Rebuilding the Carburetor

The way an engine works is rather simple. The premise for most modern day engines is this: "Suck, Squeeze, Bang, Blow". Dirty eh?

In simple terms, what happens in the engine is this: Air is sucked in through the intake, where it is then mixed with fuel. The air/fuel mixture needs to be carefully controlled in order to ensure the cleanest and best burn possible, in order to reduce emissions and to extract that maximum amount of power possible from the engine. It is then compressed, or squeezed, reducing its total volume. The fuel mixture is then ignited (bang), which causes a mini explosion to occur, rapidly expanding the gas. This is when the best (and most important) part of the cycle happens, obviously, because this is how engines generate their power, by capturing this explosion of energy. And finally, all the waste gasses need to be expelled or blown out the exhaust.

In essence, an engine is a glorified air pump.

This particular kind of engine described is called a 4-stroke engine, and nearly all engines in use today have this type. There is also a kind of engine called a 2-stroke engine, found in older small motorcycles, older lawnmowers, chain saws and pocket bikes. And then there are really wacky engines, such as the Wankel Rotary engine (most famously used in the Mazda RX-7), as well as Compression ignition engines (more commonly known as diesels)… but those won't be discussed.

I would recommend visiting How Stuff Works for further understanding of the basics of how 4 stroke engines, if you don't already know them, as it is very helpful to know at least the basic theory before committing yourself to any sort of engine related project (duh…). The internal combustion engine is truly a marvelous piece of technology... honestly, if someone asked you to be the person who invented it, would you have thought of such an idea as the internal combustion engine?

On our 1977 Honda CT70, the first thing that needs to be attended to is the engine. Once this step is done, the rest of the bike is a cakewalk in comparison (at least until we get to the electricals). The first thing on the engine that we’ll take a look at is the carburetor. So what is this?

The Basics


The Carburetor

In high school chemistry, you might have run across a term, "stoichiometry", those silly looking equations that determine the best ratio for a chemical reaction to occur. Ideally, in regular gasoline engines, that ratio is an air: fuel ratio of 14.7:1. Unfortunately, due to design constraints and real world losses, this kind of ratio is never achieved. Nevertheless, a "proper" air/fuel ratio is still required in order to extract as much performance from the engine as possible.

So, how is this air/fuel ratio achieved in the engine? Nowadays, this is usually controlled via computer, through Electronic Fuel Injection. In this kind of system, as air enters through the engine's air intake, the computer (or ECU) measures the amount of air flowing in. Using a series of logarithms measuring things such as air temperature, air speed, engine conditions, etc.; the ECU sends a signal to fuel injectors, which spray out a fine metered mist of gasoline into the incoming air stream. As the air stream continues into the engine, it mixes before it hits the cylinder, where the three other cycles take place. If you step on the gas, what actually happens is that the engine lets more air into the intake, and then the amount of fuel injected is increased as well. This huge amount of air and fuel causes the engine to speed up, and by slowing down the flow the engine runs slower.

However, in the bad old days, there were obviously no computers. On older bikes (and in fact, some newer bikes, such as the 2008 Kawasaki 250R), air/fuel ratios were controlled by something called a Carburetor. Click here and here for more info about the carburetor, but in essence, it serves the same purpose as electronic fuel injection:



Air comes in through the engine intake. The carburetor is basically a pipe (with attached “bits” on the outside) that connects to the air intake. As the air passes through the carburetor, the passageway narrows and the air flow accelerates (this narrowing is called a venturi). From grade-school physics, many of you should know about the Bernoulli Principle: that is, air that is moving very fast across a surface would "suck" that surface upwards (this is how airplane wings work). A similar thing happens in the carburetor. The venturi accelerates the air. Along the walls of the venturi, there are little "jets”, which look something like the tip of your mechanical pencil.


A variety of different jets. Like a mechanical pencil tip, one end is threaded, and the centre is hollow.

The tips of these jets are exposed to the airstream flowing past in the carburetor, while fuel enters the jet through the threaded end and travels up the hollow tube of the jet. As air rushes past the venturi and continues into the engine, fuel is sucked along and mixed in with it. The amount of fuel that flows out of these jets is determined both by how large the jets are (the larger the jets the more fuel flows in), and by controlling the amount of fuel that gets to the jets in the first place (controlled via the float and float valve). The air flow is also controlled by a butterfly valve, which closes and opens the passage of the carburetor. This valve is controlled by your twist throttle.

In a motorcycle carburetor, there are often several jets, and each one serves a different purpose. In the Honda CT70, there are in essence, two: a “slow” jet, and a “main” jet. The job of the slow jet is to deliver initial fuel flow to the carburetor, to get it idling. As you twist the throttle and the butterfly valve opens up, air flow increases and creates a more powerful suction force on the jets. This more powerful suction force starts to pull fuel from the main jet as well. With proper tuning, as the amount of air flowing into the carb increases, the fuel increases to keep the air/fuel mixture at an ideal rate. As an aside, yes, EFI does this job much better, because it is able to read and adjust the air/fuel mixture much more accurately than a carburetor ever will. In addition, changes in atmospheric pressure and air temperature all affect the air/fuel mixture. Unlike a carburetor where you would need to get off your bike and fiddle with the settings manually, EFI makes things rather stupid-proof by adjusting for these conditions automatically.

So how is the amount of fuel to these different jets controlled? This is done through the float system in a carburetor. The way the float in the carburetor works is very similar to how a float in a toilet works. Fuel enters the carburetor, and eventually it sits in the float bowl, where the fuel comes in contact with the threaded end of the carb and is siphoned up into the airstream. The level of fuel that is maintained in the float bowl is extremely important. This is because the idle (or slow) jet is set at a different height inside the carburetor than the main jet:



If the fuel level is set incorrectly, the jets won’t function as they’re supposed to. Too high of a level means that both jets would be on when they’re not supposed to (resulting in too much fuel in the mixture, a condition called “running rich”), or there won’t be enough fuel, if at all (“running lean”).

The level is maintained by the float. As you could see in the diagram, it’s very self explanatory. The float, as you might guess, floats on top of the fuel. When the fuel level is low, the float drops, opening up the float valve and allows fuel to come in. As the level rises, the float valve shuts off again and no more fuel enters. From the float bowl, the different jets pick up fuel depending on the suction force (which in turn is determined by air flow), and finally the air, which by now is (hopefully) mixed with the proper amount of fuel, enters the engine.

The diagram shown above is obviously a simplified example. Below microfiche showing an exploded diagram of the actual carburetor from a CT70:



Taking it down… and Apart

One of the early problems I noticed with the CT70 is that it seems to have an unsteady idle. In addition, there was a lot of black smoke coming out of the tailpipe. Finally, when a spark plug wrench was used and the spark plug pulled off the head, it was utterly black. This meant either one of two things: The "head" or top of the engine needs to be looked at, or the carburetor needs to be rebuilt and readjusted. All these problems could be a simple problem of the carburetor needing slight adjustments (a black spark plug could mean there is too much fuel, i.e. running rich), or simple cleaning out as the jets might be getting clogged. Since the carburetor is the easiest thing to look at first, this is where we’ll start.

Disassembly of this carburetor is fairly simple. First of all, you need to do this in a place where gas cannot ignite and is well ventilated, as you'll be dealing with leaking gasoline.

The first step is to shut off the petcock valve, and then to drain the carburetor. From the exploded diagram, you could see a drain screw located at the back of the carburetor, and a drain tube that sticks out of the carburetor and is pointed to the floor. Place a clean metal container underneath that drain tube, and then open up the drain screw. This empties the fuel that is sitting in the float bowl. If you’ve used a clean metal container you could reuse the gasoline later. The alternative, slightly dangerous (and otherwise messy), is to just drain the fuel onto the floor of your beloved garage.

The next step is to remove the intake filter in front of the carb. Um… in my case that was rather simple, seeing as someone replaced the intake on my carb with literally, a piece of swiffer clothe. In a normal CT70, this step would involve unscrewing the intake and sliding it out.

Next is the throttle cable. At the top of the carburetor, you’ll see a cable attached to what looks like a chimney. Simply unscrew the top, and then carefully take out the spring, guide and needle valve.

[picture]
Press the spring inwards, and then remove the cable from the assembly. This is what controls how the butterfly valve opens, so it’s important not to lose it!

The service manual tells you to remove the fuel lines at this point. But when I tried that, it was extremely difficult to do unless you have the carb off the bike first. So what I would recommend, to make life easier for yourself, is to take the carb off the bike first, and then remove those lines. The carburetor is connected to the engine through an intake manifold… literally, a piece of curved tubing between the carburetor and the actual engine. Simply take out your socket ratchet (or wrench), and undo the two bolts that connect the carb to the manifold. Carefully pull the carb away, and stuff a clean shop cloth (or plastic bag, whatever is handy) into the manifold to prevent garbage from going in. Next step is removing the actual fuel lines



From the diagram, there are two fuel lines in total that run from the gas tank to the petcock. Why two? Since many carbureted motorcycles did not have a fuel level indicator, the simple solution is to have two settings for the fuel tank: Main and Reserve, and there is one fuel line for each. Inside the tank, the reserve line is plugged into the very bottom of the bike, while the Main fuel line is connected to a hollow tube inside the fuel tank. This tube is set at a slightly higher than the reserve. The idea is that as the fuel level in the tank drops, it will eventually go below that of the tube connected to the Main fuel line. The rider would soon realize he is nearly out of gas, as the bike starts sputtering. The rider then flips the petcock onto the “reserve” setting, which now allows fuel from the Reserve fuel line to flow into the carb, allowing the rider to ride (hopefully) to a gas station in time.

To disconnect the fuel lines, take your trusty metal container, and put it underneath the carb (away from the engine, as you’ll soon be experiencing a “slight fuel leak”). Have a pair of sharpened pencils ready, or golf tees. Tug gently on a fuel line (or use a broad flat-tipped screwdriver… be careful not to damage the lines), and as it loosens, prepare to plug the end quickly with the pencil/golf tee as it comes off the carb (once again, I emphasize that you don’t do this near sources of ignition). Then, using a piece of masking tape, or a permanent marker, flag the fuel line on where it was connected to on the petcock. The last thing you want is to run out of fuel before realizing you switched your reserve and main fuel lines! Disconnect the other fuel line in the same way.

At this point, inspect your fuel lines. If they are cracking, or otherwise damaged in any way, it would be best to replace them. It’s a simple manner of draining the rest of the fuel from the tank in that scenario, and then plugging new lines in.

Now you should be left with a (rather dirty) carburetor in your hands… a real piece of history! The next step is to clean it up and rebuild it.


Ode-skoo yo

Taking the carburetor apart is fairly straightforward. The Clymer's manual that I used didn't give a step by step set of instructions for actually disassembling it, but all you need to do is follow the microfiche, and dig in with a pair of screwdrivers (philips and flathead).

Like a box of chocolates, if your carb hasn't been touched for the last 10 years without a rebuild, you never know what you're going to get. I've seen pictures of carbs that have interior rust in them, some that have lots of gummed up residue from gasoline sitting inside and going bad (gasoline degrades into what seems like a sort of varnish when left to sit for a period of time... when it becomes like this it is also very corrosive, eating away metal like you wouldn't believe... this is why you need fuel stabilizer when you store your motorcycle or power equipment away for long periods of time), and some that just are unusable. Even if it looks fine, check the condition of the float bowl: if you see lots of pitting, or there seems to be "coral-reef like" shadow on the bowl, then it's gone.

Fortunately, replacement carburetors are very inexpensive and easy to find, on websites like eBay or the billion or so trailbike websites. You could typically expect to pay less than $50 for a new one... however, for period correct ones expect to pay a far greater premium as they are, obviously, discontinued. If you've got a carb in good condition and you want to keep things "authentic", try restoring it first, before anything else, as spending $200 or so on a NOS (new old stock) carburetor isn't cool.

Luckily, besides a bit of griminess and wear from 30 years of use, my carburetor is more or less alright. If you're going to rebuild the carburetor, it is recommended that you get a full rebuild kit, which could be found easily at places like eBay for a low price as well. What you should get with a rebuild kit are new gaskets, jets, needle valve set, and springs. Very simply, just replace the old parts in your carb with the parts in the rebuild kit.

Before that, however, you should pick up a container of carburetor cleaner. These can be found in motorcycle shops, hardware stores and places like Crappy Tire (Canadian Tire). They usually come in a resealable container, and can be stored and used for several cleanings. Take the carburetor completely apart, set aside the old parts that are replaceable with parts from the rebuild kit, set aside all non-metal components (they will melt in the carb cleaner), and then dunk the rest of the parts in the carburetor cleaner itself. What I did was let it soak overnight, then the next day I took an old toothbrush and went at 'er, scrubbing away all remaining traces of gunk.

Reassembly the carburetor, and put everything back on the bike. At this point, you're almost ready to test the engine out again- but first, you have to readjust the pilot screw and idle screw. The Clymer's manual recommends that you actually attach a portable tachometer to the motorcycle and then set it to the correct RPM, in this case 1300 +/- 100 rpm. Unfortunately, I didn't have a tach, so what I did instead was use my ear and wing it, putting the engine idle to where I thought the idle should be. I don't recommend you doing this though =P

At this point, you're done.

Problems

Or, at least, in a perfect world everything would be done with no problems at all. Unfortunately, that didn't happen in my case.

After putting in the carburetor and trying to fire the CT70 up... nothing happened. I tried again, furiously hitting the Kickstarter a few times. Still nothing. Troubleshooting time =/

First thing I did was check to see if fuel was getting into the carb. So I turned the drain screw to see if fuel was flowing into the carb... nothing was coming out, so the fuel is clearly held up somewhere before it entered the float bowl. I unscrewed the petcock a bit, and the fuel that leaked out slightly told me that all was well there. Then I unscrewed the fuel filter portion- same thing. A little bit of head scratching later, I realized that what happened was the float level wasn't set properly- the float was incorrectly adjusted to the point where it didn't even allow fuel to enter the bowl through the float valve. A bit of twisting around, reassemble the carb, and I smiled as fuel came out of the drain tube when I loosened the drain screw.

I smiled again when I hit the kickstarter and the engine came to life on the first kick... and then my smile promptly disappeared. Coming out of the bike was a huuugggge cloud of black smoke. I readjusted the carb a couple of times to see if that would solve the problem, but it didn't do anything. On top of that, the idle was unsteady. I tried adjusting the pilot screw and air screw a couple of times, but the bike randomly revved by itself and then nearly stalled at intervals.

It's clear at this point that the problem is much more serious- something might be wrong with the cylinder head, or the actual cylinder and piston rings are screwed up.

The next step then, is to investigate these areas to see if they're the cause of the problem. To verify that it isn't indeed the carburetor's fault, a compression test would need to be done on the cylinder. Basically, this test involves removing the spark plug, attaching a meter to the spark plug hole (the meter looks a lot like those blood pressure monitors you see at the doctor's), and then compressing the engine a few times by simply kicking on the kickstarter a few times to turn it over. The reading on the compression test should show about 150 psi... if it doesn't, then either the piston rings aren't forming a tight seal between the piston and cylinder wall, or the cylinder head's valves are messed up.

I actually don't have a compression tester on me, but the old mechanic's trick is to cover the hole with your thumb and then seeing if the force is strong... it should give a general idea of the state of things. I'll perform this test soon, and pop the cylinder head off to see what's going on.