Floats stick with vibration / Wrong bowls? Wrong carb?

[Decker23]

Hey there. So I'm into the third month of trying to figure out what the devil is wrong with the carbs here.

LEAKY NEEDLE VALVE

- When I reassemble and clean everything, I hold the carb upside down and blow through the fuel intake. Or try to, can't get air through it to save my life. A good needle seal.

- But if I shake the thing a little bit, usually I can then blow through it. And shaking it some more will re-seat and stop my blowing.

- When installed onto the bike, the needle valve will behave properly until the bike has been running for a moment (seconds, a minute) and then start running gas out the air intake. If I turn off the bike and set the fuel to PRI, it will run freely out the air intake. By the time I remove the carb and get it back to the bench, the vibration re-seals the needle valve about half the time.

I suspect that the float is becoming stuck inside the bowl somehow, as it's a very close fit. Giving it the blow test (carbs upside down, bowls pointing up) and carefully lifting the bowl, I can hear the float fall back into place and make a good seal.

Any ideas why a float would stick to the sides of the bowl?

IS THIS THE RIGHT CARB?

Does the 1982 XS400 Maxim use the same carb as others, a Mikuni BS34? I ask because the one I have doesn't exactly match pictures of BS34s, and I have seen mention (a set for sale) that the carbs would fit a Maxim but not a standard XS400.

A photo of a BS34:

http://i437.photobucket.com/albums/qq93/bigfitz52/Vacuum Fuel Valve/Mikuni_Ex-bottom2.jpg

On mine, that silver plug at 12-o'clock is mssing, and is part of the bowl (which means cutting a hole when I buy BS34 gaskets). And there's a rubber plug over the pilot jet. And the drain screw isn't on the bottom of the bowl, but on the left-hand side. But I have been buying BS34 parts and they fit properly: needle valve seat, gaskets (almost), drain screws, etc.

These carbs don't have any identifying info on them besides "MIC Mikuni Kogyo" on the body. No other markings at all. They're nearly identical to a BS34, and I can't rule out that the previous owner has fussed with them somehow.

More photos here:

http://gregorthemotorcycleguy.blogspot.com/2013/06/cleaning-carbeurators-on-1982-xs400-day_28.html

 

[arfstrom]

LEAKY NEEDLE VALVE

- When I reassemble and clean everything, I hold the carb upside down and blow through the fuel intake. Or try to, can't get air through it to save my life. A good needle seal.

- But if I shake the thing a little bit, usually I can then blow through it. And shaking it some more will re-seat and stop my blowing.

I don't remember the blow test in the manual.

- When installed onto the bike, the needle valve will behave properly until the bike has been running for a moment (seconds, a minute) and then start running gas out the air intake. If I turn off the bike and set the fuel to PRI, it will run freely out the air intake. By the time I remove the carb and get it back to the bench, the vibration re-seals the needle valve about half the time.

PRI is right, Fuel running freely sounds like stuck float, don't forget to smell check for fuel in the oil dipstick hole.

I suspect that the float is becoming stuck inside the bowl somehow, as it's a very close fit. Giving it the blow test (carbs upside down, bowls pointing up) and carefully lifting the bowl, I can hear the float fall back into place and make a good seal.

I had one float that would work fine on day one of putting it back together, then day two the float would be sunk, the fuel would get booged down with fuel, and then like you said flood the air box, and the crankcase.

Any ideas why a float would stick to the sides of the bowl?

I suspect the tang that metal piece int he middle are Y out more than it needs to be. Open the manual, find the Float check method, (Look in Manual, and see the pics I uploaded on my profile, Plastic Tee fitting is overkill, (My OCD thought I can make a better mousetrap. I was wrong))

IS THIS THE RIGHT CARB?

Does the 1982 XS400 Maxim use the same carb as others, a Mikuni BS34? I ask because the one I have doesn't exactly match pictures of BS34s, and I have seen mention (a set for sale) that the carbs would fit a Maxim but not a standard XS400.

A photo of a BS34:

http://i437.photobucket.com/albums/q...Ex-bottom2.jpg

On mine, that silver plug at 12-o'clock is mssing, and is part of the bowl (which means cutting a hole when I buy BS34 gaskets). And there's a rubber plug over the pilot jet. And the drain screw isn't on the bottom of the bowl, but on the left-hand side. But I have been buying BS34 parts and they fit properly: needle valve seat, gaskets (almost), drain screws, etc.

Rubberplug is needed. Cant tell you why, I have seen the answer once, long forgotten the answer.

These carbs don't have any identifying info on them besides "MIC Mikuni Kogyo" on the body. No other markings at all. They're nearly identical to a BS34, and I can't rule out that the previous owner has fussed with them somehow.

More photos here:

Good luck

 

[Scorpio1963]

The rubber plug is need to keep gas from flowing in there from the bowls,its gets the fuel from another passage inside the carb.The bowls look like the correct ones.
As for the sticking float I think what you have going is one of the tangs from the float is not bent correctly and is catching on something.I agree with Tobie check and set your float levels correctly.

 

[Decker23]

CARB ID

What I find odd about these carb bowls, is that the bowls have that extra metal pug built into them, and not built into the carb body. Photos of other BS34s show the plug as being part of the body, and gaskets for the BS34 don't have a ring cut out for that plug, and I have to cut it myself. Also with the bowls, the drain plug isn't in the same place. Maybe a previous owner replaced the bowls at some point?

LEAKAGE

Thanks for the tip on the float height AGAIN. This is the 50th time I checked it, ... and you're right. The floats were bent ever-so-slightly too far outward (not to the side, but away from the float pivot) and under some conditions they would catch. It took me forever to finally put my finger on it, because the vibration of removing it from the bike would "solve" it.

Current status, is that it's not leaking anymore, but will run if I keep the choke/enrichener on. I'll proceed with the usual fussing and tuning, and replace those airbox boots which aren't sealing properly.

Thanks for the feedback, all.

 

[16VGTIDave]

Does the 1982 XS400 Maxim use the same carb as others, a Mikuni BS34? I ask because the one I have doesn't exactly match pictures of BS34s, and I have seen mention (a set for sale) that the carbs would fit a Maxim but not a standard XS400.

A photo of a BS34:

http://i437.photobucket.com/albums/qq93/bigfitz52/Vacuum Fuel Valve/Mikuni_Ex-bottom2.jpg

No, the Maxim uses slightly different carbs. The ones pictured here are for a SOHC.

On mine, that silver plug at 12-o'clock is mssing, and is part of the bowl (which means cutting a hole when I buy BS34 gaskets). And there's a rubber plug over the pilot jet. And the drain screw isn't on the bottom of the bowl, but on the left-hand side. But I have been buying BS34 parts and they fit properly: needle valve seat, gaskets (almost), drain screws, etc.

These carbs don't have any identifying info on them besides "MIC Mikuni Kogyo" on the body. No other markings at all. They're nearly identical to a BS34, and I can't rule out that the previous owner has fussed with them somehow.

More photos here:

http://gregorthemotorcycleguy.blogspot.com/2013/06/cleaning-carbeurators-on-1982-xs400-day_28.html

These carbs are DOHC! Note the drains on the float bowls and no rubber plugs for the pilot jets either.

 

[Decker23]

No, the Maxim uses slightly different carbs. The ones pictured here are for a SOHC. ... These carbs are DOHC! Note the drains on the float bowls and no rubber plugs for the pilot jets either.

Yep, that's what I've been finding: Maxim and Seca (DOHCs) use these, which aren't quite the same as a BS34. Do you know what model these are, or are they also called BS34 despite the differences?

 

[HoughMade]

They are still BS34s. Details were changed over the years.

 

[Decker23]

They are still BS34s. Details were changed over the years.

OK. Thanks a lot for clarifying.