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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.
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.
Is float level for yours supposed to be 14,5 or 12,5?
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.
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
[...]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 have replaced both the float and needle with new ones and it has made no difference.[...]
Quote from: hubster123 on May 18, 2022, 08:17:00 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!