Resto work on any carbureted bike or ATV is critical to the repair project. Seems that too many folks just thrown a few squirts of carb cleaner in some open orifice and think that'll cure their woes. It won't. Carburetors are fickle little gizmos which require nearly perfect mixtures of fuel and air in order for the engine to run correctly. In multi-cylindered bikes, it's even more important. One poorly conditioned carb out of a bank of four can not only cause significant performance issues, but can cause engine damage in the long run. Here's some tips on making your carb resto successful:
1. Get the service manual. It's important. It'll tell all the data that's needed like hose routing and float height.
2. Take the carb out carefully. Do not force rubber parts. Use silicone spray to help "motivate" parts in their separation. Rubber parts stick to metal after 20+ years of mating, like a long-lost relative. Mark hoses as you go. Take pictures with your phone or camera. Document stuff, it'll make reassembly a lot easier and take errors in reassembly or hose routing out of any troubleshooting matrix after the fact.
3. Use sandwich and food storage bags to separate and store small parts. I sometimes use parts drawers as well. Baggies can be written upon and keeps your jets, floats, needle seats and such from getting lost. I once dropped a float needle into a box of junk screws, nuts, bolts and such which was just below the tool bench where I was working. Almost didn't find it and the customer was needing their bike within hours. Whew. Disassemble CAREFULLY and SLOWLY. Pilot jets, for example, are hidden under factory sealed caps. The service manual will generally instruct you on how to drill out that cap and access the pilot jet. This MUST be done. That circuit is critical and must be cleaned. Often there is a very small o-ring and/or washer and spring with the jet. Don't lose them. Replace o-rings as required.
4. Wear eye protection and gloves when using solvents. I use mineral spirits or paint thinner as step one in the carb cleaning function. Make sure all rubber parts are removed first, all jets are out and everything is bagged and organized. Clean the carb with solvent and an old tooth brush. For small ports, go to your local welding supply place and pickup a welding tip cleaning tool. It'll have all sorts of small wire-like parts to poke through stubborn jets and opening.
5. Get some water based degreaser like Purple Power or similar. Mix up a small container of degreaser/Water and keep a spray bottle of 100% degreaser nearby. Wet the carb to rinse off the petrol based solvent and scrub with the degreaser. Spray heavily grimed areas directly and scrub with the toothbrush. Clean every last spot of stuff out. If it's visible, it's big enough to clog a pilot jet.
6. Rinse with clear water, thoroughly. DO NOT leave strong degreaser on aluminum for an extended period, it can damage some compounds.
7. Blow out with compressed air. Make sure all ports and circuits pass air through without any problem.
8. Replace jets when practical; clean thoroughly when not. Always replace your float needle and seat, however. It'll save you work later on when the carb starts pissing out the overflow.
9. Set the float level meticulously. I use a Starrett Dial Indicator to measure the float height. My accuracy therefore is + or - about .005 (five thousandths).
10. Change the fuel filter and clean out the fuel tank. If the tank and carb were really gunked up, ditch the fuel lines for new as well. Make sure any petcock or fuel selector is likewise cleaned or replaced.
11. On any machine with more than one cylinder (and more than one carb) it's imperative you synchronize the carbs. This is done using a manometer or vacuum measuring device. Theory is, when the carb butterflies are open at idle -- which is just a very small amount -- you have the highest vacuum at idle (that's true on any gasoline engine) and this can be measured with great accuracy. The objective is to get the vacuum between all the carbs the same, or withing a spec that will be in the service manual (for example, + or - 10 mmhg).
A final note on multi-carbed machines: on a typical in-line 4 cylinder bike with 4 carbs, they'll be mounted to a rack. Unless there is an absolute need, do not un-rack the carbs. Leave them together and service each as a unit. This is because, many used plastic "T's" and plastic connectors for the fuel and vacuum circuits between each carb and these sometimes can be nearly impossible to find on a vintage machine. Un-racking the carbs will break the o-ring seals of each and potentially damage the plastic parts. It's best to leave them alone. In this case, if there are non-removable plastic parts, I shy away from the petrol cleaners and clean exclusively with water-based solvents only. But I won't use them at 100% strength in this case, around 50% at the most. Again, these are sometimes impossible to find! Leave the carb's racked unless there is a true reason (like a broken fuel inlet or leaking vacuum port) between any of the carbs.
You need to be squeekie clean when dealing with carb resto's. It's that important. It takes a lot of time and patience but the end result is a very smooth running machine. On my multi-cylinder jobs I've done, that 80's era inline-4 sounds like a sewing machine after synching the carbs and doing other pilot-jet fine tuning.
Whether a Mikuni or Keihin carb, don't forget the internet for help. Model-specific forums and of course, YouTube can be a world of help. There are quite a few videos on carb tear down that just might be your exact carb! Watch the video and ask questions on forums. People are more than willing to help you out.
Take your time, organize, document and be confident that you can tear down that carb and rebuild it properly. It's a challenge, but it's fun, too!