Author Topic: PD46a carb problem  (Read 779 times)

Offline hubster123

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PD46a carb problem
« on: May 18, 2022, 08:17:00 PM »
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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Offline deltarider

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Re: PD46a carb problem
« Reply #1 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.
« Last Edit: May 18, 2022, 08:56:32 PM by deltarider »

Offline K2-K6

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Re: PD46a carb problem
« Reply #2 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.


Offline hubster123

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Re: PD46a carb problem
« Reply #3 on: May 18, 2022, 09:00:45 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.
Thanks for the quick reply…. Mine are 46a’s on a K3…


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Offline hubster123

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Re: PD46a carb problem
« Reply #4 on: May 18, 2022, 09:06:27 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.
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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Offline deltarider

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Re: PD46a carb problem
« Reply #5 on: May 18, 2022, 09:11:53 PM »
Is float level for yours supposed to be 14,5 or 12,5?

Offline hubster123

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Re: PD46a carb problem
« Reply #6 on: May 18, 2022, 09:29:23 PM »
Is float level for yours supposed to be 14,5 or 12,5?
14.5 … I’ve checked and re checked that setting… and tbh, if it was 12.5, things would be even worse fuel level wise..


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Offline K2-K6

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Re: PD46a carb problem
« Reply #7 on: May 18, 2022, 09:50:03 PM »
Definitely worth checking venting to ensure that's functional as designed,  it'll play havoc with fuel level if not correct.

Needs eliminating from enquiries before next steps really.

Offline hubster123

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PD46a carb problem
« Reply #8 on: May 18, 2022, 10:12:22 PM »
Definitely worth checking venting to ensure that's functional as designed,  it'll play havoc with fuel level if not correct.

Needs eliminating from enquiries before next steps really.
Yeah definitely… I can’t think where there are vents are on the 46a’s though .. nothing springs to mind from memory and they’re slightly different to the photos deltarider posted… I’ll have a look in the morning


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Offline Bryanj

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Re: PD46a carb problem
« Reply #9 on: May 18, 2022, 11:00:23 PM »
There are always vents or the carbs cant work, it has to be fuel bypassing the needle seat somehow.
Another reason to hate PD carbs

Offline hubster123

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Re: PD46a carb problem
« Reply #10 on: May 19, 2022, 06:36:49 AM »
There are always vents or the carbs cant work, it has to be fuel bypassing the needle seat somehow.
Another reason to hate PD carbs
Believe me Bryan, I despise them!!! Someone has mentioned on the 46a’s, the vents are the 2 small tubes on the intake end of the carbs. I can confirm that they are all clear. I agree it can only be fuel getting past the needle but this only seems to happen when I do the clear tube test or run the bike up.


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Offline deltarider

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Re: PD46a carb problem
« Reply #11 on: May 19, 2022, 08:40:45 AM »
[...]
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.[...]
Smell if maybe there's petrol in the oil.
Quote
[...] 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).[...]
What does the #4 plug nose look like, at idle and cruising? Have you checked the #4 plugcap is OK and there's no accidental arcing to ground? I know this is not carb related, but all CB500/550s, sooner or later, will suffer from this. And it's easily checked in the dark.
Quote
[...] I have replaced both the float and needle with new ones and it has made no difference.[...]
OEM or aftermarket?
IIRC, these carbs join one vent route, where as the oldstype carbs have two tubes, but... I could be wrong. Furthermore from memory: irregularities have been reported, which could be adressed by simply cutting the vent tube where it dangles behind the engine, near the swing arm, by an inch (or two). Make sure tube does not become pinched.
There's been said many negative things about these carbs and I fear we've now reached the stage where everybody echoes the other. There's a risk in this. It's my gut feeling these carbs can be helped to work as they should, by applying a maintenance (10-15ml) dose of fuel system cleaners, like Redex, at every fill up. More won't hurt. The stuff is perfectly safe and has no side effects. Please, do not experiment by adding doses of oil or diesel!
« Last Edit: May 19, 2022, 08:46:43 AM by deltarider »

Offline hubster123

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Re: PD46a carb problem
« Reply #12 on: May 19, 2022, 12:06:32 PM »
[...]
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.[...]
Smell if maybe there's petrol in the oil.
Quote
[...] 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).[...]
What does the #4 plug nose look like, at idle and cruising? Have you checked the #4 plugcap is OK and there's no accidental arcing to ground? I know this is not carb related, but all CB500/550s, sooner or later, will suffer from this. And it's easily checked in the dark.
Quote
[...] I have replaced both the float and needle with new ones and it has made no difference.[...]
OEM or aftermarket?
IIRC, these carbs join one vent route, where as the oldstype carbs have two tubes, but... I could be wrong. Furthermore from memory: irregularities have been reported, which could be adressed by simply cutting the vent tube where it dangles behind the engine, near the swing arm, by an inch (or two). Make sure tube does not become pinched.
There's been said many negative things about these carbs and I fear we've now reached the stage where everybody echoes the other. There's a risk in this. It's my gut feeling these carbs can be helped to work as they should, by applying a maintenance (10-15ml) dose of fuel system cleaners, like Redex, at every fill up. More won't hurt. The stuff is perfectly safe and has no side effects. Please, do not experiment by adding doses of oil or diesel!
The plug gets pretty carboned up but I’m putting that down to the fuel level moving all over the place. The coils and leads were replaced a couple of years ago and I put new plugs in last week.. The spark is fine


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Offline hubster123

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Re: PD46a carb problem
« Reply #13 on: May 19, 2022, 06:56:10 PM »
I’ll stick an update on here when I get the chance but it looks like I’ve sorted it… thanks everyone for all your help


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Offline Andy1488

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Re: PD46a carb problem
« Reply #14 on: May 25, 2022, 11:11:07 AM »
How did you sort it, and what was the problem ..?

 

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