Topic: 77-78 cb550k PD carbs tuning

[scottanderson76]

This forum is to assist with the setup of the CB550k with the PD carbs

The hope with this FAQ is to promote the unique setup of the carbs and other related issues to the 77-78 cb550k.  In this forum I would like to start off by having members with their bikes completed to display the jetting they are using and how they got to that point.  Also, with those that are just starting a project, to ellicit the help with the members that have already gone through the trial and tribulations with having to find things out blindly.  Please feel free to ask questions and link other issues that have been discussed exclusively related to this bike.

Thanks you,
Anderson

[Dave Voss]

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.

[sweet_baby_james]

78 CB550k
K&N Pods
Mac 4-1 exhaust
Stock slow speed jet
#104 Main
Floats at 12.5mm
Idle mixture screw at 1.5 turns

I just got the bike running again after a complete rebuild, motor, carbs, caliper, master cylinder etc.  It fires up great and idles nicely but seems to be running a little hot which indicates that it might be running a little lean at idle (this would be consistent with what we already know about these bikes in stock setup, and further exaggerated by the addition of the pods).  I'm going to play with the idle mixture screw, maybe adjust to 3 turns to increase the fuel at idle and see if it brings the temp down.

[Scott S]

 I have found that these "lean" carbs aren't quite as lean as everyone thinks.

 I have:

  1.00mm overbore (563cc)
  CB650 cam
  Jardine 4-1 exhaust w/ baffle
  mildly ported/polished head with match ported manifolds
  Uni-Filter in stock air box

 I started out by raising the needle one clip and going to a 105 main. This turned out to be way to rich. It wouldn't pull to redline and was sluggish in the upper RPM range.
 I also struggled with getting the idle correct and various flat spots.

 After numerous carb cleanings, setting and re-setting the float height, etc. Here's where I ended up:

 Float height at 12.5mm
 42 pilot
 95 main
 needle in stock position
 idle mixture screws at 2 turns out (I may adjust this slightly for best idle)

 I found that float height is critical for these carbs. I also found two vent holes in the carb bodies were plugged and the O-rings on the fuel feed lines were leaking by. This is important. The O-rings would not weep or leak fuel, but when we applied a vacuum to the carbs to check the needle/seat valves, it would not hold a vacuum.
 Between the plugged vent holes and the vacuum leak on the O-rings, we theorize that the float levels may not have been correct. After fixing those two issues, the idle is steady.

 Also, these carbs MUST be synched using a vacuum gauge or manometer. Bench synching just didn't cut it. There was a noticeable difference in idle quality and running after synching with gauges.

[scottanderson76]

My current setup that is working well (albeit a slight bit rich on purpose)

1978 CB550K with the PD46C carbs and slightly over 20k and no rebuild and great compression
Emgo individual POD filters
Mac 4 into 1 exhaust
45 slow jet
110 main jet
floats set at 12.5mm
IMS screw set at 1 turn out
altitude is 5000ft
needle position is set at the 2nd groove (after much back and forth finally settled on this position as it provides the best feedback with no noticeable flat spot at 5k)

This setup was achieved after all 3k service was completed.  The timing was set dead on per factory specs with a timing light and then a full sync of the carbs was completed.  The bike starts on the first kick or the first hit of the button with full choke on to start then slowly back off the choke for about a minute to set into factory idle speed.  I will be tweaking this as I go due to wanting to get rid of the POD filters.  I will be going to a stock airbox plenum with new airbox boots (thanks Paul Gabor) into a K&N filter with some sort of bracket to tie the two together (still working on that).  I hope this helps anybody that lives at altitude with this bike.  Please get in touch with any questions or comments.

Thanks
Anderson

*Updated*  Haven't been on in a while and thought I would update you on my current settings.  The only change I have made is I went down in Main Jet size due to being to rich.  I am using a 100 Main now and it seems to be the best option.

[Stilltime]

I'll have to update this as I go, but I did a little work last night and it runs amazing now.

1978 CB550K
PD46C Carbs
Keihin #105 Main
Stock #42 Slow
Mac 4-1 exhaust
Custom adapted K&N cone filter
Mixture screws 2.5T out
Untouched and unverified needle position

Untouched mixture screw (the one at the front, outside the bowl I'm guessing???).  Doesn't like to idle where I want, either around 2,500 RPM or down to like 800 RPM... 

Edit:  Carbs 1,2 and 4 mixture screws were at 1.5 turns, 3 was at 1 turn (cool trick, PO).  Set them all at 2.5 turns and WOW now I can start up fast from cold start and hold idle where it's supposed to be.  No more of the either 800 or 3,000 RPM idle, held perfect at 1,200 with no popping or backfiring like it used to.

Starting to love this bike more and more every day!

Chris C.

[brewsky]

Here's mine to date...still a work in progress, but its's getting close....
(Floats at 13.5)

[db22]

My daily rider is a 1975 CB550K that came to me with PD46A carbs and a mystery tank that came from some other Honda of the era.  Someone had assembled it from parts lying around and traded it to my previous owner, who became so frustrated with the thing that he let me have it if I could haul it away from his garage.  It hadn't been worked on in eight years when I got it.  The floats were set to the 1975 spec of 22mm, so gas gushed from the carbs when the petcock was opened.  After a summer of work and many questions answered by the generous riders on this board, it is now very reliable, fast and fun to ride.  My setup is as follows:

UNI pods
105 mains
40 slow jets
stock needle position
air screw at 1.5 turns
MAC 4-into-1 exhaust
Late 1976 rockerbox (stock '75 box was ovalizing its shaft holes pretty badly)

Current project bike is a '75 550K made the same month as my rider.  Since it had no carbs when acquired, I sourced a set of '77 PD46A carbs in the interest of interchangeability with the first bike.  Setup will probably be the same, except that I will salvage the aftermarket 4-into-2 exhaust that came on the rider.  I'm hoping the lessons I learned on the rider will allow a quick and mostly painless carb setup, when the time comes sometime next summer.

[Big Al]

Hello Everyone.
I work in Angola and have brought some PD46A carbs with me as part of a rebuild project on a 1978, approx., CB550, not sure of model but engine number is CB550E- 2027851. Like others I found that the Clymers and Haynes manuals do not have any info on these Kei Hin carbs, so I joined this forum seeking guidance and will post progress hopefully to add to the collective PD46A knowledge. I have part dismantled the carbs and found the following, which fits in with other damage found to No 4 cylinder, broken rings, bent valve stem, scored cylinder.
Stamped on carb bodies PD46A □ OCY or could be PD46A □ OCV
Setting      Carb 1    Carb 2   Carb 3   Carb 4
Float height   13.5 mm   10.5 mm   12.0 mm   12.0 mm
Air Screw      1 ½ turns   1 ½ turns   1 ½ turns   1 ½ turns
Main Jet      90   90   90   90
Slow Jet       42   42   42   42
Slide Height   34.2 mm   34.3 mm   34.2 mm   33.0 mm
Slow Jet height   14.4 mm   14.5 mm   14.3 mm   15.2 mm
Normal amount of gunge in the bowls and collected in channels, especially just above the main jet. No 4 especially dirty probably due to blow back through the carb.
Number 1 and 2 needle jet holders (emulsion tubes) were loose and No 3 had unscrewed almost completely, making it at least 5 mm lower. I thought maybe somebody had started to dismantle the carbs but on reflection the bowls were still glued on with gasket goo so the bike must have been running like this. As you can see the float heights are all over the place, I will set them at 14.5 mm (after reading sweet baby james post, thank you).
The slide height on No 4 is significantly higher, maybe because somebody tried to balance the carbs after No 4 cylinder already had other problems. Likewise the tick over is wound up so far that the choke lever does not increase throttle opening when on full choke. I will set all slides the same as No 2 to begin with (thanks lucky for slide height measuring tip) and then balance the vacuum later, and reduce initial tick over setting, set fast idle speed etc.
The slow jets are not pushed fully into the bodies and No 4 slow jet is significantly lower. I haven’t pulled the slow jets out yet but when I fit them I will make sure that they all fully seat and check that they are all the same height, maybe even locktight in place if they seem loose.
I will update with what happens when I have finished the rebuild and start the engine.

Regards, Big Al

[Big Al]

Update on PD46A 1978 CB550 carb tinkering.
Needles were all E2349F and in groove 3, so it didn’t matter whether I counted from top or bottom but which is the correct way for future reference? According to the table in the Honda supplement to CB500K3 / CB550K3 (77) my slow jets are for a CB500K3 not CB550K3. 42 instead of 38, is this to compensate for throttle shaft air leakage? Also what is an air jet? What is a slow air jet? Are they the drillings in the carb body into the bore above the air screw and slow jet? Or are they the brass tubes entering into the bodies to feed air to the needle jet space and slow jet space?

Regarding the seals around the throttle shaft and choke shaft, I found I had two different types between the carbs, some had a small lip seal and others a felt seal. Both types were well worn giving clearance around the shafts. I was considering not changing these seals but I saw a few comments about leakage affecting mixture and fitted new O rings of appropriate sizes. The way I see it working is from 1/8 to 1/4 throttle opening the slide valve cutaway together with slow jet, and to a lesser extent needle jet, govern mixture due to a two stage air pressure drop. First drop in pressure is across the cutaway (inlet) side of the slide, second pressure drop is across the downstream (controlling) side of the slide. Larger slide valve cutaways give a weaker mixture by altering the relative sizes of the two pressure drops, less pressure drop across the cutaway ie higher (absolute) pressure in the space between the two sides of the slide draws less fuel through the slow jet, and to a lesser extent the needle jet. Leakage through the throttle shaft seals allows air through the balance hole in the top of the slide, into the space between the two sides of the slide and has the same effect as a larger cutaway, weaker mixture between 1/8 to 1/4 throttle. Leakage past the choke shaft seals is not as drastic but it does allow air to bypass the air filter, which may allow dirt in and slightly raise effective air box pressure.

Bearing in mind the above comments that air leakage into the central space from above the slide will affect air pressures, when the Honda supplement to CB500K3 / CB550K3 (77) under synchronising carburettors says remove Nos 1, 3 and 4 carb tops, I think you should remove all four tops to give the same effect on each. Having one top in place must result in a slightly different vacuum reading on that cylinder. A more tedious method may be to take vacuum readings with all tops in place, adjust with engine stopped and replace tops before checking readings again.

Needle jet removal may not be necessary if there is no dirt in central small bore air channel to the annular space around the needle jet. Check bore with copper wire and annular space with a small plastic scraper made from a plastic bottle, then blow through with air. Any doubts then I suppose it is chop stick time.

Regards, Big Al

[redrider736]

Great Thread !!
Just looking for help with initial set up to my PD46C carbs.... Planning to run pods, Yoshi style 4into1 exhaust, CB 650 Cam, and Pamco ignition.  My Carbs are off my bike and in the process of cleaning & inspecting them..

Reading the above posts would a good starting point be.. Keeping everything stock, and moving the main jet up to 95?

Main Jet Size:  90 change to 95???
Air Jet Size:  120
Slow Jet Size:  42
Slow Air Jet Size:  150
Main Jet Needle:  E2349
Needle Setting:  2nd Groove
Air Screw: 1.5 turns out



How well do the Stock PD46c CB 550k Carbs work with POD filters K&N ect... 
I have read allot of info on both sohc and dohc sites that running pods can be a real B**ch to set up and get to a reliable running condition?...

[Caltrigger]

Hi Guys,

I have a set of PD46A's that were in good shape except for the choke and throttle rail seals. Like has been mentioned by previous posters some were felt and some a wiper type seal...but all were worn or torn and obviously leaking.

Big Al mentioned using correct sized o-rings....does this work? I have some nitrile ones in the exact size for the throttle and very close for the choke. Thinking that the choke seal wont be as critical?

Comments please...

[brewsky]

If the choke seals are too tight, the spring won't close the choke.