Topic: Clear tube method failure

[SamP]

So I've been chasing carb gremlins since forever. I think I've narrowed it down to fuel level/overflow.

1977 cb550K, stock airbox, foam filter, PD46 carbs, MAC4into1 exhaust. 105 mains, 42 idles.

I'm gonna skip a long story and simply say I noticed gas dripping slowly from my airbox manifold drain hose during my last chop. Didn't think a whole lot of it, but then my last post directed my attention to float height. The same frustrating thing happened to me this time that has happened in the past, which I left at "good enough" before proceeding to other things. It goes as follows:

Set float height. Turn gas on. Fill carb bowls completely. Leave gas on and observe airbox manifold drain hose - NO DRIPS. Conclusion: float valves functional.

Turn gas off. Check fuel height via clear tube method. Level looks ok, but let's turn on gas to see where it's at when the fuel is on. Proceed to turn gas on. Fuel level climbs - sometimes slowly, sometimes quickly, over the bowl height line. I tell myself "there's no possible way the fuel is actually 3 inches above the top of the bowl; how is that possible?" Then, fuel starts to drip out of the airbox manifold drain tube. Great. Turn off gas, close drain screw.

Drain some fuel from carb, close drain screw. Alright, let's double check this. Turn on gas. Gas is on for 5 minutes and NO DRIPS from airbox manifold.

That sums it up. I've tried lowering the floats, raising the floats, inverting the floats, changing float valves, and cleaning the float valve seat with a Qtip.

I don't think it's the valve seat or the valve, because it works just fine when the drain screw is closed. Opening the drain screw (a la the clear tube method) seems to trigger a runaway gas leak. I would imagine this sort of slow trickle of gas would mimic running conditions, which would explain the gas leaking from the manifold during my last plug chop.

Could it be the floats? Do they lose their buoyancy over time? That is the only thing I have not changed, but it almost seems like they are not as reactive to changing fuel levels as they need to be. Springs on the valves are definitely softer on the old ones, and very tight on the new. Both create a leak when the drain screw is opened - I just don't get why the valve is staying open enough to leak gas as badly as it's doing, especially when it's closing the fuel line at a lower level in the bowl when the drain screw is closed.

[enwri]

check the upper body vents. each carb must be able to vent to atmosphere.

[enwri]

you may have two between 2-3 as well

[greenjeans]

How are you doing your clear tube test?   I have a bowl drain screw that I have filed down to basically, a threaded barb.  I drilled it out and still have enough lip left on the screw for an o-ring.   You have to have the gas on to check the level.  I also agree to make sure ALL the vents are clear. 

[SamP]

Any closer up shots of these vents? This is the first time I am hearing of them.

Also, aren't there some tiny pinholes in the bodies of the PD carbs? I've seen posts on them before and was super grateful to learn that these were intentionally designed and not a manufacturing defect. Mine definitely have those.

[SamP]

I've got several long piece of clear tube connected to the drains in the bottom of each bowl. Once I'm sure the bowls are full, I turn off the gas and open the drain screws, lifting the tubes upward in a U shape. I let the gas level off in the tube and settle in the tube before turning on the gas again to "top off" the level in each bowl. It's at this last step where the gas just shoots up super high and then starts leaking out the back of the airbox manifold.

[scottly]

The brass tubes that stick up into the float bowls are also vents; make sure that they are clear.

[BomberMann650]

Seems to me like your float needles may be suspect.

[enwri]

in the ends of the horns facing each other between the upper carb bodies, there are tiny "pinholes" that vent the chamber.cleartubing on pd carbs with the overflow tubes and drain screws effectively blocks the lower overflow vents. relying on only the upper body vents. if they are blocked air can't get out, stopping fuel level and floats from going up . fuel keeps flowing in, gets pushed up the jets into the throat.

[brewsky]

I've got several long piece of clear tube connected to the drains in the bottom of each bowl. Once I'm sure the bowls are full, I turn off the gas and open the drain screws, lifting the tubes upward in a U shape. I let the gas level off in the tube and settle in the tube before turning on the gas again to "top off" the level in each bowl. It's at this last step where the gas just shoots up super high and then starts leaking out the back of the airbox manifold.

Put the bike on the centerstand.

Don't move the tubes after you open the drain screws.

Put a piece of tube on each drain and run them up the side of the #1 carb and tie them to the body with a zip tie. Cut them off just above the top of the carb bodies,

Then open the drain screws one by one and leave them open.

If gas in any one of the tubes rises above the top of the bowl seam, there is a problem with the float setting, float valve or overflow tube.

[ekpent]

The brass tubes that stick up into the float bowls are also vents; make sure that they are clear.

+1 and also make sure any rubber tubes connected to the vents are not blocked. Sounds like a vent issue with gas going to the airbox. After you fix it give your oil a whiff for a gas smell as well..

[DaveBarbier]

I had to think about the vent issue being the culprit and the more I think the more I think it could be right.

Right near where the fuel line comes into the #1 carb there's another nipple. That's a vent which goes down into the #1 carb. The #2 carb vents into the #1 carb, the #3 into #2 and #4 into #3 so all carbs vent out that nipple by the fuel line. They all connect via the small black rubber connecting hoses between the carbs. You can turn the fuel off, and only remove the #4 bowl. Blow into the hose that's connected to the vent nipple (that's right near the fuel inlet) and if you hear or feel air coming out the #4 carb then all carbs are clear and venting to atmosphere. And yes, the overflow tubes also allow venting. I don't know if the PD46 carbs have ANY other means of venting to atmosphere. Sam, what are these pin holes you speak of? Are you sure that's on the PD46 carbs? Post a picture please or link me to something.

Speaking of overflow tubes, they are clogged if it's allowing fuel to raise high enough to flow into the air box. Clean them all out.

Your method of clear tubing is fine but there's no reason to shut the fuel off after you connect the hoses to the drain nipples. Keep the fuel on so it fills up the bowls, connect the tubes and crack open the drain screw. Sometimes there's air trapped right in the drain nipple so you want the full force of the fuel in the tank rushing in pushing that air bubble out. Rapping on the carb body with a screw driver can also remove that bubble.

Have you thoroughly cleaned these carbs?

[DaveBarbier]

I've got several long piece of clear tube connected to the drains in the bottom of each bowl. Once I'm sure the bowls are full, I turn off the gas and open the drain screws, lifting the tubes upward in a U shape. I let the gas level off in the tube and settle in the tube before turning on the gas again to "top off" the level in each bowl. It's at this last step where the gas just shoots up super high and then starts leaking out the back of the airbox manifold.

That's a problem. I see it now. Only clear tube one bowl at a time.

Your vent that's right near the fuel inlet isn't venting to atmosphere. Fix that.

Since that's clogged (I hypothesize) the only other place to vent to atmosphere is the overflow tubes. But since you crack the drain screws on ALL bowls with tubes connected and turned up in a U shape it fills the tubes with fuel, effectively blocking air from escaping nullifying the venting ability from each carb bowl.

[SamP]

Dave,

I've tried one at a time, I've tried them all together. Either way, I get this creeping or surging overflow. Bike is on the centerstand, and I have checked the center passage on the drain tube, so I know those are clear, but I will run a pipe cleaner through there again to double check.

As for the vents, I do not have a clear photo, since getting it would require taking the carbs off (and I work two jobs, so that's not an option until the weekend). Below is a photo of my clear tube method on one carb with the gas off. In the middle of the carb body there is a little projection upwards that looks like the top of a retractable pen. The little divot on the right side is where the pinhole is. It may very well be clogged on the others, I only ever saw light through one of them. I am assuming these are the vents everyone has mentioned, since I've examined these carbs inside out multiple times, and haven't come across any other holes or vents or passages. If there is a clear photo of other vents, or of these holes I am mentioning, please let me know.



To me it makes the most sense that these holes are clogged - if the air was only escaping through the bowl drain nipple, when I close that off via the clear tube method, air wouldn't have any other way of getting out of the bowl, so it would fill indefinitely. I suppose I could check this by plugging the drain nipple with my finger and turning the gas on and seeing if this has the same effect.

Checking for photos of PD46 yields this image:



In the carb without the floats, you can see the passage leading up to where the pinhole would be.

[flybox1]

D@mn.  Look at that dirty float valve seat! 
You cant expect it to function properly if its dirty like that.   Brass jets look green/corroded 
PLEASE.....take better care of your carbs.   The whole thing looks like its in need of some care.

[SamP]

That last shot isn't mine, just a random Google image search. Mine are clean (except for some gunk blocking my vents, perhaps?).

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[dcresci68]

i am new to this site and not sure where to post this question. i have a 1968 honda p50 and i need a float valve. any suggestions?
Thanks,
dominick

[flybox1]

i am new to this site and not sure where to post this question. i have a 1968 honda p50 and i need a float valve. any suggestions?
Thanks,
dominick

CMSNL.com
remove this post from this members thread, as a courtesy, and post over in the Other Bikes page.

[flybox1]

If air doesn't have any way to escape out of a bowl, it cant fill with gas either 
How about some good internal shots of YOUR carbs so we can have a look.

[BobbyR]

Dave,

I've tried one at a time, I've tried them all together. Either way, I get this creeping or surging overflow. Bike is on the centerstand, and I have checked the center passage on the drain tube, so I know those are clear, but I will run a pipe cleaner through there again to double check.

As for the vents, I do not have a clear photo, since getting it would require taking the carbs off (and I work two jobs, so that's not an option until the weekend). Below is a photo of my clear tube method on one carb with the gas off. In the middle of the carb body there is a little projection upwards that looks like the top of a retractable pen. The little divot on the right side is where the pinhole is. It may very well be clogged on the others, I only ever saw light through one of them. I am assuming these are the vents everyone has mentioned, since I've examined these carbs inside out multiple times, and haven't come across any other holes or vents or passages. If there is a clear photo of other vents, or of these holes I am mentioning, please let me know.



To me it makes the most sense that these holes are clogged - if the air was only escaping through the bowl drain nipple, when I close that off via the clear tube method, air wouldn't have any other way of getting out of the bowl, so it would fill indefinitely. I suppose I could check this by plugging the drain nipple with my finger and turning the gas on and seeing if this has the same effect.

Checking for photos of PD46 yields this image:



In the carb without the floats, you can see the passage leading up to where the pinhole would be.

This photo confuses me. When I enlarged it you can clearly see the tube is not attached to the drain of the carb you are holding the tube against. You must hold the tube against the carb float bowl you are testing. This appears to have the tube attached to some carb in the middle of the rack.

[flybox1]

Good eyes, Bobby!

Its best to do the clear tube test while the carbs are on the bench.  Allows for the tubes to be secured and watched closely, and makes adjustments much easier/quicker.

[SamP]

If air doesn't have any way to escape out of a bowl, it cant fill with gas either 
How about some good internal shots of YOUR carbs so we can have a look.

That's true, but the air CAN escape via the tube connected to the drain nipples. But when I have the clear tube full of gas, it cuts that off, creating pressure from the full bowl, which then backs up and causes it to overflow. Since that tube is open to atmosphere, it will rise up to the height of the gas in the tank, overfilling the bowl in the process. Right?

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[DaveBarbier]

If air doesn't have any way to escape out of a bowl, it cant fill with gas either 

I was thinking that too, but if the jets are submerged in fuel and air is trapped in the bowl preventing the fuel level from raising to the height to close the needle valve fuel theoretically could be pushed up the jets.

[SamP]

Dave,

I've tried one at a time, I've tried them all together. Either way, I get this creeping or surging overflow. Bike is on the centerstand, and I have checked the center passage on the drain tube, so I know those are clear, but I will run a pipe cleaner through there again to double check.

As for the vents, I do not have a clear photo, since getting it would require taking the carbs off (and I work two jobs, so that's not an option until the weekend). Below is a photo of my clear tube method on one carb with the gas off. In the middle of the carb body there is a little projection upwards that looks like the top of a retractable pen. The little divot on the right side is where the pinhole is. It may very well be clogged on the others, I only ever saw light through one of them. I am assuming these are the vents everyone has mentioned, since I've examined these carbs inside out multiple times, and haven't come across any other holes or vents or passages. If there is a clear photo of other vents, or of these holes I am mentioning, please let me know.



To me it makes the most sense that these holes are clogged - if the air was only escaping through the bowl drain nipple, when I close that off via the clear tube method, air wouldn't have any other way of getting out of the bowl, so it would fill indefinitely. I suppose I could check this by plugging the drain nipple with my finger and turning the gas on and seeing if this has the same effect.

Checking for photos of PD46 yields this image:



In the carb without the floats, you can see the passage leading up to where the pinhole would be.

This photo confuses me. When I enlarged it you can clearly see the tube is not attached to the drain of the carb you are holding the tube against. You must hold the tube against the carb float bowl you are testing. This appears to have the tube attached to some carb in the middle of the rack.

It was hooked up to a different carb body than the one pictured. I just used that carb for the measurement against the bowl.

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[SamP]

If air doesn't have any way to escape out of a bowl, it cant fill with gas either 

I was thinking that too, but if the jets are submerged in fuel and air is trapped in the bowl preventing the fuel level from raising to the height to close the needle valve fuel theoretically could be pushed up the jets.

I think that's it - it's going up through the jets, but the vent is blocked so the air isn't being displaced, thus preventing the float from rising.

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