Author Topic: 1978 CB550 with PD46 pressed in float valve needle seat LEAKING  (Read 433 times)

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Offline Saperlipopet

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Hello and thanks to anyone who takes the time to read / respond.

I'm looking for some advice concerning a suspected leaky float valve needle seat.

Short story: Bike was sitting for years. I took the carbs completely apart and replaced everything that came in the carb rebuild kits I ordered. #3 carb only is leaking bad. The needle valve seats seem to be pressed-in and I think I might have ruined it while cleaning. Can't find replacements anywhere. In the event that I need to replace one, what do I do?

Possible solutions found on this forum to try first:

-Test bowls for leaks (overflow tubes could be cracked or not sealing properly, common issue apparently)
-Try lapping the seat using valve lapping compound and a spare valve needle then bench testing to prove good seal
-Double check float level using clear hose on all carbs (I don't suspect float level is the issue since the other 3 carbs are fine and all the floats were set to 14mm)
Adjust float level to 12mm if needed (I hope I don't have this upside down)


The long story:

The #3 carburetor on my PD46 set is leaking from the overflow hose. A lot - not a little dribble.

I have disassembled, thoroughly cleaned and re-assembled with rebuild kits (parts replaced: main jets and emulsion tubes, slow jets, float valve needles, floats, throttle needles and seats, air/fuel mixture screws +o-rings and washers, bowl gaskets and top cover gaskets from vintagecb750.com)

Floats were set to 14mm.

While cleaning, one of the carbs (I BELIEVE it was #3, will confirm tomorrow) had a particularly crusty seat. I made a contraption using a pipe cleaner to try and clean it out but didn't notice it had a sharp metal end to it... Worried I ruined the seat. It looked a bit chewed up.

I'd like to try lapping it but I'm thinking WHAT IF there's not enough material left for the needle valve to reach up and make contact with. Time to look on evilbay for another carb? :(
Previous bikes
'81 CB650
'82 CB650
'81 CB900F
'75 CB360T
'78 CB750K

Current:
'78 CB550K

Offline grcamna2

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Re: 1978 CB550 with PD46 pressed in float valve needle seat LEAKING
« Reply #1 on: May 14, 2023, 09:26:11 AM »
You may have scratched a groove into that brass(soft)seat;you may be able to test that before installing it if you remove that carb. from the rack.
75' CB400F/'bunch o' parts' & 81' CB125S modded to a 'CB200S'
  I love the small ones too !
Do your BEST...nobody can take that away from you.

Offline Saperlipopet

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Re: 1978 CB550 with PD46 pressed in float valve needle seat LEAKING
« Reply #2 on: May 14, 2023, 11:08:27 AM »
You may have scratched a groove into that brass(soft)seat;you may be able to test that before installing it if you remove that carb. from the rack.
This is what I fear. Planning on doing a bench test to see what's going on.
Previous bikes
'81 CB650
'82 CB650
'81 CB900F
'75 CB360T
'78 CB750K

Current:
'78 CB550K

Offline grcamna2

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Re: 1978 CB550 with PD46 pressed in float valve needle seat LEAKING
« Reply #3 on: May 14, 2023, 11:14:08 AM »
You may have scratched a groove into that brass(soft)seat;you may be able to test that before installing it if you remove that carb. from the rack.
This is what I fear. Planning on doing a bench test to see what's going on.

I was able to use a soft(pine?)dowel once and formed the wood into a tapered point,then along with toothpaste I would twist it to polish the permanent brass seat;caution,brass is soft and you don't want to do too much !
I hope the scratches aren't too deep.. possibly something like fine jeweler's rouge on the end of a sharpened/tapered wooden dowel?
« Last Edit: May 14, 2023, 12:01:55 PM by grcamna2 »
75' CB400F/'bunch o' parts' & 81' CB125S modded to a 'CB200S'
  I love the small ones too !
Do your BEST...nobody can take that away from you.

Offline Saperlipopet

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Re: 1978 CB550 with PD46 pressed in float valve needle seat LEAKING
« Reply #4 on: May 14, 2023, 11:29:05 AM »
Love this idea - I'm thinking I'll take something like a wooden barbecue skewer and some valve lapping compound to it and see what happens.
Previous bikes
'81 CB650
'82 CB650
'81 CB900F
'75 CB360T
'78 CB750K

Current:
'78 CB550K

Offline grcamna2

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  • Posts: 15,183
  • I love to restore & travel. Keep'em Going Strong !
Re: 1978 CB550 with PD46 pressed in float valve needle seat LEAKING
« Reply #5 on: May 14, 2023, 12:04:02 PM »
Love this idea - I'm thinking I'll take something like a wooden barbecue skewer and some valve lapping compound to it and see what happens.

No  :o I don't recommend that;soft pine and very fine liquid such as toothpaste.
Go easy because brass is very soft:less is more.

I was wrong to recommend wood at all;the thing you Don't want to do is change the shape/taper of the brass valve seat in there !   A Q-Tip with toothpaste is Best.
« Last Edit: May 14, 2023, 12:13:02 PM by grcamna2 »
75' CB400F/'bunch o' parts' & 81' CB125S modded to a 'CB200S'
  I love the small ones too !
Do your BEST...nobody can take that away from you.

Offline Saperlipopet

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Re: 1978 CB550 with PD46 pressed in float valve needle seat LEAKING
« Reply #6 on: May 14, 2023, 12:23:38 PM »
Q-tip and toothpaste it is!
Previous bikes
'81 CB650
'82 CB650
'81 CB900F
'75 CB360T
'78 CB750K

Current:
'78 CB550K

Offline Saperlipopet

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  • Posts: 36
Re: 1978 CB550 with PD46 pressed in float valve needle seat LEAKING
« Reply #7 on: May 14, 2023, 04:16:35 PM »
Update. The brass overflow tube was cracked.
Previous bikes
'81 CB650
'82 CB650
'81 CB900F
'75 CB360T
'78 CB750K

Current:
'78 CB550K

Offline grcamna2

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  • Really Old Timer ...
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  • Posts: 15,183
  • I love to restore & travel. Keep'em Going Strong !
Re: 1978 CB550 with PD46 pressed in float valve needle seat LEAKING
« Reply #8 on: May 14, 2023, 05:11:00 PM »
Update. The brass overflow tube was cracked.

Oh  :D yeah,that makes sense.. how's the brass seat ?
There's threads on this forum on guys prepping them and then lightly soldering a crack in the overflow tube.
I was reading somewhere that you just need to be careful not to put too much solder onto it otherwise it can flow into the inner part and clog the tube.
75' CB400F/'bunch o' parts' & 81' CB125S modded to a 'CB200S'
  I love the small ones too !
Do your BEST...nobody can take that away from you.

Offline Saperlipopet

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  • Posts: 36
Re: 1978 CB550 with PD46 pressed in float valve needle seat LEAKING
« Reply #9 on: May 18, 2023, 12:52:12 PM »
I tried to solder it, made a mess, cleaned that mess up, then JB welded it. I'll probably get hate for that, but I don't care. Now the bowl holds.
Previous bikes
'81 CB650
'82 CB650
'81 CB900F
'75 CB360T
'78 CB750K

Current:
'78 CB550K

Offline grcamna2

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  • Really Old Timer ...
  • *******
  • Posts: 15,183
  • I love to restore & travel. Keep'em Going Strong !
Re: 1978 CB550 with PD46 pressed in float valve needle seat LEAKING
« Reply #10 on: May 18, 2023, 01:43:42 PM »
Does the overflow tube still 'breathe' when you put a little air into it ?
75' CB400F/'bunch o' parts' & 81' CB125S modded to a 'CB200S'
  I love the small ones too !
Do your BEST...nobody can take that away from you.

Offline Saperlipopet

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  • Posts: 36
Re: 1978 CB550 with PD46 pressed in float valve needle seat LEAKING
« Reply #11 on: May 22, 2023, 07:22:20 AM »
It does!
Previous bikes
'81 CB650
'82 CB650
'81 CB900F
'75 CB360T
'78 CB750K

Current:
'78 CB550K