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PD46a carb problem

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hubster123:
I’m wondering if anyone has encountered this issue before, or has any ideas how I can rectify it.
I’m no mechanic, so please bear with me.
I own a 1978 CB550 with PD46 carbs.
I won’t bore you with the details running up to some fuelling problems but I ended up carrying out a clear tube test on the carbs.
Everything was fine on carbs 1-3 but no matter what I do, the fuel will not sit right on carb 4 in the clear tube, with it always creeping way too high up the tube.
I checked the float height and it was correct at 14.5mm.
Basically what happens, no matter how high up I bend the tang on the float, when I open up the bleed screw at the bottom of the bowl, the fuel rises up the clear tube and keeps on rising past the join where the bowl meets the body of the carb until fuel starts to overflow out of the brass pipes on the inlet of the carb.
Strangely, if I don’t do the clear tube test, let the fuel fill up the bowl and let it sit for however long, then the fuel will not start leaking out of the overflow tubes.
But when I start up the bike, carb 4 clearly isn’t running well (header temperature is less than half of the other 3 on tickover).
I then do the clear tube test on carb 4 and again, the fuel rises up and up and comes out the overflow pipes.
It seems like doing the clear tube test on this particular carb causes it to go haywire.
I have tested that the float/needle are working correctly by holding a clear bowl up to the carb and it shuts off the fuel as soon as the float starts to rise.
I have replaced both the float and needle with new ones and it has made no difference.
I have checked to see that the float is not catching on the inside of the bowl by isolating the floats on 1-3 and moving the carbs around… it doesn’t  catch.
I have spent hours and hours trying to problem solve this issue with the majority of this time, the setup has been off the bike utilising the bikes fuel tank with the carbs in a clamp.
I have ran this issue past quite a few people with a great amount of knowledge on motorcycle mechanics however I have always drawn a blank.
If anyone has any ideas, it would be greatly appreciated.


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deltarider:
Could there be a blocked bowl vent? That would hinder the bowl to fill up to the right level in normal use. Once you attach the test tube (IMO a mediocre and cumbersome test method), the bowl has the ambient atmospheric pressure as it should. Once the vent is cleared, we could limit our focus on what causes the overflow. Model 1978 means nothing. Is it a K3 or a K4? IIRC these models had different vents. Either the carbs have a vent tube or a vent opening as shown below.

K2-K6:
I thought the same as delta, that it could have two faults present.

Checking for clear ventilation in normal installation with all hardware fitted and correct that if impaired.

Then look further at shutoff valve taper and seat to see what could be wrong there that allows fuel past when closed.

hubster123:

--- Quote from: deltarider on May 18, 2022, 08:45:11 PM ---Could there be a blocked bowl vent? That would hinder the bowl to fill up to the right level in normal use. Once you attach the test tube (IMO a mediocre and cumbersome test method), the bowl has the ambient atmospheric pressure as it should. Once the vent is cleared, we could limit our focus on what causes the overflow. Model 1978 means nothing. Is it a K3 or a K4? IIRC these models had different vents. Either the carbs have a vent tube or a vent opening as shown below.

--- End quote ---
Thanks for the quick reply…. Mine are 46a’s on a K3…


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hubster123:

--- Quote from: K2-K6 on May 18, 2022, 08:50:47 PM ---I thought the same as delta, that it could have two faults present.

Checking for clear ventilation in normal installation with all hardware fitted and correct that if impaired.

Then look further at shutoff valve taper and seat to see what could be wrong there that allows fuel past when closed.

--- End quote ---
Thanks… mine runs on velocity stacks.. Not great I know, but I’ve never had this issue before.
When I have the bowl off and I lift the float to 14.5mm, the fuel flow stops and not a single drop comes from the valve.


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