SOHC.co.uk Forums > CB750

PD carb query

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MarkCR750:
Please could someone with PD carbs have a look at their No1 carb and tell me if there should be a small brass blanking plug or similar in the small hole in the side of my carb, it's in the centre of the photo below and you can see a light coloured vapour mark on the carb body just below it, my tickover dropped earlier today, I just squirted carb cleaner in the small balance pipes in the back of the carb and it came out of that hole, it's never done that before! 😀.

PS I can see the same feature on the No2 carb, squirting carb cleaner in No2 does not result in it coming out of the side, so the question now is should it?, are these holes vents to atmosphere to achieve a balance between the intake and atmosphere?, maybe?, confused at present.

K2-K6:
Just had a look at some on F2 750 and they are different in that the part you are looking at has a much longer nose sticking out on the example here to about 10\15mm further than yours.

I can't quite see inside the casting to give an accurate idea of what's in there and the photo I've taken is in shadow down inside there, will try to investigate further.

K2-K6:
Still can't get camera flash and lens to point down the casting hole to get you a picture,  but if I put a tube onto that hole you can pull a vacuum on it and it flows no air so on that basis it should be sealed as far as I can see.

Hope that helps.

Nigel.

MarkCR750:
Thanks Nigel, it's odd the way that the casting slightly "hoods" the hole, like it's supposed to be open, I'll see if I can find a diagram that shows it.

PS I don't think the answer to my question is in the link below but it is a brilliant how to on PD carbs.


http://www.motopsyco.com/2013/08/09/vintage-piston-valve-keihin-carburetor-overhaul/

K2-K6:
It's good when people go to that effort to show others through something.

I know there's not alot of people like the PD carbs as they are fiddly to work with but they did work well originally in my experience, I also think they are definitely the right philosophy in that they run under constant load with a leaner mix and just richen for the acceleration demand. Means you get less fuel unburnt to contaminate the oil along with probably less bore wear.

This is after all what weber twin chokes on cars do and the American pumper carbs on older V8s which nobody complains about.

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