So here's a question: Do you know what the carburetors actually do on your motorcycle? Well, if you answered "No!" then I will try to give you my interpretation of their function. If you said "Yes!" then read on and feel free to punch holes in my explanations if need be.
This is the way it works out in my head: the carburetor's function is to measure and deliver the right amount of fuel to enable the engine to run at it's most efficient and ensure maximum power. But what is the right amount fuel? Well, there is an actual optimum ratio of fuel to air that means everything will get burned up in the combustion chamber. It's called the Stoichiometric Ratio, and for a gasoline engine it's a ratio of 14.7 parts air to 1 part fuel. You can read more about it here.
It's worth getting some of the terminology right before we get too deep into carburetor tuning:
If an engine is running RICH, then there is more fuel in the combustion chamber, therefore the AFR (Air to Fuel Ratio) is less than optimum, i.e.: <14.7
If an engine is running LEAN, then there is more air in the combustion chamber, therefore the AFR is more than optimum, i.e.: >14.7
You've probably heard these terms before, Rich and Lean. But what do they actually mean for the engine?
Running Rich will result in some of the following problems:
* Black, sooty, fouled spark plugs;
* Poor fuel economy;
* Excessive smoke from exhaust;
* Sluggish and flat acceleration.
A rich running engine will have excessive carbon deposits in the combustion chamber, decreasing the overall life of the engine. Plus you'll be sending plumes of black sooty smoke into the neighborhood; not good for the environment or keeping your neighbors happy.
Running Lean will result in some of the following problems:
* Engine will knock/ping, or overheat excessively;
* Acceleration is slow to respond;
* Popping or backfires from the carburetors as the throttles are closed;
* Similarly there maybe popping or backfiring from the exhausts.
A lean running engine is the most dangerous of situations. Slightly lean engines will be problematic to ride, if the lean situation becomes severe then you risk overheating the engine and seizing/melting the pistons. Not good at all.
So how does the carburetor do the measuring of fuel to air? Well, it's pretty simple really. If you think of the engine pistons moving up and down, they draw in air from the carburetor side, the spark fires, the fuel air mixture ignites and the exhaust gases are expelled. It's that moment of the pistons drawing down into the cylinder that creates a negative pressure on the engine side. And as you know from high school air will flow from high to low pressure. So air rushes through the carburetor and in doing so it draws a certain amount of fuel with it by the force of the Venturi Effect. How much fuel is determined by a series for 'jets' (think of these as holes of different sizes where the fuel can flow into the stream of air); each jet is responsible for delivering fuel at a certain amount of air flow, or throttle position. If you think about the air flow at idle, it will be relatively small. With a wide open throttle (WOT), then the air flow is at it's maximum. Because these two extremes require two vastly different amounts of fuel, there are different jets for different throttle positions. These are the following components of the carburetor that we will be dealing with:
Pilot System: The consists of the Pilot Jet, Air Screw, and Slide Cutout.
Pilot Jet: this controls the amount of fuel that is being delivered at idle. Think of it as a trickle of fuel to keep the engine running.
Air Screw: this allows you to finely adjust the idle AFR after you have got the Pilot Jet set approximately. It is a screw on the side of the carburetor that increases/decreases the air allowed into the mixture. On the Mikuni VM34's, turning the airscrew in makes the engine run richer, conversely turning it out makes the engine run leaner. All you are doing to the opposite of increasing/decreasing the pilot jet size, but in a smaller, more precise way.
Slide Cutout: If you look at the slide on a VM34 you see that on the air side of the carburetor the bottom of the slide has a section cut out of it. The larger the section, the more air will be flowing into the carburetors. They are numbered, with the larger number meaning more cutout, thus a leaner condition.
So at idle to 1/8th of the throttle being used only the Pilot Jet and Air Screw are working. (The Pilot Jet and Air Screw are on the engine side of the slide, so they are pulled by the vacuum of the pistons moving. At idle there isn't sufficient air flow to draw fuel from the other jets.) When the throttle is 1/8 to 1/4 wide, then the Pilot Jet and Air Screw are working to maximum capacity, then the Slide Cutout comes into play. It allows some air over the main jet (which we'll get to later) and fuel begins to flow.
Needle Jet and (Jet) Needle:
So I find myself always getting confused about which one I am talking about with these. The best way to think about is not the mention the Jet in Jet Needle; just call it a Needle, which it is. These are tapered, very precisely machined pieces of metal that resemble a needle. They sit in the Needle Jet, which again is a precisely machined "hole" of exacting diameter. The Needle moves up and down with the slide, thus when the throttle is held wide open, the slide is all the way up allowing maximum air to flow, and the needle is all the way out of the needle jet. At different throttle positions the Needle sits in different heights in the Needle Jet, so different amounts of fuel are allowed to flow. By using different Needles you can change the AFR at different throttle positions.
The Needles have a series of small notches at the top of them. This allows you to change their relative position in the Needle Jet by altering their relative height. It's simply a matter of moving the circlip up or down, to rich or lean an engine. If you think about it, moving the needle up allows a larger diameter of "hole" where fuel can flow. It's all about surface area.
The Needle Jet and (Jet) Needle effect engine performance from 1/4 to 3/4 throttle position. After that it's all about the Main Jet.
This is last part of the carburetor that effects motor performance. At 3/4 to WOT, the Main Jet supplies all the fuel that the engine needs. Because of this, it's the easiest of all the parts to tune, but it does also effect what happens below it, so at the very beginning of tuning you should put a really large Main Jet in the carburetor. Think of it this way; if there was too small a Main Jet then there wouldn't be sufficient fuel flow for the Needle Jet and Needle to work, so that would totally screw with your results. The Main Jet is below the Needle Jet.
Okay, so now we have the basics covered for the various components effecting the AFR. But there are some things we should talk about before you even start to think about jets etc.
* Engine free from leaks. Make sure all the carburetor's rubber boots are crack free and securely mounted. If there is air getting into the carbs aside from the air filter/pods then you are screwed to begin with;
* Air filter/Pods clean and oiled to manufacturers specs;
* Engine compression is good;
* Spark plugs in good condition and properly gapped;
* Fresh fuel and the correct grade;
* Floats are set correctly;
* Carbs synced with each other. This can either be done with a vacuum sync tool or by bench syncing them. I didn't have a vacuum sync to start with (not that it really matters as the VM's have no place to attach the vacuum pipes.) so I took off the carbs off and used the shaft of a drill bit to make sure the slides were at exactly the same height on the work bench. Once I remounted them, I checked again with the drill bit to make sure they heights were identical after reconnecting the throttle cables, and that they both took off at the same point. There is no substitute for fingers in determining take off synchronization. Just put thumb and finger in each respective carb slide and twist the throttle. You can immediately tell if one is moving faster than the other. Adjust throttle cables till they match.
* Engine warmed up and at correct idle speed.
If you are still with me so far, then I will now explain how I tuned Natasha's carbs. If you've read my previous build entry about building a custom AFR setup then you'll know that I am all about the science and using the numbers to support my findings. I am sure there are many excellent engine tuners who can do all this by 'ear'. That's great if you have twenty years experience behind you, but for Joe Blogs working in his shed I would recommend buying a basic AFR meter.
One of the first things I discovered when trying to get the Pilot Jet right was no matter what I did, going up or down in jet sizes, I just couldn't get the bike away from a severely lean engine. I must have changed jets a dozen or so times before I realized what was going on. (Note: You will become incredibly adept at pulling off and tearing apart your carbs. I would recommend setting aside some dedicated tools just for the job. The right sized screwdriver is essential not to wreck bolt heads.)
So the AFR sensor was mounted in a special holder at the end of the exhaust. Because the relative small engine capacity compared to say a car engine means that Natasha doesn't actually produce a lot of exhaust gases. Plus, there is a certain amount of vacuum pressure between cylinders firing that draws air back into the exhaust. So the AFR meter was constantly reading lean from outside air readings. What it took was removing the special bracket and just pushing the AFR sensor all the way down inside the exhaust. BOOM. I suddenly got meaningful readings at last. (Later on I got some tubing and extended the bracket so that it would be drawing exhaust gases from near the engine. That's the images below. This worked a treat when out on test rides, as having the AFR sensor loose in the exhaust was not such a great idea: it fell out on the first test ride! :( )
Once I could see what the engine was doing I quickly had the Pilot Jet dialed in, and set the Air Screw mixture so I was getting exactly 14.7 AFR.
With the whole setup attached to Natasha, I took her out for a test ride. The AFR data logger would record everything, but specifically I wanted to see the AFR against throttle position. As I rode Natasha, I held 1/4, 1/2, 3/4 and WOT throttle positions for as long as I could to get stable readings. Easier said than done on these tiny island roads, but with repeated runs I got some meaningful data.
Plugging these results into an Excel sheet that I built gave me a visual clue as to what to do next. The key to carb tuning is to only change one thing at a time! So after each run I would change to a different jet and see what would happen. Sometimes the results would be as expected, other times I would be confused. You see even though all these parts in the carburetor effect different throttle positions, they all effect each other to some degree or another.
As I built up more data from each run it was easy to get lost in the figures and not actually listen to what the engine was doing. I started to drop the idle AFR to something slightly richer, this made pulling away from a standstill so much smoother. Even though 14.7 is the magic number, it's not the number you really want across all throttle positions. Ideally a cruising 1/4 throttle will be somewhat lean, for best fuel economy. When you want to pick things up a bit, so say 3/4 to WOT then a rich engine is better for maximum power. Balancing all these needs and trying to pick the perfect set of jets and needle combinations is near impossible.
After nine runs I needed to get Natasha ready for the Distinguished Gentlemen's Ride, so this is where I am at right now. It's not quite perfect but pretty damn close. She has a surge of power from 1/2 to 3/4 throttle. WOT is perhaps a little off, but how often do I really need to open up on these island roads? Saying that, I still want to tweak Natasha and get her on a Dyno to see if my little homemade AFR data package reflects what a professional multi-gas meters reads.
Feel free to download the Excel tuning spreadsheet. I have some useful tabs in there listing data of all the jet needles etc. Plus you can use my settings to start you off in the right direction assuming you are using Mikuni VM34's, pods, and a similar free flowing exhaust system. Here is a useful link about Mikuni Tuning; I found this very helpful in figuring out what was going on. Best of luck!