15% less energy per pound than the old fashioned stuff. In many cases the engine system is working fine, the problem is in the fuel itself, for which there is no easy fix.
The metering jet and the aerator tube work with the pressure drop across the venturi… ('across' meaning from end to end, inlet to outlet, rather than side to side… which wouldn't make much sense) …across the venturi to determine how much fuel gets sucked down the throat of the carb and I'll tell you right off the bat, it's not a perfect system.
Your carb can only provide a stoichiometric mix across a very narrow range of air flow rates. Sorry Charlie, but there it is. (And if 'stoichiometric' has you scratching your head, lookitup. Like 'venturi,' you won't find 'stoichiometric' in the typical spell-checker, most of which are tailored for about a sixth-grade reading comprehension level. [And you thought computers were way kewl, right?] :- )
At lower rates of flow, such as when the engine is running at a slow speed, the mixture will tend to be too rich. At flow rates ABOVE the stoichiometric 'window,' you'll be too lean. People who design carburetors know all this stuff and have done everything they can to widen the window, such as causing the accelerator pump to act as an auxiliary jet at high air-flow rates and designing a separate, leaner burning circuit for low speeds. Unfortunately, with Volkswagens the Instant Experts trash all that by using the wrong distributor, altering the low-speed air-flow characteristics by riveting shut the bleed-hole in the throttle plate and then hogging out the jets to some ridiculous size that guarantees you'll be running rich virtually all the time. Which is probably what they want, since running rich means the engine wears out faster. Built-in job security.
Running 'on stoke' means a perfect ratio of fuel to air. It also means minimum emissions, maximum performance, highest mileage and lowest wear. With 'ideal' air and 'ideal' fuel the ideal stoichiometric ratio is something like 14.7:1
Here comes the Big Joke: Your engine will run on any mixture between a super-rich black-smoky 8:1 all the way up to a welding torch blue-flame 20:1. The majority of engines I see in cars owned by kiddies are over- carbed and running way too rich.
Now read that again. Note that I said 'run,' not 'run well.' In fact, due to a number of limitations imposed by the design of the Volkswagen engine, most of which have to do with its ability to cool itself, your best mixture is going to come in somewhere between 13.3 and 13.8:1.
…and here's the punch line for the Big Joke: What's YOUR air/fuel ratio? No, don't tell me; I don't wanna know. If you're getting between 28 and 32 miles per gallon in a VW sedan, running light at a steady 50 mph, you're doing okay. But if you're not…
MAKING IT MO' BETTA
By their nature, passenger cars require a variable-speed engine. (The alternative is a constant-speed engine with an infinitely-variable gearbox.) Most other internal-combustion engine applications, such as airplanes, water pumps, boats or racing cars use engines optimized for a narrow range of rpm and use a carb matched to that stoichiometric window. But if you want a car you can DRIVE, the carb must have a fairly wide stoichiometric range, a feature lacking on virtually all single- barreled carbs.
So how do you widen the window? One method is to add more barrels to your carb, each optimized for a different rpm range. That's why all modern car makers used dual-barrel progressives. The primary barrel provides an ideal mix up to a certain rate of flow. Above that, the secondary begins to open up, extending the stoichiometric window well beyond what can be achieved with a single-barreled carb, no matter how many bells & whistles you hang on it. (Of course, that can't be right since the 'technical editor' of one of the VW-specific magazines couldn't get his Weber to work… with his centrifugal-advance distributor and after-market exhaust system. Gotta be the carb's fault, right? :-)
If you want perfect stoichiometry across your engine's entire rpm range you gotta toss the carb and go to some form of fuel injection, something folks started doing in the late 1800's as soon as they understood the limitations of carburetors. It worked pretty well for Diesels but lightweight engines using gasoline presented some serious problems. By the mid- 1930's they had come up with solutions but they were more expensive than a simple carburetor. Practical fuel injection systems for gasoline fueled engines didn't arrive until we added a computerized combustion management system to the equation; the so-called 'EFI' (electronic fuel injection). This has improved both fuel efficiency and engine durability; one econo-box gets nearly 80 miles per gallon (!) and a quarter of a million miles of service from a fuel-injected engine is commonplace. This latter fact should give you some idea of what it costs to run rich all the time. (As a point of historical interest, Volkswagen was the first auto maker to introduce an electronic fuel injection system in a production vehicle… back in 1965.)
In Part Thirteen I'll tell you about a coupla things you can do that will improve your mileage, make your engine last longer and allow you to grow long, silky blond hair all over your body. But for now, let's give your carb a bath.
CLEANING YOUR CARB
Go buy a gallon of carb cleaner. It's expensive but it's reusable. Read the warnings on the can. Carb cleaner contains methyl chloride. If you get it on you it will cause you to have two-headed babies, act strange and die young. In the meantime you'll smell bad and girls will avoid you. (Darwin was right.)
Go down to Home Depot or whatever and buy a gallon of paint thinner. It should say 'Mineral Spirits' on the can. Cheap.
Get yourself an empty three-pound coffee can.
Got an air compressor? Okay, howzabout pumping your spare tire up to about sixty pounds and using one of those hose-adapter thingees? No? Then swing past Office Depot or a computer store and get yourself a couple of cans of 'Dust-Off' or whatever.
You also need some cardboard. Or newspaper. Something to cover your table or desk. (Methyl Chloride is the active ingredient in paint stripper. DON'T get it on your mom's table, okay?) Plus an egg carton, saucer, ashtray or whatever. It's to hold the screws & stuff so they don't get away from you. Paper towels. Can of WD-40. Some silicone lubricant. Good lighting.
The overhaul kit for your particular carb. This is the important part because THE INSTRUCTIONS ARE IN THE KIT!
From this point on it's a no-brainer. You take the carb apart, soak it in the carb cleaner for twenty-four hours OR MORE, rinse it in the coffee can half-filled with mineral spirits, inspect it for wear then put it back together using the new parts from the overhaul kit, which is just the float valve, the two diaphragms, a couple of springs and some gaskets.
But if it's so easy who do so many people have trouble with it?
The most common error is failure to let it soak long enough. Some of the internal passageways are really tiny and if they're gunked up the carb cleaner can only attack the end of the gunk. That means it can take three or four days for the carb cleaner to dissolve all the gunk, after which you rinse and blow until you're sure ALL of the internal passageways are clear and the check-valves – those balls you'll hear rattling around – are in fact rattling around and not corroded, gunked or rusted shut. (Shake it; listen for the rattle.)
How do you tell if the passageways are clear? You squirt WD-40 through them. Squirt it in here, watch for it to come out there, pretty good evidence the passageway is clear. Be sure to squirt in the proper direction; you want to push the ball-type check-valves off their seat.
How do you know about the passageways? You read the manuals! And the instructions that come with the carb kit.
(Why does it get gunked up? Two main reasons: Not being driven enough and using modern gasoline. Modern gasoline contains lots of WATER along with other chemicals that are very corrosive. Your carb is made from a zinc-aluminum alloy called Kirksite, often called 'pot metal.' As metals go, its pretty good stuff, almost as strong as mild steel. But what makes it really useful is it's low melting point and near zero coefficient of expansion; it casts beautifully, machines easily and holds up rather well. It also corrodes like a bitch in the presence of water. Modern gasoline contains alcohol and other hygroscopic chemicals; it ABSORBS water. Leave it sit in your carb for any period of time and the volatile components will evaporate [remember, the float bowl is open to the atmosphere] leaving you with a nice layer of water and sediment to gunk up your carb.)
Another common problem is that having cleaned the carb, a lot of folks don't bother to inspect it. Carbs got moving parts; they DO wear out. If you detect any play in the throttle shaft it will have to be re-bushed, otherwise you'll be sucking too much air at low speeds, never get your idle right.
Carb cleaner totally destroys grease. (And paint. And old Levis, leather and most other stuff.) After soaking & rinsing your carb you gotta replace the lubricant on the moving parts. And WD-40 is NOT a lubricant (actually, it is a 'water dispersant' – that's what the 'WD' stands for). To lubricate the carb, use the silicone stuff. You let it wick into the throttle- shaft bore and the choke-plate bore and any place else that needs lubrication, such as the shaft from the vacuum diaphragm that opens the choke when you accelerate (or the vacuum piston if you got an old carb).
Probably the most common carb-related error is failure to replace the gasket between the carb and the manifold. Although the manual sez to replace it only if hardened or cracked, in fact it's pretty much a one-time- use gasket. Once the carb has been torqued down, the gasket's life is over. If you remove the carb, you gotta replace the gasket. Otherwise you'll get vacuum leaks. (Yeah, I know; nobody replaces it; they just tighten it down more. Why do you think the studs are always coming out?)
Or they use the WRONG gasket. See the notches? Some carbs tap-off manifold pressure at the gasket flange for use in the low-speed circuit or for relieving the choke. What's 'relieving the choke'? When the choke is on, it limits the amount of air that can be sucked down the throat. Once the engine starts it will create a very strong vacuum under the throttle- plate. The choke's vacuum diaphragm senses that negative pressure and OPENS the choke accordingly. ALL Solex carbs use manifold pressure for choke relief. Early carbs tap into the throat below the throttle plate but the later models tap-off the vacuum at the mounting flange. Use the wrong gasket, the carb doesn't work properly. This is one of those funny ones because even when you SHOW them the difference in the gaskets there's a lot of people who insist that such a little notch couldn't be very important… and go ahead and use the wrong gasket. (It's kinda like an IQ test :-)
WHAT IT'S ALL ABOUT
Your Volkswagen doesn't run on gasoline. It runs on nitrogen. Hot nitrogen. You only use the gasoline to get the nitrogen hot. (This is all Bobby Boyle's fault so blame him, not me.)
Your engine sucks. Air, mostly, if you got a good air cleaner. The carburetor adds a little gas to the air. Not much, just a tad. For every fifteen pounds of air or thereabouts that gets sucked in, the carb adds a pound of fuel.
Stoichiometric balance. That's when you add EXACTLY the right amount of gasoline so that when it's ignited there's nothing left over. Perfect combustion. Maximum power.
Most folks don't understand these things. When you don't understand the fundamentals you become a Victim-in-Waiting; you are a 'mark,' someone easily conned.
Take air cleaners, for example. Your engine needs clean air. If each cubic foot of air carried just one particle of dust it's enough to wear out your rings & valves in about 20,000 miles. But a cubic foot of air usually carries THOUSANDS of particles of dust. So you gotta use an air filter. An EFFECTIVE air filter. The oil bath type is one of the most effective air filters ever designed.
What's the worst? Those gauze & window-screen jobbies they sell to all the kiddies. Followed by those nifty foam thingees.
The poor air filters provided with the typical after-market carb kit is the main reason you don't see everyone running dual carbs or center- mounted progressives. They guarantee the engine gets dirty air and that causes it to wear out long before it should.
A modern car is a transportation appliance. Just jump in and go, no thinking required. Computers do all your thinking for you, right down to turning on a little sign that sez 'Engine Service Required.' But an antique Volkswagen is not a modern car. And YOU are the Mechanic in Charge. You need to know some science and engineering and math and words of more than one syllable (like 'stoichiometric'). Knowing this stuff gives you a big advantage when you're trying to keep your antique running right.
-Bob Hoover -4 June 2K