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Cv carb diaphragm cap question

thebrandonbeezy

thebrandonbeezy

Forum Mentor

This is the cap off of one of my bst38ss carbs, it has that vacuum nipple.. what is it for and where does it go. On my fzr one of the caps was missing for who knows why.. well these caps bolt on but the fzr has the vacuum routed into the chamber of the cap instead of to a nipple. Just wondering what the difference is and the purpose of this is
 
Might have to ask that on an FZR forum, because none of the GSes have a cap like that. :-k

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Im still waiting to have access to the fzronline forum but i think its dead haha. Anyways the cap pictured is off a suzuki i believe early 90s not sure what though. Im missing one fzr cap and the suzuki cap bolts on but i needed to know how those vacuums were ran.
 
thebrandonbeezy said:
Anyways the cap pictured is off a suzuki i believe early 90s not sure what though.
Click to expand...
You might have to spend some time browsing parts fiche to see if you can find one like that, then look up a service manual for that bike.

That is the kind of challenge I enjoy, but my employer has other plans for my next 6 hours or so. :-\\\

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Ive done a little Googleing and it appears theyre off of a 93 94 ish gixxer 750 and the vacuum nipple gets capped off on them. Looks like its tapped right into the throat of the carb behind the throttle butterfly on the gixxer. Now since the fzr is routed back into the chamber of the cap would that be for better throttle responce? I may try the gixxer caps with the tubes capped off on the fzr since i cant find any fzr caps on ebay unless i but an entire rack of carbs (not anywhere close to being in my budget)
 
Can you tell if the port simply vents above the diaphragm? If it does, and you connect to engine vacuum (between the throttle plates and the engine), that would simply hold the slides up all the time. Might be good on a race bike, but destroys the basic operating principle of a CV-type carb. The slide is supposed to move in relation to the airflow, to allow higher velocity at lower speeds to maintain crisp throttle response. If you hold the slides open and crack the throttle open, the engine will fall flat on its face.

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the brass nub protruding through the aluminum body is where the cap nipple is located. But rerouted to the cap chamber on fzr
you can see the tube runs down to the throat of the carb and meets with another tube
 
it joins in with this and this.
And then joined to the bowl aswell. now i ran the one cap on the fzr open just to hear the bike run, and it ran good but i couldn't ride it cause it was stuck between gears. Now im thinking of just running all four zuki caps and capping off the nipples like it would be on the zuk? Idk
 
OK. :-k

Apparently, whatever is in that port never gets to the space above the diaphragm.
In your first picture, it shows that the brass nipple is not connected to the vacuum side of the diaphragm. Would be nice to know where it goes. Can you spray some carb cleaner in there and see where it comes out?

In the second picture, it appears that the vertical tube comes down to where the "choke" plunger lives, but I don't think it joins there, it might come down and forward from there. Does it open into the carb throat?

In the third picture, that is a great shot down the "choke" circuit, but it appears that the plunger is pulled out, indicating the "choke" is ON.

In the fourth picture, that appears that might answer my question about the second picture. That might be the bottom end of that vertical tube. Can you verify?

The fifth picture seems to be a duplicate of the second picture, both of them seem to show that curved tube that goes from the bottom of the carb body to the fuel inlet of the "choke" circuit. The BS carbs in our GSes have that passage built in on the carb with just a short section of brass visible. There is no picture that shows it, but I will guess that there is a tube extending into the bowl below the end of that curved tube?

Now, the only truly "unknown" is the purpose of the port on the cap that appears to go unrestricted to the intake tract downstream of the throttle plate. My guess is that it might be part of some sort of emission control system. Because it feeds unregulated air directly into the engine, there must be some sort of valve system that controls it, like maybe only on deceleration (high vacuum) and maybe over a certain RPM, but that is strictly a guess.

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Its weird, im almost willing to bet that the nipple is California emissions. Seeing as all the bst38ss on ebay have the same nipple but with a rubber cap over it. I probably will run the bst caps on the fzr carbs with them plugged like it would be on the gixxer.. untill i find the correct unit. Steve, soon as i get home ill test what all it connects with. The exup is all electronic and reads rpm from my understanding, I'd like to experiment with that on a gs.. maybe one day haha
 
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