Topic: 78 550k carb float height PD46C...and other help!

[lucky]

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.

QUOTE...
"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."

LUCKY-
The slides on this model are opened closed manually with a cable and is NOT a vacuum operated carb.
Removing the top of #2 carb will make no difference. Check that out yourself.

IF the throttle shaft bushings whatever material they are, do not work then that WILL affect the vacuum readings.

[Black 750K8]

Best way to check the level is to use a clear plastic pipe attached to the drain on the float bowl..bend it up and open the drain screw....on my PD46C carbs one of the POs set the levels as high as the float bowl gaskets...might be a bit high but no problems with running....

This is incorrect information.
The float height is MEASURED when the carb is upside down or on its side with the float tang just touching the float needle. FIRST.

THEN AFTER.....AFTER that you can use the clear tube method if you want to.

This is incorrect information
You are right the information you just gave is incorrect

All the clear tube deal does is tell you if you need
 a new float needle because it is not working right.
All the float needle does is control the fuel in and out of the bowl.

The float height is MEASURED when the carb is upside down or on its side with the float tang just touching the float needle.

No lucky you do not measure it with the carb upside down.
ScottS is correct.

MEASURE them like the book says to. Forget the clear tube.
I cannot stress that enough.

To late you have already stressed it too much.
  Don't start the wining bully crap.
I like everyone make mistakes some man up and some don't.

Now back to the op.

The float is actually heavy enough to start to compress the needle. You want the float to be touching the needle, not sitting on it.

That is right Bankerdanny 

Does it run on a prime/starting fluid sprayed into the carbs? If so, then your problem is more than likely with your carbs.

If not, then your timing may be way out of whack. I would go through that whole system and make sure its functioning properly. The factory manual available for free here has a great walk through, including specs.

Best thing here is to just make a checklist and walk through your systems one by one and make sure they're in order. 

Yes try some starter spray if it fires better then look into fuel delivery issues

[Big Al]

Thanks for the feedback Lucky,

Quote

LUCKY-
The slides on this model are opened closed manually with a cable and is NOT a vacuum operated carb. Removing the top of #2 carb will make no difference. Check that out yourself.

IF the throttle shaft bushings whatever material they are, do not work then that WILL affect the vacuum readings.


« Last Edit: October 12, 2013, 07:45:13 PM by lucky »

Removing the carb top has exactly the same effect as leakage past the throttle shaft seals, both allow air into exactly the same space. If one adversely effects the vacuum readings then so will the other.

Regards, Big Al.