Honda-SOHC
SOHC.co.uk Forums => CB750 => Topic started by: MarkCR750 on May 28, 2016, 02:50:36 PM
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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.
(http://i1269.photobucket.com/albums/jj582/POR1983/Mobile%20Uploads/image_22.jpeg) (http://s1269.photobucket.com/user/POR1983/media/Mobile%20Uploads/image_22.jpeg.html)
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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.
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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.
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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/
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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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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.
Compared to PD carbs, the old Holley's on my mustang many years ago were great, you could probably use a broom handle to clean jets, and the accelerator pump could probably pump the titanic out to refloat it in about 10 pumps!
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One of life's great pleasures, sitting behind a nice V8 with the throttle wide open.
I've never owned a big yank V8 but spent alot of time with rovers and currently have a 3.9 on injection in a old Range Rover.
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Dude they some of the cleanest pd carbs I've seen. Mine are in a box and don't look that clean!
Sent from my iPad using Tapatalk
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Dude they some of the cleanest pd carbs I've seen. Mine are in a box and don't look that clean!
Sent from my iPad using Tapatalk
That's cos yours aren't painted 😄
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Hi Mark
I have been fiddling with my carbs for while & used plenty of carb cleaner as you , but I have not had any leakage of any kind from the very small hole that you are asking about. on my carbs that small hole looks like its sealed.
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I've googled it a fair bit but can't find anything , I actually think they should be open to atmosphere, that's because of the way the alloy is moulded above the hole, like a hood, really not sure though , I'll do some more googling.
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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.
(http://i1269.photobucket.com/albums/jj582/POR1983/Mobile%20Uploads/image_22.jpeg) (http://s1269.photobucket.com/user/POR1983/media/Mobile%20Uploads/image_22.jpeg.html)
There is a piece of alloy that looks like a tube connector missing off yours. The butcher has been at work there. I will try and get a usable photo of mine if I can. It should defo be blanked and not leaking.
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Managed to find a picture of mine, you can just see the tube type alloy piece here. All 4 carbs have a similar thing between 1&2 and 3&4 that face each other and look like they were at one time connected with a pipe, these have a brass insert to blank them. Only carb 1 has this extra hose fitting on the outside. It is completely sealed and the casting has never been drilled. Hope that helps.
[attach=1]
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Harrisluv from the US site has spoken, looks like its specific to the 77 model, thanks for your replies all
HARRISLUV
On the 77 PD carbs, there is no vent tubing between the carbs. Instead, each carb has its own individual vent hole. It's really tiny. It should be clear and open.
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I don't agree with that. My carbs in that picture are PD41B circa 1977 F2. The later carbs are PD42B circa 1978 onwards (possibly K7 1977). The bodies on both the F2 & K7 are the same just the jetting is different. That nozzle casting on mine has never been drilled and is airtight. Defo not a vent of any kind, the vents I believe are the 4 inner tubes facing each other.
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I don't agree with that. My carbs in that picture are PD41B circa 1977 F2. The later carbs are PD42B circa 1978 onwards. The bodies on both the F2 & K7 are the same just the jetting is different. That nozzle casting on mine has never been drilled and is airtight. Defo not a vent of any kind.
According to Harrisluv mine are PD41A, he seems to know his stuff, he rebuilds carbs as a sideline, he may have answered in brief not bothering to mention changeover dates etc, anyway I'm sure he's correct re my carbs due to the casting being slightly different , just got to unblock no2 and maybe 3&4 I haven't checked those yet, always something to do on a CB!.
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Here you go Roy, I found this image on google of a 41A body, the vent is the same as mine, so mine hasn't been adapted, about the only thing that hasn't !😄
(http://i1269.photobucket.com/albums/jj582/POR1983/Mobile%20Uploads/image_6.jpg) (http://s1269.photobucket.com/user/POR1983/media/Mobile%20Uploads/image_6.jpg.html)
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I stand corrected. The bodies on the F2 & K7 carbs are not the same (always thought they were). Your PD41A carbs are off a K7 and the settings also differ to F2 ie. float height, main jet, air screw, needle setting. You learn a bit more about these old girls every day.
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Here you go Roy, I found this image on google of a 41A body, the vent is the same as mine, so mine hasn't been adapted, about the only thing that hasn't !😄
(http://i1269.photobucket.com/albums/jj582/POR1983/Mobile%20Uploads/image_6.jpg) (http://s1269.photobucket.com/user/POR1983/media/Mobile%20Uploads/image_6.jpg.html)
Mark
I have just checked mine again with a magnifying glass & torch & it looks like mine has a tad of brass blanking it off, I certainly can't see or get a very fine piece of fuse wire in any hole...
I don't usually have a problem finding the hole... ;D
[ my carbs are the exact same carbs as yours]
have you found anything further
Greebo
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No I haven't had time to look today Greebo, out at 6am for a ride, breakfast at 8 with friends, straight back to clean the T250 for tomorrow now crashed out on the sofa, if there's a brass plug in there it sounds intentional, I'll ask Harrisuluv if they were modded later in production .
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Mark
Ask Matt Harper here : http://m.harpers-ultrasonic.com/
He refurbishes these carbs & he is very helpful, give him a call
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That's who did my carbs he is very helpful .