Topic: Carb overflow nightmare!

[Swiss Toni]

1978 CB550K. Morning, guys (and gals). Recently bought a US import 550, and busy re- commissioning it. Carbs (PD46C) taken off, and completely cleaned, and new o rings fitted on assembly. Everything spotlessly clean! Float height set at 14.5mm at highest point of float, as opposed to 12.5mm on the PD46A’s. Bench tested for leaks before re-fitting to engine. Perfect! No leaks. On fitting them to the bike, however, and coupling up to a half full tank, the overflows on no’s 2 & 4 are literally pouring out! The float needles looked fine, no ridges or grooves, and the seats were meticulously cleaned, and spotless! Any ideas?

[M 750K6]

Have you tried tapping the bowls with a screwdriver handle. Possible the floats are hung up from being held at angles while fitting.

[Deltarider]

Could it be that some rubber rings still need to swell up to their sealing size? Do all carbs vent? There have been K3/K4 reports of these tiny vent openings being blocked.

[Swiss Toni]

Have you tried tapping the bowls with a screwdriver handle. Possible the floats are hung up from being held at angles while fitting.

Yes. Tried that straight away. no change.

[Swiss Toni]

Could it be that some rubber rings still need to swell up to their sealing size? Do all carbs vent? There have been K3/K4 reports of these tiny vent openings being blocked.

Yes, Deltarider. Did that as well. Apparently, there's not a lot of info on this model carbs but, the info I did find, specified cleaning those vents out. And, I can't remember how to insert multiple quotes, hence the two posts! 

[newday777]

What's the inside of the tank look like? Any surface rust? Just a little rust in the tank will cause lots of problems as it will dislodge tiny flakes that can prevent the float valves from sealing fully. Remove the float bowls and inspect for rust sediment. Remove the petcock screen sock that is up in the tank of your 550, replace it if there are any holes in the screening.
I had a friend's K4 750 in last summer to clean the carbs. The bowls had orange sediment in them. Popped open the tank to find fine film of surface rust in the tank, so I cleaned the tank with 5 gallons of Evaporust to fill the tank(some people seam to think that they only need 2-3 gallons because they don't want to spend the money to buy 5 gallons to fill their tank, but I've tried that and you don't get all the rust out doing that. The top of the tank has to be clean of rust too, I've found rust on the top of the tank using a bore scope after cleaning because I didn't burp the air out of the tank when full of Evaporust and had to do another cleaning, so check the top of the tank too not just what you see when you open the lid) When cleaning your carbs, be sure you also have a clean tank.
I rebuilt the petcock too while cleaning the carbs.

[Swiss Toni]

Good call, Newday. Will check it out tomorrow and let you know!

[Swiss Toni]

Inside of tank pristine! Filter has detached itself from petcock inside tank, however! Anyway, opened up the carbs to see if anything was obviously wrong to cause the four carbs to overflow. Nothing. Float needle tips in very good condition and, I cleaned the needle seats thoroughly. Again!
Only thing I can see was, 2 of the needles spring loaded plungers were a little bit weak. Engine was only firing on 1 & 4. Swapped plugs over, no change. Not plugs or coils causing problem. Incidentally, when I stripped, cleaned and re-built the carbs, I re-set the float height from 12.5 to 14.5mm, which is supposedly the correct height for the PD46C carbs.

[Deltarider]

If the float parts are aftermarket like Keyster, float height may need to be adjusted to that particular brand. A member in the German forum who rebuilt his CB500K1 has reported on this.
As far as the ignition, have a look at the plate whilst running and check for this:

At the breakerpoints a tiny bit of sparking is normal, excessive sparking or arcing is not good and can indicate a bad condenser. So in this vid the left condensor (1+4) is bad, right condensor (2+3) is good.

Also, make sure spark 2+3 is not 'leaked' from the plugcaps and/or HT wires to the head. This is often the case and best noticed in the dark. Check plugcaps are in good order and connect well.

[rotortiller]

I too had a float bowl nightmare this week. One of the fuel line filters sprung a leak so I proactively removed them all and ran new fuel line. The thing started overflowing on one carb with another to follow. So I cleaned the carbs and added new filters with similar results. Mucked and fiddle farted with float levels using fuel medium for the set. Worked fine on the bench as usual. Every time I removed a leaking carb I wiped and checked the needle and seat then retested. Turned out the so called chemical resistant line on the fuel line rack at the automotive store was not so chemical resistant. Went to the bike shop for more fuel line and problem was solved. The garbage some places sell, took the vendor specs off the plastic line, did some research and they specifically stated not to use it for gasoline. #$%*, #$%*, #$%*, #$%*, #$%* lol

[flatlander]

verify with the "clear tube method" what actually happens in the carbs. you can search the forum for these terms, there should be plenty of posts describing it.
essentially, you'll be attaching clear tubes to the drain ports. they will let you see how the carb fills, when it stops and where the fuel level will remain. you can even check with engine running and blipping the throttle to see it under running conditions.

especially if you use after market parts or any modifications this is the easiest and most reliable way to tell what goes on inside the carb.

[Deltarider]

IIRC on the PD carbs CB550K3s and K4s have, you can simply attach a clear tube to the overflow pipes and then open the drain screws. On monitoring: give fuel time to reach its level.

[Stev-o]

especially if you use after market parts...

+1...verify you have genuine Keihin parts.

[Swiss Toni]

Good information, guys! I'll keep at it!

[Sw1ssdude]

Make sure that the floats dont bind up on the side of the float bowl (this usually results in no flow at all, but could do the opposite as well). You can switch the floats and bowls from a leaky carb to one that seals, and see if the problem follows.


Are you from Züri? In that case i could helf you aus with the carbproblemlösig...

[Swiss Toni]

SW, they're all leaking!  I'm in the UK.

[Don R]

 Well, that sucks. I've removed the floats in place and allowed fuel to flow out of the open float valve to flush out anything in there. I just got allen screws to make it easier to remove the bowls in place.

[HondaMan]

Well, that sucks. I've removed the floats in place and allowed fuel to flow out of the open float valve to flush out anything in there. I just got allen screws to make it easier to remove the bowls in place.

As a modification to this approach (worked out using a CB650, PITA to open up the float bowls in-frame) I have also drained a given float bowl, then turned the gas back on to try to flush a grit-bit out, into the bowl. Sometimes it works (it did this time!).