Topic: 750K4 Keihin floats and needles are eating my lunch

[Maltboy]

After rebuilding a set of 750K4 Keihin carbs, I'm having hell getting all of them to stop overflowing. A new set of Keyster needles/holders that came with the bike were installed but some leaked.  The floats (all original) were checked by putting them in a container filled with gasoline to make sure they all floated at the same height after a day of soaking. Float heights were carefully adjusted, and checked to make sure they weren't rubbing or getting stuck.  Replaced needles/holders with old Keihin parts that I polished including the holder bore and the needle flutes that rub against it.  Needles/holders were hand lapped (like lapping valves only really small) with polishing compound to make sure they sealed right.  The little spring-loaded tits on the end of the needles move freely and are still springy. Overflow tubes were all checked for cracks and leaks.  There were none.  Everything seems to seal fine until the bike sits for a few hours, then the leaking starts.

I don't know what to do next. Has anyone else fought this monster before?

[vames]

It really makes sense to develop a habit of turning the petcock off each time you shut down the bike -- it becomes second nature. I've had bikes where the float valves are virtually perfect, and even those would sometimes piss out a tank of gas with no warning.

[seanbarney41]

yup, if the engine is off, the petcock is closed.  If you continue to have leaks with the petcock off, time to fix the petcock.  If you continue to have leaks with the engine running, you still have float valve problems.

[Maltboy]

Thanks guys.  Lesson learnt.  It's really a pisser though.  I mean literally.

[kerryb]

The only way I could get new aftermarket float needles to seal,  to check float height ( clear tube method), was to put in new  oem needles and seats (from Honda dealer) at $40 / carb.  I've also read here many times that keyster jet needles usually don't match the shape of the keihin needles.
Pic is from a set of keihin carbs off my goldwing

[david 750f]

The floats can hang up on the float bowl gaskets. Are yours aftermarket or Honda? They might need a slight trimming.

[754]

The tang adjusted  from tge side and not parallel to pivot, can cause float to hang up against post sometimes.

[Maltboy]

The gaskets are pretty skinny, but I will check to make sure.

Frank, do you mean the tang should not be a different angle than the pivot like a < ?  So it's supposed to be more like a sideways U ?

[754]

The flat of the  tang shouod be parallel to pivot pin. If you bend it from the side ut can be at an angle to the pin.

[Maltboy]

The flat of the  tang shouod be parallel to pivot pin. If you bend it from the side ut can be at an angle to the pin.

Got it. Thanks.

[jonda500]

Not disagreeing with the good advice given, but, except when removing the tank I have never bothered to turn off the fuel in any of my honda's - if it is gonna leak from worn out needle&seats or something stuck in there, then it's gonna leak anyway and I'd rather it leaks on the concrete in the garage where I'll see it than the rear tyre while I am riding it! (ok, I have actually done the toggle the fuel tap on/off riding my bike while I was waiting for new needle&seats to arrive)
John

[Maltboy]

The carb overflow issue seems to be solved.  I paid special attention to the bowl gaskets to make sure they weren't rubbing on the floats.  All the float tabs looked fine, but all of them had a pip mark on the tab where the needle "tit" contacts it.  It seems like that could possibly cause a float to stick so it was removed with a mini file and sandpaper. I also polished the float pins and cleaned up the hole where the pin slides through the floats with a drill bit.  I'm not sure what actually fixed the problem, but it's working so far.

[ekpent]

Not disagreeing with the good advice given, but, except when removing the tank I have never bothered to turn off the fuel in any of my honda's - if it is gonna leak from worn out needle&seats or something stuck in there, then it's gonna leak anyway and I'd rather it leaks on the concrete in the garage where I'll see it than the rear tyre while I am riding it! (ok, I have actually done the toggle the fuel tap on/off riding my bike while I was waiting for new needle&seats to arrive)
John

What I worry more about with a bad leaky petcock or needle valve is getting gas down in the engine and contaminating the oil and also hard on any rubber bits. Raw gas getting into the air box can swell and ruin the seal between the two halves on a 750 also.

[Maltboy]

The airbox got soaked a couple of times.  It almost ruined a brand new K&N filter, but it "recovered".  I'm sure a bunch got in the engine too. Ugh.  I make absolutely sure to shut off the fuel now.  The problem is I keep forgetting to turn it back on! 

[Don R]

 I learned to tilt the carbs left then right and check the float clearance each way. I've had them hang up on seemingly nothing. I agree on turning the fuel off. It's not if one will eventually leak, it's more when.

[jonda500]

Unless the overflow brass pipes or rubber hoses are blocked the only place fuel from over filling float bowls will end up going is out the bottom of the overflow hoses? (or out of the seams if the float bowl gaskets are buggered or missing)
John

[Maltboy]

Unless the overflow brass pipes or rubber hoses are blocked the only place fuel from over filling float bowls will end up going is out the bottom of the overflow hoses? (or out of the seams if the float bowl gaskets are buggered or missing)
John

They leaked gas out the overflow hoses too. The problem was the hole in the brass overflow tube is so small that it couldn't keep up. So the fuel level rose up into the carbs and got into the airbox and maybe the engine too.

[HondaMan]

The general design of the carbs requires the fuel be shut off when parked. The manuals refer to this, too. The reason: whenever the wind blows, the local air pressure drops, disturbing the pressure of the floats' tangs against the float valves. This cycle repeats also as the bike is cooling (after being run) as the carbs cool. The result is overflow, naturally. That's why the extensive nipples and drain tubes, clips and all, with even a gathering clamp for them, is on all these bikes. That's a lot of expense for Honda, and they never spent a Yen they didn't need to on these bikes!

[Don R]

Old guy story but maybe someone can learn from it,    I bought an old 850 yamaha with a vac petcock that (I learned later) didn't work. I thought the carbs were holding fuel so I dumped a couple gallons of gas in the tank. That was when I learned the carbs weren't holding anything and those petcocks have no off setting. On, reserve and prime.  I had to roll it out in the driveway and shove an oil change pan under it.  It was heating season and I thought my shop was going to blow up. That's why the furnace is up by the ceiling. 

[Maltboy]

The general design of the carbs requires the fuel be shut off when parked. The manuals refer to this, too. The reason: whenever the wind blows, the local air pressure drops, disturbing the pressure of the floats' tangs against the float valves. This cycle repeats also as the bike is cooling (after being run) as the carbs cool. The result is overflow, naturally. That's why the extensive nipples and drain tubes, clips and all, with even a gathering clamp for them, is on all these bikes. That's a lot of expense for Honda, and they never spent a Yen they didn't need to on these bikes!

That makes sense, and it also explains why they didn't leak when I was working on it, but they did leak at night when the shop cooled down.  I guess that's one of the downsides of a gravity-fed fuel system.  Live and learn.

[Bankerdanny]

My experience with the Keyster float needles is that they are shorter than stock, so your floats may not be at the correct height. I only use genuine Honda needles on any of my bikes any more. On my '75 550F I had issues with the float hanging up because of the dimples on the tang. Light sanding and polishing to remove it definitely helped, although they would still hang up sometimes.

[Maltboy]

I think I have that covered, but I'm learning how much I don't know every day it seems. 

The original needles were cleaned up and re-used. They were each hand-lapped with the holders to get a good seat. All the spring-loaded "tits" on the needles were checked to make sure they moved freely and were still springy.