leaking issues bs38's, man what a pain!

[lil2lo]

Hi there,

So I recently bought a 1970 xs650 cafe racer. It has a 1977 engine with 1976 carbs. It has been sitting for a while so I'm doing some minor work on it to get it going.
The carbs where the first things on the list. They were leaking fuel. I cleaned them in a ultrasonic machine, cleaned everything again, checked, etc, etc.
After setting the floats, I put them back together and tested them in a setup on my workbench. With a clear tube from the float bowl I measured the fuel level. The new viton tipped needles are holding the fuel nicely. But now the problem. I measured the fuel level at 8 mm below the lower edge of the carb body. This is to low so I adjusted the floats. Then it was at 3 mm, so I was kinda happy with this, but after a while fuel was starting to leak from the carbs. It appears to come from the top of the float bowl. Both carbs are leaking. It's not much, but any fuel leak is to much in my opinion. With the fuel level at 8 mm the carbs didn't leak.

So how can carbs leak with the fuel level at 3 mm? Do I set them at 8 mm or will this be to low?

 

[peanut]

Hi and welcome .!

You're going to find those BS38s a real challenge at first. You'll soon find that you need to be really meticulous with cleaning and setting them up before you can get them to work reliably.

There is a comprehensive carb guide in the Tech section put together by some of the more experienced members but be warned its a lot of words to wade through first time round.

The fuel needle jets are obviously letting by and not sealing consistantly so that is where you should start resolving the flooding issue . Once you have the float needles shutting off reliably, your flooding issues will stop. Don't whatever you do be tempted to over- tighten the fuel bowl screws to try and seal the leaking or you'll strip those delicate aluminium threads for sure.

There are a few videos on youtube that might be worth watching but the bottom line is be really really meticulous in sealing everywhere on the inlet rubbers and carb bodies to prevent air leaks and make sure that the carb throttle plates are in sync .

 

[lil2lo]

Thanks peanut, I read the carb guide front to back and then again, it's very usefull. I always clean carbs with an ultrasonic cleaner and compressed air. So there's no doubt everything is clean.

You say that the fuel needle jets are letting by, but if that's the case the level in the clear tube should also rise. And that's not the case it stays at 3mm below the carb body edge. So I'm starting to think they might leak somewhere else. But as you mentioned I'm not going to tighten the bowls any further or I will strip the threading.

 

[gggGary]

It's possible a dreaded PO overtightened the bowls and bent the corners where the screws are. Use fine paper on a flat surface see if those corners show sanding first. Did you take a steaming hot cup of water and quickly dip the floats look for bubbles? I find well over half the floats I check are junk. The BS38s did not come with viton tipped needles, are you sure you have float needle sets that are designed to fit each other?
BS38 float bowls also seat the emulsion tube to the carb body, which has a really small o-ring for the seal. IIRC the o-ring is right under the base flange on the 76-77 carbs. If you bought a kit be SURE you have the right emulsion tube the oring location changed for different years some orings supplied may not be the correct size. It can be a bear to get even the right parts fully seated, it's possible that o-ring is keeping the float bowls from fully seating.

 

[peanut]

Thanks peanut, I read the carb guide front to back and then again, it's very usefull. I always clean carbs with an ultrasonic cleaner and compressed air. So there's no doubt everything is clean.

You say that the fuel needle jets are letting by, but if that's the case the level in the clear tube should also rise. And that's not the case it stays at 3mm below the carb body edge. So I'm starting to think they might leak somewhere else. But as you mentioned I'm not going to tighten the bowls any further or I will strip the threading.

Just because neither of the needle jets appear to let by when you conducted the float level test doesn't mean they don't let by intermittently at other times. If you watch the video through you'll see that its common for the float pivots to be too loose which throws out the float level setting by up to 5mm . Most owners wouldn't even notice or be aware of this .Also the needle jet can rub against one side of the needle jet chamber if the tang is at too acute angle.

Where else on the carb do you think could be leaking if the float needles are not letting by ??? if the float needle jets are sealed there is no fuel anywhere else in the carbs unless the engine is running and sucking fuel out of the bowls
Try using a piece of tissue paper to find the leak source

 

[lil2lo]

Thanks guys,

Use fine paper on a flat surface see if those corners show sanding first.

@gggGary
I will try that, wet sanding with some 2000 paper.

I find well over half the floats I check are junk

You're right about that. I forgot to mention that the original floats were leaking, so I replaced them. I tested the new ones first, just to be sure

are you sure you have float needle sets that are designed to fit each other?

Yeah I know, in an old post on this forum I read that you can use viton tipped needles from GL1100's. So I have those in, but I'm not so sure about them. Maybe I replace them with aftermarket new ones, but you know about the quality.

It can be a bear to get even the right parts fully seated, it's possible that o-ring is keeping the float bowls from fully seating

I think you mean the o-ring on the needle jet (correct me if I'm wrong). I've installed new ones and the fit nicely. The needle jets fit right in.

@peanut

Where else on the carb do you think could be leaking if the float needles are not letting by ???

Yeah that's what's puzzling me too. Actually when the fuel level stays where it should be there is no way it can leak through the gasket between the body and the bowl. But that's where the fuel comes out.

Just because neither of the needle jets appear to let by when you conducted the float level test doesn't mean they don't let by intermittently at other times.

Well yesterday I let I sit overnight and check once in a while and it appears that the level is staying the same, but then again it couldn't be leaking without me noticing it.

For now I'm gonna call it the day, maybe I tinker with it a bit further tomorrow. A bit off topic but here's a pic off the bike. Rumour has it this is the oldest XS650 in the Netherlands. But again a bit off topic.

 

[gggGary]

the early BS38s have brass overflow tubes in the float bowls that have been found cracked a time or 2.

 

[weekendrider]

But the leak wouldn't show at the top of the bowl, it would appear at the bottom. A weep that produces a drop or three probably wouldn't show in a float bowl sight line.unless the line has increment marks.

 

[peanut]

gorgeous bike.
I think your next step should be to bench test the carbs using a suspended fuel bottle and observe the carbs under good light so that you can check the carbs all round .

If both the float needle jets are sealing then there shouldn't be any way fuel can leak from the top of the fuel bowls so I suspect the leak must be coming from your fuel line/s or where they join the carb bodies

 

[peanut]

Yeah I know, in an old post on this forum I read that you can use viton tipped needles from GL1100's. So I have those in, but I'm not so sure about them. Maybe I replace them with aftermarket new ones, but you know about the quality.

EDIT
I just noticed this in your last post

gggGary said: ↑

are you sure you have float needle sets that are designed to fit each other?

Yeah I know, in an old post on this forum I read that you can use viton tipped needles from GL1100's. So I have those in,

The float needles and seat should be fitted as a matched pair . Did you fit the GL1100 needles only ? or did you fit them with the matching GL1100 seats as well ?

Your carbs are a frankenstein mix . I would have recommended the original brass floats were repaired and retained and a complete float valve set fitted for XS650 BS38 carbs

 

[lil2lo]

I hope I find some time tonight to work on the carbs again. On my to do list is sanding the sealing surface of the bowls. Just a good clean up with fine paper. The viton needles are going out and new bs38's ones go in along with new seats. Sadly the old floats are beyond repair. So I replaced them. I will also check the fuel lines.
I will keep you guys posted on the progress.

 

[gggGary]

Peanut gives good advice.
However repairing old floats is not the way to go.
Take a magnifying glass and examine them closely, if they aren't leaking now they soon will be.

This one was "really" bad but fix one leak, new ones will open. I find "cracks" around all the old floats I look at. Buy genuine mikuni floats, the cheap knocks offs have serious deficiencies in the pivot area resulting in unreliable fuel levels. So far I've not had any problems with knock off float valve assemblies.

 

[peanut]

must be close to a solution now good luck.

ggg those floats look terrible !.... I'm used to quality brass floats on old classic British vehicles

 

[TwoManyXS1Bs]

the early BS38s have brass overflow tubes in the float bowls that have been found cracked a time or 2.

This is what gggGary's talking about:

71XS1B Carbs Overflow03 It gets worse. Not only were bubbles emerging from the base of the overflow tubes, but from vertical splits as well.

• TwoManyXS1Bs
• Oct 25, 2015

From this album:

http://www.xs650.com/media/albums/1971-xs1b-bs38-carburetor-rebuild.2684/