Carb float question

[jjc839]

Keihin PD26 carb from a CB125T. Bike sat for many years, cleaned and rebuilt carbs, all new parts except floats. Bike ran great for the first couple of short rides.

Now, left side carb floods as soon as you turn on the fuel tap. Disassembled, check float height, all good (12mm). Float needle moving free. With carb off and float bowl off, gently blew into a hose connected to fuel port while moving the float up and down, and float needle appears to be sealing fine.

Removed float, immersed in water, no bubbles. Dropped in a bucket of gas, floats.

If the float needle seals (it does), and the float floats (it does), what am I missing?? This is a plastic type float, bear in mind it sat for many years. Could the float have "gained" some weight somehow over the years due to porosity or something? But the fact that it floats in petrol means there is no reason it shouldn't be closing the needle?

Tell me I'm missing something obvious, because I'm losing sleep over this. And of course new floats seem unobtainable.

[awayatc]

You can connect clear hose to bowl drain.
Raise end of clear hose above carb.
Now you can see till what level fuel goes before needle seals and shuts of fuel....

[OddDuck]

Rust in the tank getting in between needle's rubber taper and seat?

Tiny spring plunger in needle moving freely or jammed with old petrol varnish?

Float is jamming in bowl somehow?

Needle valve rubber tip gone hard over time, seals sometimes and not others?

All i can think of at the moment, sounds like a bit of a puzzle.

[husaberg]

Keihin PD26 carb from a CB125T. Bike sat for many years, cleaned and rebuilt carbs, all new parts except floats. Bike ran great for the first couple of short rides.

Now, left side carb floods as soon as you turn on the fuel tap. Disassembled, check float height, all good (12mm). Float needle moving free. With carb off and float bowl off, gently blew into a hose connected to fuel port while moving the float up and down, and float needle appears to be sealing fine.

Removed float, immersed in water, no bubbles. Dropped in a bucket of gas, floats.

If the float needle seals (it does), and the float floats (it does), what am I missing?? This is a plastic type float, bear in mind it sat for many years. Could the float have "gained" some weight somehow over the years due to porosity or something? But the fact that it floats in petrol means there is no reason it shouldn't be closing the needle?

Tell me I'm missing something obvious, because I'm losing sleep over this. And of course new floats seem unobtainable.

Tap the side of the float bowl while its leaking with a soft rubber hammer.
order a new carb kit anyway they are not that expensive.
Cb125 single or xr200 floats likely fit.

[jjc839]

Thanks all.

Tried the clear rubber hose attached to the drain, turned on tap and fuel goes up to the level of the carb throat (where it obviously leaks out the jets).

New float needles, so rubber in good shape and spring is free and functioning. Carb kit was installed however it didn't come with floats.

Float needle seals when blowing in fuel inlet with air, and there is an inline filter so shouldn't be any rust. Float bowl is clean.

Tried tapping on float bowl, no change.

It really is as though the float isn't moving at all. I don't know how it could be jamming up, everything is aligned, nothing bent, etc.


The only thing I can think of is that they are heavier than original, but still just light enough to float in petrol. I'll have a look for some XR200 floats, cheers.

[Askor]

Is the float binding on the new bowl gasket?

Are the bowl vents clogged?

Wack the carbs harder, might dislodge the floats :eek:

Worst case, buy new float valves and seats (if the seats are replaceable).

[jjc839]

Is the float binding on the new bowl gasket?

Are the bowl vents clogged?

Wack the carbs harder, might dislodge the floats :eek:

Worst case, buy new float valves and seats (if the seats are replaceable).
Bowl gasket is o-ring style, so no chance of binding.

Fuel comes out the vent when the tap is turned on, and I can blow through it, so it's clear.

Float valves are new. Seats are non-removable.

[awayatc]

Does valve close when you hold carb upside down and blow through fuel line?

[Akzle]

back to the clear tube thing. if the float has changed bouancy, change the height. bend the tab so it's lifting sooner and closing before flooding.

[jjc839]

back to the clear tube thing. if the float has changed bouancy, change the height. bend the tab so it's lifting sooner and closing before flooding.
Valve closes and seals when carb is held upside down.

Good idea on the change of buoyancy. I'll try to increasing the float height to counter it and see if that helps.

Cheers

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[jjc839]

Update:

Just because floats float, and don't appear to have any holes/cracks, doesn't mean they are any good.

With the carb on the bike, I removed the float bowl and put a clear plastic container in its place. Turned the fuel on, and sure enough the float was almost completely submerged before it pushed the needle up, which resulted in the fuel level being right up to the flange.

Adjusted the float height from 12 to 15 mm, and although still running rich, at least it isn't flooding. New floats on the way.

Thanks for all the help.

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[awayatc]

Good to hear you got to the bottom of it...well done

[Akzle]

your welcome.

as to running rich. where in the range?
worn jets likely.

[jjc839]

your welcome.

as to running rich. where in the range?
worn jets likely.
Runs good, hard to tell, bit of a stumble midrange. Stock size pilot and main, new.

Fuel level is still too high so I'm sure that has something to do with it. Will reassess with new floats. Plugs sooty.

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[FJRider]

Check for a blocked/dirty air filter. that will cause it to run rich .... and the path from the filter to the carb for leaks too.

[jjc839]

Leak free and brand new filters. I'm 99% sure it's fuel level. I'll update when the new floats are in. Everything is stock on the bike.

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[Akzle]

Runs good, hard to tell, bit of a stumble midrange. Stock size pilot and main, new.

Fuel level is still too high so I'm sure that has something to do with it. Will reassess with new floats. Plugs sooty.

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a) plug gap. b) plug chop.

it's been long since I played with carbs but there really isnt much to fuck up.

new needles too?

mid range would be as vacuum is decreasing and it rolls off one jet to the next... drawing too much fuel... clip up.
hows your ign timing?

[jjc839]

a) plug gap. b) plug chop.

it's been long since I played with carbs but there really isnt much to fuck up.

new needles too?

mid range would be as vacuum is decreasing and it rolls off one jet to the next... drawing too much fuel... clip up.
hows your ign timing?
New plugs, heat range and gap as the manual says. New jets and needles. Timing checked, all good.

When I cranked up the float height, it basically got to where it wouldn't flood anymore, but it's by no means where it should be. I never tried to increase it any further. If it's still rich with the new floats I'll clip up. But it's set on the second groove as the manual states. I'll keep you informed.

Cheers

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[jjc839]

New plugs, heat range and gap as the manual says. New jets and needles. Timing checked, all good.

When I cranked up the float height, it basically got to where it wouldn't flood anymore, but it's by no means where it should be. I never tried to increase it any further. If it's still rich with the new floats I'll clip up. But it's set on the second groove as the manual states. I'll keep you informed.

Cheers

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What is "plug chop?"

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[Akzle]

What is "plug chop?"

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god damn, son. you'd been doing really well up until now.

I suppose everybody deserves one.

http://lmgtfy.com/?q=what+is+plug+chop

[jjc839]

god damn, son. you'd been doing really well up until now.

I suppose everybody deserves one.

http://lmgtfy.com/?q=what+is+plug+chop
Ya well if I was smart I wouldn't be messing around with a 40 year old 125 now would I?

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[Akzle]

au contraire

[Askor]

If you haven't replaced it already, check that the emulsion tube isnt worn - specifically where the needle enters it. Fairly common problem on older high km bikes, the orifice gets ovalised causing a rich condition midrange

[husaberg]

If you haven't replaced it already, check that the emulsion tube isnt worn - specifically where the needle enters it. Fairly common problem on older high km bikes, the orifice gets ovalised causing a rich condition midrange

Especially Keihin's as they normally are not replacable, esp in the two strokes unlike a Mikuni.
Normally the only cure is a new carb or a fatter needle.