BS38 on 80/81. Mount/Boot issue.

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wmurray

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Hey everyone. Got another silly one for ya.

My engine number tells me I am running a 1980 Special II running and I'm running bs38s. I took the carbs off to clean after I bought it, and now they don't fit back in. I originally thought that I was just doing it wrong, but upon further research (combined 3 hours of cussing, finagling, mallet work, and 2 bruised palms), it appears I may have the wrong mounts/boots on the engine.

They have the barb for the line to link the boots for the vacuum, and I believe the tank uses a vacuum petcock based on its layout (prime, on, reserve - clockwise right to left). My question is this: am I better off replacing the boots with the 78-79 bs38 ones from mikes and also replace the vacuum petcock with a manual one, or should I try to use the pre-78 mounts with the vacuum/sync barbs or will they even fit?


Thank you for your help. Appreciate everyone fielding my newbie questions.
 
78-79 BS38 and all BS34 mounts came with barbs from the factory. The carbs are tight fit in the mounts. But the 80 SPII would have come from the factory with BS34s so it's possible someone muscled the 38's in them. Surprised they didn't split......

My trick for getting the carbs back on...
Smear carbs and holders with a bit of silicone grease. then put the bottom of the carbs in the holders, they will be angled down with space at the top of the connection. (this is easier on a lift) stand on the right side of the bike put right arm around front of engine inside LH pipe, reach back and grab carb rack. Now with one hand on each carb and some serious leverage. pull rack up and forward. They will pop in IF you have the right holders.
 
Thank you for the info!

Found this thread as well which confirmed my suspicion:

http://www.xs650.com/forum/showthread.php?t=32212

The intake on my carbs is 46mm OD and the ID of the boot is 41mm - indicating to me that PO somehow squeezed the 38s into the 34 boots.

Trouble is, I am anxious about buying these based on gggGary's recommendation, as I am concerned they will not properly fit: https://www.mikesxs.net/product/48-7479.html

The only others available (on Mikes) for bs38 carbs do not have the air barbs. If there are other places to find them, then I am all ears - I have been checking the good ol' ebay for a while.:banghead:

Additionally - I am unsure the mounts were installed properly to begin with :doh:. When I took the carbs off, I noticed the air barbs where pointed down and in (downwards toward center of bike) and linked together with a hose. this mounted the carbs at an angle which was lower toward the front and higher toward the back. Isn't this the opposite of how they are supposed to be mounted/aren't the barbs supposed to be open (if running a vacuum petcock)?
 
I just installed jbm manifolds on some 78 bs38s and they don't come or recommend drilling them for barbs (makes em weak) so I ordered 6mm vacuum adapters from motion pro to fit to each side of carb....for vacuum petcocksView attachment 44323View attachment 44323
 

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Pardon my ignorance payaso13, but since mine was originally set up incorrectly I am unsure about this - where do those hoses for the adapter kit attach, the petcock(s)? When I picked the bike up, my tank is attaching to the bowl of one carb, and the two carbs are linked together by another hose (attached photo for reference). The barbs on the mounts were also linked by a hose.

Now I feel silly... Stuck in one of those situations where "I don't know what I don't know." Thanks for bearing with me.


uiCka7q.jpg
 
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Yeah I'm not an expert on it but mine is linked in middle like yours and the hoses above that are for fuel...looks like they plugged on side and use the hose in middle to feed both carbs. Mine has two hoses from carbs (middle of your pic) running to each petcock...one on both sides of my tank. Then behind both petcocks is a vacuum line...attached there to each side of carb.
 
Also I just re-read you said barbs were facing down???? I believe there supposed to be pointing up about 10 and 2 o'clock...maybe your manifolds are mounted upside down? They should have an L and R stamped on them
 
Your carbs are set up correctly to run BS38 carbs on a single petcock BS34 tank. But that tank would have had a vacuum petcock from the factory, that needs vacuum from the LH barb, the RH barb gets capped off.
 
They definitely were, if they are supposed to point up. Another problem I am seeing is the were linked together instead of to a vacuum line on the petcock. I think I saw one hidden under the tank - about 2 or 3 inches of hose connected to it and just dangling inside. Seems like there is a lot of un-screwing-up I need to do haha.

The part that I am concerned about now is the tank is a single-petcock tank, which leads me to believe there is probably only one vacuum line. Is it possible to split the one vacuum line to both manifolds?

Am I better off ditching the vacuum petcock and installing a manual one instead so I don't have to worry about the barbs on the boots/splitting the vacuum line?
 
gggGary -

I see. So the side connected to the tank gets the vacuum line on the manifold, the other side just gets capped? Makes sense. Panic subsided a bit. Glad I have a second length of hose ready... would hate to have to wait for it to show up.

Now the trouble is going to be sourcing the manifolds... JBM sounds good but that's a bit steep. Also, did you have to permanently mod your carbs for that vacuum adapter to work?
 
You don't split the vacuum line, you just run it off one of the barbs and cap the other. You may want to investigate the function (or lack there of) of your petcock. If it is an original vacuum type, it's not supposed to flow fuel unless it's hooked to a vacuum line. Well, in the "On" and "Res" positions anyway. It will flow with no vacuum line in the "Pri" position. There's a problem with running that way though. The "Pri" position draws its fuel from the reserve pipe. When or if you run out of gas, you're all out with no back-up.
 
Heads up that in the past Mikes manifolds have given lots of trouble with poor quality rubber that quickly splits.
You might find someone willing to swap 34 for 38 manifolds. caveat; the factory manifolds are 35 years old and many are dry rotted.
 
5twins said:
You don't split the vacuum line, you just run it off one of the barbs and cap the other. You may want to investigate the function (or lack there of) of your petcock. If it is an original vacuum type, it's not supposed to flow fuel unless it's hooked to a vacuum line. Well, in the "On" and "Res" positions anyway. It will flow with no vacuum line in the "Pri" position. There's a problem with running that way though. The "Pri" position draws its fuel from the reserve pipe. When or if you run out of gas, you're all out with no back-up.
Click to expand...

Yep - that explains a lot about why it was kind of funky when I bought it. He said it only ran when on Prime, no vacuum line hooked up, barbs on left/right manifold linked and pointed inward, wrong diameter mounts.

gggGary said:
Heads up that in the past Mikes manifolds have given lots of trouble with poor quality rubber that quickly splits.
You might find someone willing to swap 34 for 38 manifolds. caveat; the factory manifolds are 35 years old and many are dry rotted.
Click to expand...

Fair enough - may be worth it to throw down for the JBM ones. Just concerned about modifying the carbs as mentioned to make the vacuum work.

This has been very helpful so far. I appreciate you all dealing with these silly questions haha. I am effectively trying to build a puzzle without seeing the whole image or knowing what it is. The advice is great! Hope I don't wear you all out before I get this guy running...
 

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