Big fuel leak from rebuilt Carb

[grusvag]

Just rebuilt a pair of carbs of the BS34III vintage, put them back on the bike, set the petcock to Prime and not long afterwards a lot of fuel came pouring out the right-hand carburetor. I had the air cleaners off and the fuel was coming out the back of carb and would have gone right into the air cleaner so it appears to be an internal leak.

Before I pull them back off the bike and start breaking them down again, does anyone know where the leak might be coming from and where I should be looking in order to fix this problem?

[drfong]

Must be the needle and set or float in the carb that is overflowing.

[grusvag]

Must be the needle and set or float in the carb that is overflowing.

I knew enough to take them apart and clean them but most of the inner workings are still a mystery to me. I'm guessing that when fuel fills the float bowl it raises the floats which then make the needle valve assembly close off the flow of fuel into the float bowl. Do I have this right?

That would make sense because the floats were way beyond spec - they were too high - when I put the carbs back together. Shortening this distance would make the needle valve close off the fuel flow sooner, right?

[drewpy]

I knew enough to take them apart and clean them but most of the inner workings are still a mystery to me. I'm guessing that when fuel fills the float bowl it raises the floats which then make the needle valve assembly close off the flow of fuel into the float bowl. Do I have this right?

That would make sense because the floats were way beyond spec - they were too high - when I put the carbs back together. Shortening this distance would make the needle valve close off the fuel flow sooner, right?

bending the tag outwards ie opening the gap, will get the floats to close the fuel off sooner

[grusvag]

Just want to make sure I'm doing this right.

To raise the float height and shut the fuel flow off sooner I should bend the "tongue" of the float:

So that this distance measures 1.26" (according to the Haynes manual):

And just to be clear, this is done when the carbs are off the bike, and upside down.

Do I have this right? It's a big change from where it was.

[Cy Welch]

Just want to make sure I'm doing this right.

To raise the float height and shut the fuel flow off sooner I should bend the "tongue" of the float:

So that this distance measures 1.26" (according to the Haynes manual):

And just to be clear, this is done when the carbs are off the bike, and upside down.

Do I have this right? It's a big change from where it was.

The bs34 III's are 1980 models and unless the jets have been changed the float height would be 1.1 inches rather than 1.26 like on model II's.

[grusvag]

The bs34 III's are 1980 models and unless the jets have been changed the float height would be 1.1 inches rather than 1.26 like on model II's.

So I guess I'm bending the right thing to adjust the float height?

And is it ok to use the BS34 IIIs on my 1979 bike?

[drewpy]

So I guess I'm bending the right thing to adjust the float height?

And is it ok to use the BS34 IIIs on my 1979 bike?

The float height is measured at fuel shut off NOT bottomed out as in pictures!!

bend the tongue down towards carb to shut fuel off earlier and lowers the level of fuel in carb bowl

are you sure they are 111's? the tickover adjustment is on the outside and have plastic floats

[grusvag]

The float height is measured at fuel shut off NOT bottomed out as in pictures!!

bend the tongue down towards carb to shut fuel off earlier and lowers the level of fuel in carb bowl

are you sure they are 111's? the tickover adjustment is on the outside and have plastic floats

Now I'm really confused!

1. I thought the floats shut the fuel flow off when they were bottomed out as in the pictures (or raised when in their proper orientation when mounted on the bike). When the floats rise, they push the needle valve assembly up so that the rubber cone seats and closes off fuel flow, right?

2. I'll reset float height to 1.1" while holding the float at it's maximum height (while upside down on the bench).

3. I was told that they're BS34 IIIs from a 1980 and later US vintage because the idle mixture screw is capped off as show in this picture:

[Cy Welch]

Now I'm really confused!

1. I thought the floats shut the fuel flow off when they were bottomed out as in the pictures (or raised when in their proper orientation when mounted on the bike). When the floats rise, they push the needle valve assembly up so that the rubber cone seats and closes off fuel flow, right?

2. I'll reset float height to 1.1" while holding the float at it's maximum height (while upside down on the bench).

3. I was told that they're BS34 IIIs from a 1980 and later US vintage because the idle mixture screw is capped off as show in this picture:

Don't hold it at max height, but make sure that the little button on the bottom of the needle valve is not comressed at all. Generally if you just let them hang by their own weight when upside down that will get you the right measure.

Yes, those are III's and as far as I know they will run fine as long as you adjust them like you would a 1980 since the jetting and all are different.

[drewpy]

Now I'm really confused!

1. I thought the floats shut the fuel flow off when they were bottomed out as in the pictures (or raised when in their proper orientation when mounted on the bike). When the floats rise, they push the needle valve assembly up so that the rubber cone seats and closes off fuel flow, right?

2. I'll reset float height to 1.1" while holding the float at it's maximum height (while upside down on the bench)

3. I was told that they're BS34 IIIs from a 1980 and later US vintage because the idle mixture screw is capped off as show in this picture:

1. correct so when you open up the tang it shuts the fuel off "earlier" and gives you a lower fuel height. You hold the carbs right way round and gently blow though the fuel inlet whilst raising the floats. when the air stops, that's where the measurement is taken.

2. I thought 111's were for the dohc xs400? correct me if i'm wrong.

3. I don't do inches only metric

[grusvag]

1. correct so when you open up the tang it shuts the fuel off "earlier" and gives you a lower fuel height. You hold the carbs right way round and gently blow though the fuel inlet whilst raising the floats. when the air stops, that's where the measurement is taken.

Thanks, I'll try that.

2. I thought 111's were for the dohc xs400? correct me if i'm wrong.

Here's what Cy Welch said about them from another thread:

"Actually the 79 model year shouldn't have the plug like that but rather a screw with a limit cap on it. The plugged one would be a 1980 or later US model with the BS34 III rather than the BS34 II on the 79's. I haven't seen one from that age range that didn't need drilled out and adjusted, and you may need to replace the o-rings on the adjuster to get it working well."

3. I don't do inches only metric

Just curious - what are the metric specs?

[drewpy]

found a supplementl section in my official Yam service manual, here goes;

model id 3F9-00

main jet #135

needle jet Y-2

Pilot jet #42.5

starter jet #35

jet needle 5GZ9

float height 27.3 +/- 0.5mm

air jet main #45

air jet pilot #155

[grusvag]

found a supplementl section in my official Yam service manual, here goes;

model id 3F9-00

main jet #135

needle jet Y-2

Pilot jet #42.5

starter jet #35

jet needle 5GZ9

float height 27.3 +/- 0.5mm

air jet main #45

air jet pilot #155

Awesome, thanks.

Is this for the model II or model III BS34?

[Cy Welch]

1. correct so when you open up the tang it shuts the fuel off "earlier" and gives you a lower fuel height. You hold the carbs right way round and gently blow though the fuel inlet whilst raising the floats. when the air stops, that's where the measurement is taken.

2. I thought 111's were for the dohc xs400? correct me if i'm wrong.

3. I don't do inches only metric

The III's are on pretty much everything US delivered from 1980 - ???. Each model of bike got different jetting, but the base carb model was the same. 650's 750's 1100's and 400's for sure, I'm not sure how many of the others got the same carbs as well.

[drewpy]

Awesome, thanks.

Is this for the model II or model III BS34?

for an xs400sg

[Cy Welch]

for an xs400sg

Drewpy was that the 79 or 80 model? The 79 should have the II and the 80 would have the III.

[drewpy]

Drewpy was that the 79 or 80 model? The 79 should have the II and the 80 would have the III.

the service manual says it is 1st edition may 1981 from engine # 3F9-000101

[grusvag]

So do you think these specs would be good for my 1980 BS34 III carbs?

[Cy Welch]

So do you think these specs would be good for my 1980 BS34 III carbs?

Float height matches that for my 1980 BS34 MkIII's. From memory the rest match up as well, but those I don't keep in my head (float height I do though as I have played with it so much).