So, You Wanna Jet Your Bike, Hmm?
Lots of people think carburettors are some arcane and mystical creature and that getting them right is akin to finding a unicorn or a pot of gold at the end of a rainbow. Nothing could be further from the truth.
Carbs are miracles of the mechnical art and deserve respect for doing something inherently very complex, as well as they do. Now, for those of you that don't have degrees in Fluid Dynamics and just want to jet your carbs to match you new pipe or clear up the mistakes the factory made; here's some guidance.
First, you are going to have to get to the carbs. This has varying degrees of difficulty, depending on the bike. A ZX-11 is a pain for example, due to enclosed bodywork but, a ZRX-12 is fairly straightforward. Usually, the procedure is, remove seat, remove tank, remove airbox and you're there. To remove the tank, shut off the fuel, unbolt it, and move it so you can disconnect the fuel and vacuum lines (if equipped) from it. Carefully lift the tank off and set it somewhere safe on something soft, so it won't get nicked or bashed.
Removing the airbox also varies. Typically, there are a number of bolts holding down the lid and when you have those off, the ones holding the base are revealed and can be removed. There may also be a seperate plate held to the carb mouths or it may be integral to the airbox but, you've got to clear all this away so, you are looking at naked carbs.
What you'll see when you've done this is the carb mouth. You'll be able to see a slide with a needle attached to it. The needle goes into what's called the emulsion tube. Just inside the carb mouth will be 2 small visible tubes or holes, likely in the lower part of the carb body, one in line with the needle and one just off to the side. The one in line is the main air jet and the one to the side is the pilot jet. On the bottom of the carb you'll see a casting held on by screws. This is your float bowl which contains, no surprise, a float as well as a needle a seat assembly.
Now for some definitions and functions:
Pilot jet: This controls fuel flow mostly for idle and low speed operation. Through the combination of the pilot jet, the pilot air jet and the pilot screw position, this is richened or leaned.
Main air jet: This controls the flow of air to the emulsion tube/needle assembly and is primarily responsible for midrange fuel flow. The air enters the emulsion tube and blows the fuel into a fine mist; "emulsion" means, at it's simplest, "mixing".
Now you'll need to remove the carbs to proceed. Usually you can install a jet kit without seperating the carbs or removing the throttle cables but, not always. You'll almost certainly have to remove the choke cable. Unscrew its holder from the top of the carb it's attached to and rotate the cable 'til it comes out of the holder/lever arm.
The carbs need to be drained before you can go further. Usually, you'll see a little pipe sticking out of the float bowl with a screw near it. When you undo the screw fuel will drain from the float bowls. Use a small container or a length of vacuum hose to a container to capture the draining fuel.
The carbs themselves will likely be held onto their respective manifolds by worm clamps of some kind. Loosen all of them. At this point, check to see if there are any vacuum lines or electrical connections to the carbs and disconnect and label them, so you know where they go and don't come crying to someone like me because you forgot where a line or two go!
The carbs will usually need a little rocking to loosen them and then try to pull the entire rack off smoothly and carefully. Flip the rack over so the float bowls are up and you are ready to install your jet kit.
Stuff rags or paper towel down the intake manifolds to prevent junk from entering the engine. Look carefully for a little brass or plastic cap near the front (throttle plate side) of the carb. This covers your mixture screws and will need to be removed. I use a "pin vise", a tiny hand held drill available at any hobby shop, and a small tap to pull the plug. Be careful! You don't have to go far in to punch through the plug and you don't want to bugger up pilot screws behind it.
Undo the bowl screws but, pull only one bowl at a time. Inside you will find a couple of metal tubes screwed into the top of the float chamber, and a plastic float. The bigger tube is the emulsion tube and will have a little orifice screwed into its bottom. This is your main jet. The smaller tube is your pilot jet. Near the float hinge you'll see a tiny little metal cylinder attached to the hinging float. This is the float needle.
Time for more definitions and functions:
Emulsion tube: Flow through this is controlled by the needle rising and falling and is responsible for midrange performance, primarily.
Main jet: Mounted in the bottom of the Emulsion tube, this little orifice controls mixture at wide open throttle (WOT), primarily.
Pilot jet: Delivers the fuel for idle and low speed running conditions. It works in combination with the pilot air jet and the mixture screw position to fatten or lean the low-speed mixture.
Float: Like the water level sensor assemby in your toilet, it tells the carb its float bowl is full.
Float needle: This is attached to the float and opens or closes by pressing on a seat. This allows or shuts off fuel flow to the float bowl.
Your typical jet kit will contain main jets, emulsion tubes, needles, clips and washers for the needles and possibly pilot jets as well.
I start by removing the pilot jet, if required. This is usually equipped with a cut for a slotted screwdriver. Unwind it, carefully place it aside where it won't get lost and install the new pilot jet. Then set the pilot screws you uncovered when you pulled the little plugs to the starter setting you jet kit suggests.
Next, I move on to the main jet and emulsion tube. Remove the main jet first with your slotted screwdriver, being careful as these are made of soft metal and are easily distorted. Set the jet where it won't get lost and remove the emulsion tube with a 1/4 inch ratchet and what is most likely an 8 or 10mm socket. Replace the tube with the one in the kit and replace the jet with the suggested starting point jet from your kit. 5-10 inch/pounds are all you need to install these. Do not overtighten them or you'll strip the carb body and have to buy a new carb.
Next comes setting the float height. Usually, you'll have to rotate the carb do the hinge is upward and just touching the needle. Also usually, you'll measure from the bowl flange to the base of the float. I use a meachnics caliper for this but a decent millimetric ruler will suffice. A good tool shop can supply a ruler that is ruled from the very edge of the ruler, not 2-4 mils in as your elementary school rulers are. The right setting will vary but, usually 4-5 mm is what's required. If it's wrong, remove the screw holding down the float hinge, remove the float, remove its needle and carefully bend the tang on the float the required way to correct it. This is a pain in the butt as it may take a few tries to get right and you have to reassemble it every time to check it. Be careful not to drop the needle, the mounting screw or the float hinge bar. They are easily lost and hard to replace individually.
Float height is often overlooked and can cause low speed running problems if set too high or high rpm power problems if set too low.
Once this is done, replace the bowl and repeat for all the carbs.
I'd suggest a short break here; no coffee or beer though; these beverages don't go with jet kit installs very well!
Once you're back and ready for action, flip the carbs over right side up and you'll be looking at some big caps held down with screws. These contain your slide diaphragms, slide springs, the slides themselves and the needle.
Further definitions and functions:
Slide: Using engine vacuum and slide spring tension, the slide opens and closes like a choke on the air horn of an old car carburettor. What this does is, essentially, make the carb variable in terms of size so you can get clean throttle response at low rpms/light throttle and yet have a carb volume large enough to feed your ravenous 12,000 rpm monster at WOT.
Slide diaphragm: Corralls the engine vacuum moving the slide. It is usually made of a nice nitrile rubber.
Slide spring: Forces the slide into its naturally closed position and helps control the rate of slide opening.
Needle: This is hung off the bottom of the slide, is tapered and goes into the emulsion tube. It rises and falls with the slide, allowing fuel to leave the emulsion tube, to particularly enrich or lean the engine in midrange conditions.
Unscrew one of the caps, being careful here as the spring will want to fire stuff into the darkest recesses of your garage, never to be seen again.
Put aside the cap and spring and you'll see the diaphragm. Grasp whatever hard part is in the middle of it and carefully pull it straight up and out. Make sure you don't tear that diaphragm or you may have stress similar to when you tear the other type of diaphragm many of us are familiar with!
Typically, there will be a clip in the middle of the diaphragm and you'll need to undo this. This will allow you to carefully tap the slide top onto a surface and the needle will fall out, possible in accompaniment with a washer. Make sure that the washer goes back in if that's what your kit calls for.
Take the needles from you kit, install the e-clip provided in the starting point groove at the top of the needle, the washer if it fell out, and drop them into the slide hole. Do not apply any side force to the needle you can avoid. Then put the clip back and reinstall the slides. Repeat for all the carbs.
Now you'll need to reinstall the carbs. I'd suggest going to the pharmacy and buying some Astro-Glide or, you can use silicone brake grease. Don't use any petroleum based grease, including Vaseline!
Smear a little Astro-Glide inside the rubber manifolds the carbs were attached to. This will ease their entry. Push straight and square and slide the carb rack back onto the manifolds, connect up the choke cable and anything else you removed, move the throttle to ensure normal operation and reinstall the airbox assembly.
You will now need to synch the carbs. A very nice synching tool is the Morgan Carbtune II. I recommend it. You simply cannot skip this step.
Hook your synch tool up to the ports on the manifold. You may have to remove a bolt or two and install some vacuum hose barbs. These are generally provided with any synching tool.
Supply fuel by either reconnecting the tank temporarily or using a small fuel supply you fab yourself. Turn the fuel on to fill the bowls, fire the bike and set the idle as low as it will run without heavy stumble (usually 800-900 rpm).
Synch the carbs as 1-2 and 3-4 pairs first, then use the centre synching screw to synch the pairs together. The screws you are adjusting are between the carb bodies usually and have little springs on them under the screw heads. It may take a bit of time to get this right; it's worth it.
Reinstall the tank permanently and any bodywork and take it out for a spin.
Stay tuned...for an article on tuning your jet kit! The fun has just begun!