Topic: 1977 CB550K Carb Leak Issues

[Maraakate]

Hello,

A few years ago I acquired a 77 CB550K off a family friend who had it sitting outside for over 20 years.  Amazingly the bike was in decent shape.  Cleaned the points and sprayed some starting fluid down there and it fired right up.  At that point I cleaned the carbs and noticed it had a bit of a gas leak.  Ordered a Sumo Moto kit from ebay and it still had a slight leak but put the bike away for another day.  Fast forward 4 years later and I again bought the same kit and replaced the float needles and bowl gaskets.  The carbs are leaking out of the drain tubes.  I had someone who is an experienced motorcycle mechanic (mostly with Yamaha's) help me check the float height.  They were pretty far off, at about 21mm.  I reset them all to 14.5mm (as I have seen some threads claim 12.5 or 14.5).  They still leak at all 4 drain tubes.

At this point I have tried the following:

* Replace the o-rings on the fuel banjos connecting the carbs (as one of them actually was leaking I noticed later).
* Reset the float height to 14.5mm.
* Put all 4 floats in a bucket of gas and verified that they do, in fact, float and that they are all floating evenly.
* Cleaned the brass seat with some toothpaste and a cotton swab multiple times for all 4 carbs until they came up clean.
* Take a single carb off, and stick gas line over the banjo and turn the gas on.  I did this for the outer carbs.  Both end up leaking around the bowl and the drain tube.
* Bench tested for these leaks with no bowl gasket to be certain that they are not swelling and blocking the float (I've noticed these bowl gaskets expand in size greatly once fuel has been applied to them.  You appear to only get one chance to use them).
* Replace the o-rings on the drain screws.

At this point I am out of ideas.  My thoughts are that the Sumo Moto needles themselves are garbage.  I unfortunately no longer have the original float needles so I do not know if the height of them are different from the OEMs.  Another possibility is someone bored the float seats, but I really doubt this.  If I take the bowls off and force the float needle to push up to seal then there are no leaks so I know it's not coming from another location inside the carbs.

I did some additional experimentation with setting the float level to something ridiculous, like about 23-26mm and this will stop the leak, but this is clearly wrong.  I set the float height by sitting the carbs on a flat surface at about a 45 degree angle until they start very lightly pushing on the seat.  I think this is the right way?  I can take pictures or make a video of this process.

I also have NOT used the jets, emulsion tube, etc. from the Sumo Moto kit.  Just the gaskets, o-rings, and float needles.  However, I am confused what the tiny o-ring in that kit is for.  I have not seen it used anywhere in the carbs, but maybe I am not looking hard enough?  And just so we're clear, the needles and floats have not been exposed to carb cleaner.  The floats do not feel "heavy".  I think the floats are in OK shape.

Thanks in advance!

[scottly]

I did some additional experimentation with setting the float level to something ridiculous, like about 23-26mm and this will stop the leak, but this is clearly wrong.  I set the float height by sitting the carbs on a flat surface at about a 45 degree angle until they start very lightly pushing on the seat.  I think this is the right way?  I can take pictures or make a video of this process.

The problem with setting the float height that way is weaker springs in the needle plungers will result in higher fuel levels in the bowls. Try turning the carbs upside down, so the weight of the floats is pressing on the needle, and checking the measurement again.

[Maraakate]

I did also check just by having them upside down.  Even at 14.5mm they still leak.

[scottly]

But at 23-26mm they don't leak? Picture, please.

[Maraakate]

I can get you a picture tomorrow.  Will be about 14 hours until I can do so.  Just from googling "bad aftermarket float needle" for fun I have seen other anecdotes that the plunger springs are weaker on aftermarket needles so having the floats raised higher can be necessary.  I have not tried the clear tube test, but will also do so (with pictures) tomorrow.

My money is on the cheap Sumo Moto crap.  Does anyone have any experience with these float needles?  Is there a more reputable aftermarket brand?  I'd prefer OEM, but at $30 per float needle x 4 that's getting kind of nuts.  Unless someone knows a better price.

[Maraakate]

Forgot I did take this picture earlier.  Proof that the floats actually float.

[scottly]

Oh, I believe you. I went through the same process some years ago: the floats float, so why the heck are the carbs pissing gas??

[Maraakate]

I will get some detailed pictures.  Another possibility is that they are bent slightly funny and catching in the bowl which would be hard to determine since we don't have clear bowls for these lol.

What was the solution for your gas leak issue?

[Maraakate]

Pictures unnecessary but I can post them if needed.  I set the floats to about 26mm.  Once the carbs were on the bike it was a #$%* to get started and would die once in gear and trying to give it some throttle.  Took the bowls off while the carbs were still on the bike (super fun!) and bent the tabs a little so the floats are probably about 24-25mm.  The bike runs pretty well now.  So for future reference for those out there... avoid the Sumo Moto kit.  I wouldn't trust the jets in it, but the float needles must have weak springs.  The bowl gaskets are garbage too.  They swell immediately and can't be reused.  The K&L bowl gaskets seem to work much better.

[bryanj]

Is it an F model or a K model as carbs are very different, alternatively what is the id number on the carb flange?

[Maraakate]

1977 CB550K.  I believe it said PD46A, but I can't recall for sure.  Bike is not nearby at the moment.  If you want me to check for you I can.

[Deltarider]

1977 CB550K.  I believe it said PD46A...

PD46 carbs with a float level of 24-26 mm... that's a bit odd, isn't it?

[Maraakate]

Exactly!  That's why I created this thread.  I really do think those Sumo Moto float needles are so far whacked.  And this was absolutely necessary to stop the leaking and the clear tube test!

[1976cb750f836]

Could your brass tubes be cracked? I chassed a similar problem once, I now inspect them close.

[Maraakate]

I don't think the brass tubes are cracked.  It leaks from all 4 of them.

Anyways, now that I have it on the bike and not leaking I have been having a bit of trouble getting the idle nice.  I do have some sync tools.  Cylinder 3 always seems to "hang" a bit longer than the rest.  I forgot to replace the o-rings in the pilot screws so I'll do that and see where I'm at.

I did spray around the boots and all that fun for air leaks with some PB blaster and it doesn't appear to change anything.  One thing I did notice though was the "polyurethane element" was complete soot.  I cleaned that out, but should I replace this or will the bike run fine without it?  It looks like it was just to catch some blowby.

During carb sync I also noticed that the right side is running fairly rich.  I can tell by the pipe on that side (the bike came with some aftermarket 4-to-2 straight pipes).  The bike also seems a bit hard to start.  I haven't pulled plugs yet, but plan to do so Thursday or Friday.

However, I do plan on ordering an OEM float needle this week to see if it works at the proper float height.  I still think everything in this Sumo Moto is total garbage and I probably should not use anything from it except the o-rings (and even that may be stretching it).

[jdip16]

I'm running into the exact same issue. Bought the bike (same year same model) with carbs removed and partially disassembled. Installed Honda rebuild kits and put everything back to factory settings. Slapped carbs back onto the bike, starting fluid, and some throttle work I got a good start.

Eventually the bike would start on its own without starting fluid, but wouldn't idle. After 5 minutes of working the throttle, the drain tubes started pissing fluid. I figured that was a good stopping point and called it a night.

I will update if I find any solutions.

Sent from my LM-G710VM using Tapatalk

[Maraakate]

Update to this:

I was having various small problems the more I started test riding the bike 10 miles here, 10 miles there.  Hard to keep idle, then one of the cylinders appeared to not be firing properly as my RPM was very limited.  Almost like anything above 1/2 throttle didn't matter.  I also had an issue when the sync tool was hooked up whenever I blipped the throttle cylinder 3 took longer to "fall down" indicating an air leak.  Initially I could not find the air leak, but it ended up being the clamp on the intake side of cylinder 3.  It was fairly tight, but where it joined at the screw down there it was not as tight as it could be.  Tightened that up and the air leak went away.  Make sure you spray carb cleaner with a straw on top, bottom, and sides of these boots!  Spraying on the top with a lot of cleaner did not show this leak!

At this point, I took the plugs out, did a compression check, all (at dead cold for 3-4 days) was about 155psi.  They were all within about the same of each other.  Put brand new NGK D7EA's in it.  Replaced the oil and brand new OEM filter and o-rings, as it was old.  Bought OEM float needles.  Set the float height to the proper 14.5mm.  Put new OEM o-rings on the intake manifolds.  When I took the carb off I noticed one of the main jets was floating around in the bowl... which explains the 1/2 throttle issue.  It also explains why it was a total #$%* to start.  Reset the pilot screws to 2 1/4 turns out.  I only did one o-ring down there because the other 3 were so hard to get out.  They do need replaced, but I will do that over the winter when I have more patience.

Put it all back together, seems to run great now.  Carbs need resynced and I will have to play with the pilot screws.  Power is restored.  Seems to idle mostly OK now.  Still need to do valve spacing (waiting on the angled feeler gauges) and checking point gap.

This bike has some 1980s-1990s era aftermarket straight pipes.  4-into-2.  Not sure of the brand.  But, I did notice one side is a bit lean, the other side is a bit rich.  This was when I had the bike on the stand on first startup and the RPM was kinda high (like 3000-4000 RPM).  I will have to pull plugs later and adjust the needle height after the carb resync.  However, I do recall that carb 4 does not seem to have a nylon spacer at the choke linkage.  Must have evaporated into goo many carb cleans ago.  Nobody seems to sell these, not even 4into1.  Does anybody have something on what I can purchase to replace these?  I'm assuming it's fairly important, but maybe livable without it for the time being.

So again, to everyone out there.  Avoid the sumo moto kit!  Those float needles were awful quality apparently.  I know it's expensive for the OEM float needles, but it was worth it for me.  Maybe those K&L's are good as well, but I have never bought them.  Their float bowl gaskets are good though.  They held up with me removing the bowls quite a few times.

Next steps: Get a new air filter, may cure part of the issue with the unbalanced mid-to-high RPM range.  However, I am pretty sure whoever put these pipes on this bike 20-30 years ago never adjusted the needle height.  Check and adjust the valve spacing.  Check and adjust points gap.  Resync carbs.  Get pilot screws dialed in.  Fab up some filter element for the blowby element at the bottom of the airbox.  In the winter when I have some extra cash I will put all new OEM clamps on the boots.  I don't trust them to stay viable over the long term.  This will get kind of pricey and they aren't causing leaks right now so I will leave this be.  In the winter I will do the other 3 pilot screw o-rings.

I think I can do the carb sync now since this only affects low idle and low range RPM correct?  The book says don't bother with the pilot screw adjustment until carb sync is done.

[Maraakate]

I'm running into the exact same issue. Bought the bike (same year same model) with carbs removed and partially disassembled. Installed Honda rebuild kits and put everything back to factory settings. Slapped carbs back onto the bike, starting fluid, and some throttle work I got a good start.

Eventually the bike would start on its own without starting fluid, but wouldn't idle. After 5 minutes of working the throttle, the drain tubes started pissing fluid. I figured that was a good stopping point and called it a night.

I will update if I find any solutions.

Sent from my LM-G710VM using Tapatalk

I was chasing my tail with multiple leaks.  If you have not replaced the o-rings on the connecting fuel banjos do it now.  If you have not replaced the fuel line and made absolutely sure it is clamped TIGHT do it now.  I had leaks from both of those places, but it would only show its face in one spot.  Fixing one moved the leak around.  You can replace the banjos without entirely splitting the carbs.  Just remove the plate that connects all four carbs and remove the butterfly choke plates from 1 and 4.  Remove the screws on the top of 1 and 4 for the slides and you should be able to separate the outer carbs.  Take the tubes out, replace the o-rings.  Either use some fresh gas or some light oil on the o-rings and put the tubes back in.

I removed carb 3 as well to get the tube to 2-3.  You will have to remove the spring from the choke linkage to get it to separate if I remember correctly.  Be mindful how it goes back on.

Have you replaced the o-rings on the fuel drain screws as well?  Check this passage for blockages as well.  If any of these are plugged then it will overflow.  First time running I had one carb with very slight drip-drip and thought I could just plug it off with an old jet in the drain tube to test ride it.  This will make it overflow someplace else.

[jdip16]

For the blowby filter element, I've heard you can use a green scotch Brite pad and it works just fine. For my fuel leak issue, I used some metal polish on the brass seats for the needle valves. Discovered some serious pitting on the number 1 seat. Cleared up the fuel leaks on all carbs but number 1.

Sent from my LM-G710VM using Tapatalk

[Maraakate]

If that's the case you may need to source another carb.  The PD46 series does not have the needle seat available as a separate option.  It is pressed in and I don't believe it is easily removable.  At least, I have not read of anyone doing it successfully on here.