Honda-SOHC
SOHC.co.uk Forums => CB500/550 => Topic started by: Neill C on March 07, 2023, 07:06:46 PM
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Hi there. I've been bench testing my carb after suffering from float bowl overflow leaks. One bowl slightly overflows after about four hours. Float levels are set at 22mm as the book says. Would you consider that overflowing after 4 hours is perfectly OK seeing as you should turn the petcock off if the bike is going to be left on its stand for 4 hours anyway?
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Should not leak no matter how long it’s left.
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Of course. I'd get the fuel that's in the pipe after the petcock on my garage floor every time I park her up otherwise. Thanks.
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Left on side stand or main stand?
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Have you tried tapping the float bowl to see if its the float valve is leaking? Maybe the spring has had it.
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The carb is on the bench in a vice. I've tapped the bowl but still overflows. I'm beginning to think I'll have to set thr float slightly lower than the specified 22mm
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I would be thinking it's the float needle & seat.
I'd do what was suggested earlier & try to lap the seat before replacing it assuming the float is all good.
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Have you replaced the float needle and seat by any chance
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Yes. Could the float just be slightly less buoyant than the others? They are 45 years old after all.
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It’s most likely the needle and seat, well known for aftermarket needles and seats to be sub standard compared to genuine brass. I’ve seen some where the point on the needle isn’t even central.
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What's the answer there then. Do I keep replacing the valve and seat until I find one that doesn't leak?
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Neil try a cotton bud in a drill with a little tooth paste and polish up the valve seat. I’ve been lucky never had a problem with these seating. Careful inspection with magnifier too at tip of valve.😉
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I'm going to try that. Thanks
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As the carbs are on the bench, try swapping the valve etc over with one that doesn't leak and see if the overflow moves with the valve. Just a thought.
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I'm going to try that. Thanks
A drill is too much and asking for problems. Do it by hand. It takes very little, maybe one turn.
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So I've now done a valve and seat swap as well as a light seat and float pin polish and the only way I could prevent it from overflowing was by ever so slightly lowering the float. It hasn't overflowed for over 24hrs now so I'm going to do a clear tube level check to see what the actual levels are. I reckon it'll end up being my inability to set the floats at the right height in the first place thats the problem! Thanks for all your advice.
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This may be of help.
http://www.sohc.co.uk/index.php/topic,23233.0.html
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Neill, the clear tube method in practice is not as simple as often presented. Therefore I advise anyone to do this method first, to check if anything is wrong at all. Often there's nothing wrong, but there will be after people have 'adjusted' the floats. My test will not lie and is very simple. First check if your carbs are fed fuel the same way as mine, knowing: one tube feeds carb 1+2, the other 3+4. http://forums.sohc4.net/index.php/topic,185754.msg2151647.html#msg2151647 But this method also applies when carbs are fed by one tube. If you understand the principle, you can perform it, be it with a minor variation ofcourse.
Know that all CB500/550s are very sensitive to have the fuellines in exactly the right dimensions and that extra inline filters are a no no.
If you've find one carb that is off and tapping the floatbowl with the stump end of a screwdriver didn't help, then it's time to remove that floatbowl for further inspection. Succes!
Oh, I cannot be of help with any aftermarket parts, as I have no experience with them.
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I readjusted the float levels and did a bench test
No overflow for a couple of days. I refitted carbs and still no overflow. Then I went out for a 20 mile ride. One carb overflowing like you wouldn't believe. I head home, park her up, turn the fuel on and no overflowing. I want to cry!😭😭
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I feel your pain - was it the same needle & seat that you moved?
If it was me I would be fitting a new float plus the needle & seat assuming the originals or a decent new aftermarket replacements are available.
Just a long shot but is it fresh fuel i the tank?
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Hi, Neil.
It is of course possible that the fuel overflow was caused by some debris, now cleared? Stick with it.
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Yes. Fresh fuel. When it's not raining I'm going to let the bike sit for a few hours with the fuel on and then to go out for another ride to see of it overflows again. I just can't see what else it can be other than debris. If the float had a hole in it it would have carried on overflowing when I got home so it can't be that. Even if the level in that bowl is incorrect why would it overflow one minute and not the next. Onwards and upwards!
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What one interprets as the 'wrong float level', can have several causes.
Debris that prevents the valve to close is one. A floatneedle that sticks in the valve is another.
Correct working order:
1. Tap floatbowl with stump end of screwdriver. If that doesn't help >
2. Remove drainscrew and flush floatbowl by opening and closing the petcock. Collect gas ofcourse.
3. Combine 1. and 2.
If 1 or 2 (or combination) doesn't work, remove floatbowl for further inspection.
Consider 'readjusting the floattang' as the last, the very last option. I never had to do it on no CB Four ever. My method (see above) is a proven and safe method for diagnosis.
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Another possibility is a bent float rod, it turns as the bike is in motion due to all the vibrations etc, as it turns it jams the float so it leaks, eventually the rod turns again and the float becomes free again.
You could try removing the rods and checking them and also polishing them, that's been known to help as well.
You could also make new rods, they sell the brass you need on Amazon.
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Another possibility is a bent float rod, it turns as the bike is in motion due to all the vibrations etc, as it turns it jams the float so it leaks, eventually the rod turns again and the float becomes free again.
You could try removing the rods and checking them and also polishing them, that's been known to help as well.
You could also make new rods, they sell the brass you need on Amazon.
If you mean the swivel pins, on our models they can't get dislocated. Honda carefully shaped the carbs thus, that they stay shielded in the right position. Also our type of floats cannot leak. When maintenance is needed, cleaning the swivel pins is recommended. A little naphtha or WD-40 will do. Do not use anything abrasive. Swivel pins are best removed using your fingernails. If you must use pliers, I repeat must, do not pinch the pin!
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If it comes out and isn't pinned in place it can move DR. They ain't a tight fit, I can pull them out with my fingertips. As they are made of brass they are also easily bent.
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I have previously polished the pins. I push them out with a piece of stiff wire. If it overflows again I'm going to remove it to see if it's bent as that would make sense.
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I had problem with overflowing carbs that had just been cleaned and rebuilt using new brass float needle valve seats.
After a lot of cursing it turned out that the new brass wasn't the same dimensions as the old brass and therefore the o-ring wasn't sealing properly
I put some old brass in and the torrential overflow problem stopped straight away, so maybe check the valve seats are all the same?
Old on left, new on right. O-ring groove at slightly higher position on the new.
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That's very helpful. Thanks. Maybe it seeps past the o ring with the vibration of the engine.
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If it comes out and isn't pinned in place it can move DR. They ain't a tight fit, I can pull them out with my fingertips. As they are made of brass they are also easily bent.
IIRC, when the carb is assembled, like in normal daily use, the design of the bowl prevents the pin coming out.
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Correct that man👍
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DR, please read what I said, I said it could vibrate round NOT out, so if the pin is bent it can jam the float from moving, it then vibrates loose and the float can either stick in place or start to work as it should.
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I've ordered a new float and pin as I'm taking all the advice I can get. Thanks
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Are there only 2 air mixture screws on a cb550 carb. One on the left and one on the right or am I not looking hard enough for the other two,
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There is one on each carb body, so you will have 4 in total. Just checking your carbs are Keihin 069a's and not Keihin PD's?
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069a.
I've looked for the other two but can't find them!
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069a.
I've looked for the other two but can't find them!
Have you put the carb bodies on in the correct order? I'm not sure if you can't but if you can find the screw on 2 of the carbs, the other 2 are in the same place on the other 2.
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I haven't taken them apart but when I had the carb off I looker for them but couldn't see them. I'll have to try again!
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I haven't taken them apart but when I had the carb off I looker for them but couldn't see them. I'll have to try again!
Ill take a photo in the morning if you still haven't found them.
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Thanks
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Thanks
The air / fuel mix screws are these brass ones.
[attachimg=1]
[attachimg=2]
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Neil, did you sort your overflow issue?
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Excellent. I found them hiding behind the fuel hose. Thanks for your hepl
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Yes. The float bowl overflow went away after a couple of hours riding. It was intermittent at first then went away completely. I'm hoping that the pin on the float sorted itself out and somehow has straightened itself out.
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Glad it sorted itself out but I have to wonder if the problem was some debris between needle and seat clearing through eventually. Saw earlier in the thread that in line filters in the feed from tank are problematic. To avoid this very problem I was thinking of using them in my rebuild. Is it the consensus view that they cause too much difficulty with flow rate?
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In my case they did. Here's the variety I've tried. All resulted in issues. Basically there are two problems a float valve can have. A needle that's sticking in the valve. Tapping can help. Also a fuel system cleaner additive can help. A needle not perfectly shutting the valve is the other problem. A tiny particle of dirt can cause this. Again: tapping and flushing by opening the drainscrew and opening and closing the petcock. With the OEM fuel filter in good order, there's no need for any extra filtering.
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By oem fuel filter I assume you mean the filter on the fuel tap inlet inside the tank ?
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Yes.