YZF750/1000R - The Fly 5

Prepping for the valve clearances. I was able to get started last night. You have to peel the onion to get the the valve cover.
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Last but not least. I built two Raspberry Pi3's over the holidays. This one will be a gateway to the internet for security cameras and a stateful firewall for the Wifi. Fun stuff.
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Hoping you can help me understand the proper function of the Mikuni BST36SS carbies I am using on my triple. Understand that the YZF uses a different carb, but they appear to be similar.
There are 2 hoses coming off your carbies, just under the diaphragms, I have highlighted in red in the photo.
It appears they would go into the airbox? Do you know what function they perform? Others have suggested that I blank them off, but I would prefer to understand their function before I do that.
 

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XS750AU said:
Hoping you can help me understand the proper function of the Mikuni BST36SS carbies I am using on my triple. Understand that the YZF uses a different carb, but they appear to be similar.
There are 2 hoses coming off your carbies, just under the diaphragms, I have highlighted in red in the photo.
It appears they would go into the airbox? Do you know what function they perform? Others have suggested that I blank them off, but I would prefer to understand their function before I do that.
My F650 comes with a pair of BST33's. Those hoses are part of your power jet circuit. It adjusts the mixture for high rpm delivery. I don't want to claim credit as some expert on these, so here is a good article I recently read that does a good job explaining...

http://oldskoolsuzuki.info/archives/531

If they don't go to the airbox, then they could be a breather for the float chambers (explained at the end of the article).
 
XS750AU said:
Hoping you can help me understand the proper function of the Mikuni BST36SS carbies I am using on my triple. Understand that the YZF uses a different carb, but they appear to be similar.
There are 2 hoses coming off your carbies, just under the diaphragms, I have highlighted in red in the photo.
It appears they would go into the airbox? Do you know what function they perform? Others have suggested that I blank them off, but I would prefer to understand their function before I do that.


I'm not sure this will help but on this bike, they are BDST38's and they do not go to the airbox and are used as bowl breathers. These carbs lay flat on the bike rather than cv end up on a standard bike. Likely the reason for the breather configuration. They have a sort of screened check valve on them also. In the pic you referenced I had moved them. They actually lay like this and just out of the way:
 

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they are BDST38's and they do not go to the airbox and are used as bowl breathers.
Thanks, that is a great help. I guess the BDST is for the horizontal version of the BST vertical carbi - they look very very similar.
Don't block them off. They serve a purpose.
That is exactly my thoughts, hence the question. I just wanted to know if they needed to vent or pick up a vacuum out of the airbox!
Next question - there is one hose coming off the diaphragm cap (black plastic) on one carbie only. The other 2 carbies have the port capped off. It appear to route down through the choke and into the vacuum side of the venturi. Is this for carbi function or is it just supplying vacuum for some other engine control on acceleration?
If it is for carbi function then why are 2 out of 3 carbies capped off?
Thanks for your help.
 
XS750AU said:
Thanks, that is a great help. I guess the BDST is for the horizontal version of the BST vertical carbi - they look very very similar.
That is exactly my thoughts, hence the question. I just wanted to know if they needed to vent or pick up a vacuum out of the airbox!
Next question - there is one hose coming off the diaphragm cap (black plastic) on one carbie only. The other 2 carbies have the port capped off. It appear to route down through the choke and into the vacuum side of the venturi. Is this for carbi function or is it just supplying vacuum for some other engine control on acceleration?
If it is for carbi function then why are 2 out of 3 carbies capped off?
Thanks for your help.
Did you see that link I posted up there in my first comment? That's a write up on these BST carbs and explains all the tubes.
 
Yeah, Factory Pro did cool stuff for the YZF.

http://www.factorypro.com/Prod_Pages/prody73.html
 
Did you see that link I posted up there in my first comment?
Sorry missed that!!!
Thank you both for your great help. I think I have now worked it out. It looks like all Mikuni BST carbies are not the same, as the Triumph BST's are different to the ones Factory Pro referred to.
Now for the explanation of how I think the Triumph BST36SS carbi Power Jet works.
The first photo shows the bell mouth, which has 2 air vents at the bottom, one for the pilot circuit and the other for the float bowl/main circuit. Up the top is the main vent for the diaphragm to lift the slide during acceleration.
The next photo shows the powerjet tube going from the float bowl up to the point where the fuel enters the air inlet between the butterfly and the cylinder head. The interesting point here is that the vent also extends up to the diaphragm cap to a hose barb. If the hose barb is not capped then the engine will just suck air and severely affect the carbi function.
The last photo shows the powerjet port coming in just behind the butterfly where there is max vacuum. By the way this also appears to be where the choke circuit also enters the carbi. The top highlight is the cap I have placed on the diaphragm cap hose barb. I guess Triumph had some kind of valve to control the vacuum at this point which I do not have on a 1978 XS750/896.
I am thinking that as the powerjet is pulling vacuum behind the butterfly that it will still function with the low impedance airfliter?
I think this is how they work - if I am wrong please let me know.
Canyon - sorry for highjacking you blog, but thanks for your help.
 

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XS750AU said:
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Thank you both for your great help. I think I have now worked it out.
I think this is how they work - if I am wrong please let me know.
Canyon - sorry for highjacking you blog, but thanks for your help.

No worries man, glad it helped.
 
Waiting on a thing or two I forgot to order. Looks good under the valve cover so far.
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I made a quick spreadsheet to log all the important info: (yes, some have surprisingly little clearance)
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Then lots of time with the feelers. It's a royal pain to get some of them in the right position to measure. The lobes on end of each cam are a "gimme" at least.
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clem said:
Is it looking pretty tight in there or do you have some slop?


26,017 miles total, and about 1400 of those are mine. It doesn't look it's been opened since Mr Yamaha built it. It's pretty tight.
 
It's good that nobody had been in there and monkey fingered the insides but they are way too tight. The book says valve clearances are to be done by 26k miles. (I think it's about every 14k on the Zrx.) It's a good thing I didn't try to squeeze more miles out of it before doing this job. The chart show my initial run through the measurements. I always do it twice on the initial check.
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