Update on PD46A 1978 CB550 carb tinkering in Angola.
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. Also from the same table what are air jets and slow air jets? Are they the drillings in the carb body above slow jet and air screw?
All the settings I have found so far are.
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
Needle E2349F E2349F E2349F E2349F
Needle Groove 3rd 3rd 3rd 3rd
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
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.
