I'm glad to see this section of the FAQ setup, I hope my information can help others get their carbs dialed in.
I bought my '78 CB550K in stock (though somewhat neglected) condition, and it came with the original stock and correct PD46C carburetors. In trying to get the engine to start and run reliably, in became obvious that he carburetors were in need of attention, so I removed them for a thorough cleaning and inspection. I found that everything inside was stock original, and not really very dirty, so the clean-up was pretty straightforward.
First, I checked the float bowls to make sure the internal tubes were not cracked and the drain screws still sealed as intended. If either issue exists, it needs to be addressed in order for the bowl to be able to maintain the correct level of fuel. Thankfully, all of mine were alright.
Next, I removed the floats and float valves for inspection. The floats should be checked for distortion, and carefully bent as necessary to ensure that neither lobe is higher or lower than the other. The stock float valves are the rubber tipped style, which tend to last a long time, but can distort and not seal as well if they are very old or have spent too much time dried out. Because they are rubber tipped, the socket part of the float valve tends to not wear as much as the earlier all brass style.
Then, I removed the main jets from the emulsion tubes by unscrewing them using a flathead screwdriver, and removed the emulsion tubes from the carburetor bodies by unscrewing them using a socket on a nut-driver handle. Once removed, I cleaned them using carburetor cleaner, spraying through all of the individual holes, and looking through each for light to confirm they were open and not clogged with any varnish or debris. If any of the holes are relunctant to clear using spray cleaner, a small diameter wire can be useful to get started.
After that, I removed the slow (or idle) jets. These are pressed into the carburetor body, and so they need to be removed by gripping them with a set of soft jaw pliers, and pulling straight out. If they are stubborn, they can be easier to remove by twisting them slightly as you pull, but be careful to pull straight away from the carburetor body to avoid distorting the bores or galling the jets. The slow jets have a very small hole in them that tends to become blocked if they are not used for long periods of time, so I made sure they were clear by chasing the hole with a very fine wire and following that up with lots of spray cleaner.
Finally, I cleaned the carburetor bodies of all debris, light surface corrosion, etc. and verified that all of the individual passages were clear by using more spray cleaner. Since my carburetors were in relatively good condition, this part was easy, but if yours are very dirty or more heavily corroded, take whatever time is necessary to make them as clean as possible.
On the top side, I removed the covers and inspected the slides and linkages, and everything looked good, so I left everything up there as-is. The fuel lines between carburetors also looked good and didn't leak, so I also left those alone. If leaks develop there, the o-rings at the end of those lines are probably due to be replaced, and that requires separating the carburetors from each other, not a big deal to do, just more disassembly than I needed to do so I skipped it.
Reassembly is pretty straightforward, be especially careful not to overtighten the emulsion tubes or jets to avoid distorting or stripping the threads. The slow jets are easily reinstalled by setting them in their bores and giving them a mild rap with a screwdriver handle, as they only need to be tight enough to not fall out.
Setting the float level was next, and a preferred method is to position the carburetors vetically so that the floats are hanging downward with the pivot shaft and float valve at the top. Carefully tilt the carburetor slowly while watching the spring loaded ball in the float valve for compression. The point at which the tab on the float is touching (but not compressing) the ball is where the measurement should be taken. The early carburetors with round floats are set at 22mm (measured from the carburetor base to the farthest point on the float) but the PD46A carburetors (1977 model year) are to be set at 14.5mm, and the PD46C carburetors (1978 model year) are to be set at 12.5mm.
When setting the float levels, in my experience it is actually more important to set them all at exactly the same level, than it is that they match the recommended level. In other words, if all four are set to exactly 13mm (for instance) the engine should run fine, but if there was a 1mm difference between one or more carburetors, the engine won't run quite as smoothly. Also, not all service and repair manuals contain the correct information regarding float levels, for instance the Honda service manual states 14.5mm for 1977-1978 models, but later issued a suppliment stating that 12.5mm was the proper level the the 1978 models, and the Clymer manual doesn't include any information on the later style PD carburetors at all, leading some owners to mistakenly adjust their float levels to 22mm. This was the case with the CB550 that I bought, which explained why it wouldn't idle, as the slow jets were being starved for fuel.
Here is a listing of information for the stock 1978 CB550K4 carburetors:
Carburetor: PD46C
Main Jet Size: 90
Air Jet Size: 120
Slow Jet Size: 42
Slow Air Jet Size: 150
Main Jet Needle: E2349
Needle Setting: 2nd Groove
After completing my carburetor inspection and cleaning (and adjusting the floats to the correct level) the engine started easier, revved better, and ran well enough to begin riding around. Not long afterwards, I noticed that the engine was running kinda lean at part throttle, and much more lean at half-to full throttle, enough to actually overheat the engine after about 15 minutes of crusing on the freeway at around 6k rpm.
After quite a bit of reading through older posts on this forum, and getting some great comments and advice from experienced members here, I realized that my 1978 CB550K was not quite as stock as I thought. First, my 4-into-4 exhaust system was stock alright, but from a 1974-1976 model year (with the narrower trumpet shaped tips) which are less restrictive than the 1977-1978 equivalent (with straight megaphone shaped tips). Also, my air cleaner element was an aftermarket UniFilter cartridge, which is also a bit less restrictive than the stock air cleaner element. In combination, my engine was breathing more freely than it did when totally stock original, and this was causing the carburetors to run lean, especically so past half throttle.
To remedy this condition, I made two changes. First, to richen up the mixture in the 1/4-to-3/4 throttle range, I adjusted the main jet needle clips from the 2nd groove to the 4th groove (counted from the end), as the farther from the end the clip is located, the higher the needle will be held up from the main jet, and the larger the area will be between the needle and the main jet, allowing a bit more fuel to flow into the carburetor. Second, I replaced the stock size 90 main jets with size 95 main jets, as the main jet size has the biggest contribution to the overall mixture in the 3/4-to-Full throttle range. With these changes, the engine runs just a little bit smoother at idle and part throttle, but is noticeably more responsive above part throttle, and does not run nearly as hot on extended rides at speed with half throttle or above.
Like all multi-carburetor setups, synchronizing is very important to achieving a smooth running engine, and the PD46 carburetors are no different. As with other applications, perform all other maintenance and adjustments first, like setting valve clearances, tensioning the camshaft chain, adjusting the point gaps, setting the timing, and dialing in the idle speed. There are differing opinions about the best method of synchronizing, and I use a Morgan CarbTune to perform the synchronization at about 2k rpm, which is above the normal idle speed, but in my experience provides a more stable running condition to dial-in any differences, and provides very smooth results.
The PD46 carbs are interesting, in that the No.2 carburetor has no provision for adjustment, and is therefore designated as the 'reference' carburetor that the other three are to be adjusted to match. Removing the top cap on each carburetor exposes a hex nut and slotted screw post just like a valve tappet, and they are to be adjusted the same way, until the vaccum being pulled through each of the four carburetors is the same (or at least very close, it can be difficult to make them all 'exact').
Here are a few things to consider while synchronizing your carburetors. First, it takes time, so be patient, if you are in a hurry, it will show in the results. Second, many folks like to remove the fuel tank and temporarily supply fuel from an alternate source, but I find that by propping the back end up on a block of sorts provides sufficient clearance to make the necessary adjustments. Third, remember that your engine is air-cooled, and extended periods of idle running will cause it to run hotter than normal, and this will affect the results. I like to remove the carburetor caps, position the tank, and connect my CarbTune ahead of time, then start the engine and let it warm up for a few minutes, set the idle to 2k rpm, and immediately begin making any adjustments necessary, with the goal of being finished within 5 minutes or so. If it takes longer than that, I turn off the engine and let it cool down for a while before I resume with completing the synchronization.
Well, I hope this information is helpful to others, I've enjoyed figuring all of this out, in part on my own, and in part with the help of those who shared their knowledge and experience with me, for which I am greatful.
