Topic: 78 550K leaking fuel after carb service

[Venomcustomchoppers]

Hi everyone, 

I'm Nico, the metric technician at a shop down here in Florida.  I've had a 75 CB550K once many years back, this 78 came in to me two days ago for carb work and new rubber, with the customer stating it leaks fuel from two carburetors.   Thus far I've spent a day on it, and really not gotten a lot farther, it's still leaking fuel. 

A little background on what I've done so far, (prior to being brought in, the carbs were rebuilt) I've removed, disassembled, cleaned and examined EVERYTHING, set the float height to 14.5mm, still leaking. All the needles and seats are clean, the overflow tubes have some corrosion, but are not corroded through nor cracked. 
Right now, the only two things I can think of are a defective float, or there is something I'm clearly missing in these remarkably simple carburetors.

Any help from you guys would be appreciated immensely. I don't often get stuff this classic, nor this beautiful in for service. It even rode like a dream when I took it for a test ride to determine what all I'm looking at doing.

[DaveBarbier]

I’d probably swap bowls with ones that aren’t leaking to be doubly sure the bowls and overflow tubes are good. Then I’d see if there are any serious dimples in the float tang where it rides against the valve. Dimples can cause the float to stick down. You can solder a piece of brass sheet on there to give it new life, but will have to reset float height.

It’s possible that the bowl gaskets are moving out of place when you’re installing the bowl which could interfere with the float movement, but that’s unlikely as it seems your issue is with two bowls all the time.

Lastly, look up the “clear tube method” of setting float height. Very easy on your PD46c carbs. That will let you know if your fuel level is rising too high and going out the overflow. Fuel level should be about 3mm from the carb bowl seam.

You could try and just wrap on the offending carbs with a screw driver handle while the fuel is on. The floats might just be temporarily stuck down and need some agitation.

Some others might have ideas.

[BRG-BIRD]

Make sure the fuel inlet “T’s” are not leaking as well.

[Bankerdanny]

Where are they leaking from? If it's the overflow then you have an issue with the float system. Did you reuse the original float needles and seats? If you replaced them did you use genuine Honda or Keihin needles and seats? If you used a generic kit the float needle and seat are often not quite OEM size and so using the factory float height doesn't always result in a correct setup.

Finally, I had an issue with my 750 where I thought it was leaking at the bowl seam, but it was actually leaking out the T connector between the carbs.

[Venomcustomchoppers]

Thank you!  I now have at least something else to try on these carbs.    I sometimes forget just how much I have learned, and more importantly, have yet TO learn about older carburetors. 

Bankerdanny,  I don't know what the owner rebuilt the carbs with, whether OEM, K&L, or chinese.  I just have no idea honestly as it was done before it came in for service.  In true to proper care fashion though, and my love of saving old bikes, I wholeheartedly intend on giving the owner the best recommendations for what else this gem needs to be alive for another 40 years.

I'll give these ideas a shot today, and report back my findings.  Award for most helpful motorcycle forum definitely goes to this one!  Like the ad said back in the old days, "you meet the nicest people on a Honda"

[brewsky]

Probably the o-rings in the fuel joint tubes as said above,, although I don't believe they are "T"s in these carbs, just straight tubes.

[flybox1]

Where exactly is it leaking from?
Fuel "T" inlets? 
Bowl gasket seams? 
Overflow tubes?

[Bankerdanny]

Thank you!  I now have at least something else to try on these carbs.    I sometimes forget just how much I have learned, and more importantly, have yet TO learn about older carburetors. 

Bankerdanny,  I don't know what the owner rebuilt the carbs with, whether OEM, K&L, or chinese.  I just have no idea honestly as it was done before it came in for service.  In true to proper care fashion though, and my love of saving old bikes, I wholeheartedly intend on giving the owner the best recommendations for what else this gem needs to be alive for another 40 years.

I'll give these ideas a shot today, and report back my findings.  Award for most helpful motorcycle forum definitely goes to this one!  Like the ad said back in the old days, "you meet the nicest people on a Honda"

The OEM Keihin float seats (and the jets) will have a stamped K logo that is shaped like a lop sided star.

[Venomcustomchoppers]

Well, an update.  this carb rack seems perfect, yet it keeps leaking fuel.  I'm starting to honestly wonder if I don't just have bad floats on those two cylinders, which is seeming more and more probable.

[BRG-BIRD]

If you are suspicious of the floats maybe move the floats to different carbs to see if the problem follows? Are they brass floats or hollow plastic? Fibrous material? If they are some form of fibrous material they could have absorbed ethanol or other contaminates making them too heavy. Had this happen on a float gauge assembly in a Triumph Sprint ST and nobody on Triumphrat forum believed it until I posted a photo of the float clearly not floating in a jar of gasoline.

[flybox1]

if you cant answer where the leak is coming from, how can we help you troubleshoot? 

Only the hollow white plastic, or brass floats go bad(cracks and filling with fuel)
Your solid black plastic ones do not.....

[brewsky]

this carb rack seems perfect, yet it keeps leaking fuel

Did you separate the rack?

[Venomcustomchoppers]

So, to answer where the fuel is leaking from, it's coming out of the overflow tubes on cylinders 2 and 3.     I haven't pulled the rack itself apart as there is no need to do so. It wasn't coming from the cross tubes, which is one of only two reasons I split carburetors, being complete rebuilds, or to replace missing/broken parts.   I'll look more Monday and see what I see. I have to get some clear tubing, since nobody seems to even have any in the shop.

[MikeSimon]

The overflow standpipes in the bowls are prone to cracking. Causes fuel leaks through the overflow.

[brewsky]

The overflow standpipes in the bowls are prone to cracking. Causes fuel leaks through the overflow.

Or....corroded bowl drain screws will do the same

[Venomcustomchoppers]

The overflow standpipes in the bowls are prone to cracking. Causes fuel leaks through the overflow.

Or....corroded bowl drain screws will do the same

Thanks! This is something I haven't considered even, Noted for Monday, I'll examine this and see. If they're corroded I presume the only real fix is new screws and/or bowls depending how damaged the surfaces may be?

[DaveBarbier]

Again, swapping bowls to different carbs will quickly determine if the bowls are the issue.

Did you wrap on the carb bodies when the fuel is turned on? Did you check for dimples in the float tangs?

[Bankerdanny]

The PD series carbs on the 77-78 550's were the only ones that the drain screws can allow you to drain the carbs through the overflows. I had completely forgotten about that. One of the overflow pipes in my 77 cracked after I had serviced it, at least I believe the crack wasn't there when I cleaned them. At any rate the cracks can be very hard to see. If the are cracked you can fix them several ways. I used JB Weld, but you can also solder or fit a brass sleeve. A good hardware store should carry small enough tubing in its craft section.

[Venomcustomchoppers]

Well, a farther update, I'm at the shop right now examining these carbs.  The bowl overflows look fine, so I don't think they're an issue, but I don't totally understand what you mean dimples in the float tangs. 

[Bankerdanny]

Look at where the tang touches the needle. Over time pressure from the float forms a tiny dimple will that looks like someone pressed a sharp nail against it. If it gets deep enough the float can hang up and not move through its full range.

[crazypj]

wear from vibration where the shut off valve plunger fits. Actually it's irrelevant as setting float level takes it into account. More likely you have something moving around in fuel line, when you invert carbs it isn't visible. Have you 'back flushed' fuel passages? Never particularly liked the PD carbs. O-rings on drain screws good?

[Venomcustomchoppers]

I believe the O rings to be good, drain screws in good condition. This idea was tested via removing the bowl, and putting fuel in, didn't leak. So I'm thinking something to do with the needle at this stage.
 
these carbs make my SRAD GSXR carbs feel simple in contrast. finnicky buggers.

[DaveBarbier]

You’ll get fuel everywhere, but you can remove the bowl gently, lift the float with your finger and see if the fuel stops flowing.

[BRG-BIRD]

I would move your floats to different carbs to see if the problem follows the floats. Someone mentioned the floats are “solid” yet to float they must trap air in the solid material to float, they can absorb ethanol and become too heavy. I experienced this with the Sprint fuel sender float mentioned before and a 70’s carbureted car, it’s carb float became too heavy and holding the new float in one hand the old float in the other the difference was noticeable. Both had “solid” floats. They may not be the problem but at least you can eliminate them.

[dave500]

stick your thumb on the overflow hose,if the seats are leaking the carbs will fill up and leak elsewhere,if it stops you have very fine cracks in those brass overflow tubes.