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Do you know what causes gasoline to drip out of the over flow tube of your carburetor?

The short answer is that a tiny piece of dirt is most likely stuck in the needle and seat of the float valve, preventing the valve from fully seating. Or else your float valve may need to re re-adjusted, or replaced (due to wear on the needle valve and seat), to lower your fuel level in the carburetor float bowl.

However, I'd like to give you a bit more background on this important question. I used to think that the float valve in the carburetor operated like the float valve in the tank of a toilet: i.e., when water in the tank got low (as in after flushing), the valve would open until the tank filled and then close again. However, it is important to realize that (unlike our toilet tanks) the needle in the carburetor float valve is always closed. What varies is the pressure that the carburetor float exerts on the needle: relatively high as the fuel level increases, and relatively low as the fuel level decreases.

This means that the fuel pressure (on the inlet side of the valve) is in constant equilibrium with the pressure exerted by the needle (on the carburetor side of the valve), so that a very constant level is maintained in the float chamber.

Understanding that the needle of the float valve is always being pressed against the seat is important in understanding why any tiny bit of dirt can cause so much trouble. Once a piece of dirt gets caught between the needle and the seat, it will be held there and interfere with the needle valve's ability to control the fuel level, until such time as fuel is drained from the carburetor and the piece of dirt is flushed away. This is the reason for the flushing procedure of the fuel system, by removing the drain plug from the bottom of the float bowl and let enough fuel run out, which will usually rinse off the the debris that is on the needle valve and seat, as fuel is running out of the float bowl drain plug hole.

To help prevent this problem from happening, again. It's a good idea to put a good inline fuel filter between your gas tank and carburetors that keeps your fuel system very clean going to your carburetor's float valve and seat, which also prevents having other problems in your carburetor, too.

In addition to dirt causing the valves to stick open, I have seen a few cases where the brass seat of the float valve needs to be polished (smoothed out) by pressing and rotating a short pointed hardwood dowel into its small orifice. Dowels of 3/16” diameter work well. I have even had to clean up brand new seats in this fashion before they would stop leaking.
 

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The Cruising Gunsmith
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FYI:

Even if there is no dirt in there, a metal-to-metal seal is not going to be dead tight. If you don't turn off the fuel at the petcock, the constant downward pressure of gravity will cause gas to seep past the needle valve and fill up the bowl if the engine is not running. You can get a bit of overflow seepage even under the best case if the petcock is ON. The moral is to always turn off the petcock when you park it or you may see a puddle of gas on the ground.

As for the brass seat: brass tarnishes (oxidizes) which is why they start out shiny and after a few months will look dull and rough. It is a good idea to polish the seat regularly to keep the mating surface smooth (the area that mates with the needle).

It's also a good idea to put the needle in a hand drill and polish the tip up real smooth. That gives you the best seal you can get between those parts, but they can still seep gas if the petcock is left on.
 

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The oring around the float..

needle body is cracked/hardened, not allowing a good seal. Common on older bikes. I just pull the float valve body and smear a thin coat of Permatex copper sealant around the oring and replace the valve in the carb. I am too cheap to buy a repair kit. This works for me and the copper sealant is easy to remove once a new valve is obtained to correctly fix the problem....chris3

Forgot to mention, look down the hole where the float valve body was and clean out any crud that may be there from the fuel tank....chris3
 

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well what about the tip of the float needle that over time swell up due to corrosion formming under the tip and causein no fuel concern is there a pre service you can do to this to prevent that from happening? -corey
 

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In my 40 years..

as a mechanic, I have never seen a corroded float needle. The only way this could happen is if the bike sat for a long time and had water in the tank/carbs. chris3
 

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well surprise surprise i must of had the only one my float needle was claen and the seet but the tip had corrosion under the rubber tip...huh wierd one -corey-
 

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Hiya, Gang!

Speaking of rubber-tipped float (needle) valves, the KZ305 has them. (At least mine does.) I've also seen them on a slew of Hondas, even the little Z50R in my garage.

The rubber-tipped valves are great for absolutely stopping fuel flow. The downside is that, over time, they can harden (due to leaching of the vinyl chlorides, usually due to exposure to ozone and/or sunlight). My 1982 KZ305 sat in my garage for over 20 years and, to my utter amazement, the rubber tips on the valves were in very good condition when I finally disassembled the carburetors to clean them. Perhaps I was just lucky, or maybe the quality of components the factory used was better than I generally give them credit for.

I've seen replacement rubber-tipped valves for sale at various places on the web, some asking $6-$7 for a single valve and one place with a much better price but only selling valves 6-to-a-pack.

At any rate, that's my 2 cents worth.

Thanks!

Andrew....
 

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Do you know what causes gasoline to drip out of the over flow tube of your carburetor?

The short answer is that a tiny piece of dirt is most likely stuck in the needle and seat of the float valve, preventing the valve from fully seating. Or else your float valve may need to re re-adjusted, or replaced (due to wear on the needle valve and seat), to lower your fuel level in the carburetor float bowl.

However, I'd like to give you a bit more background on this important question. I used to think that the float valve in the carburetor operated like the float valve in the tank of a toilet: i.e., when water in the tank got low (as in after flushing), the valve would open until the tank filled and then close again. However, it is important to realize that (unlike our toilet tanks) the needle in the carburetor float valve is always closed. What varies is the pressure that the carburetor float exerts on the needle: relatively high as the fuel level increases, and relatively low as the fuel level decreases.

This means that the fuel pressure (on the inlet side of the valve) is in constant equilibrium with the pressure exerted by the needle (on the carburetor side of the valve), so that a very constant level is maintained in the float chamber.

Understanding that the needle of the float valve is always being pressed against the seat is important in understanding why any tiny bit of dirt can cause so much trouble. Once a piece of dirt gets caught between the needle and the seat, it will be held there and interfere with the needle valve's ability to control the fuel level, until such time as fuel is drained from the carburetor and the piece of dirt is flushed away. This is the reason for the flushing procedure of the fuel system, by removing the drain plug from the bottom of the float bowl and let enough fuel run out, which will usually rinse off the the debris that is on the needle valve and seat, as fuel is running out of the float bowl drain plug hole.

To help prevent this problem from happening, again. It's a good idea to put a good inline fuel filter between your gas tank and carburetors that keeps your fuel system very clean going to your carburetor's float valve and seat, which also prevents having other problems in your carburetor, too.

In addition to dirt causing the valves to stick open, I have seen a few cases where the brass seat of the float valve needs to be polished (smoothed out) by pressing and rotating a short pointed hardwood dowel into its small orifice. Dowels of 3/16” diameter work well. I have even had to clean up brand new seats in this fashion before they would stop leaking.
Hi Greaseyfingers
I read your post with great interest about fixing float seats with wooden dowel.
I've got a 84 GPZ900R that I can't stop flooding. I've polished seats with cotton buds in a drill with metal polish about 4 times, bought new needles, but can't get them to seal. Could you please explain in a bit more detail how to fix seats with dowel. Just twist by hand or in a drill? Soft or hard pressure?
Would greatly appreciate any advice :unsure:
 

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I would like to start by making a general statement. I am a retired professional mechanic and I follow multiple forums. Auto, motorcycle and aircraft. There is much valuable information from very competent posters. However there are also posts from those who have perhaps rebuilt a couple of sets of carbs, adjusted valves and perhaps rebuilt their clutch. These people make all encompassing statements of fact, in which I believe are all made in complete sincerity, but are simply false. One of the above posts states that no metal to metal valve can produce a complete seal. Wrong. For decades one of the final tests for a valve job is to fill the port with solvent, if it leaks past the valve you have more work to do. no leakage, good to go. The valve to valve seat is a one hundred percent metal to metal seal. Very hard for those who are just learning to separate accurate advice from that given by those who think they know but do not truly understand the systems they are commenting on. 84 Kaw, at this point I would suggest you purchase new inlet valves and seats. Install, set float level and go from there. Also when cleaning carbs it is important to always blow compressed air in tn the opposite direction of normal flow.
 

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Alizard's assessment is correct about the metal-to-metal stuff. The leakage when the motorcycle is parked is probably one of the reasons Kawasaki installs vacuum actuated petcocks on most of their motorcycles. In real world conditions a perfect metal-to-metal contact seal isn't always attainable and lessens the older a motorcycle is, for various reasons.
 

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I am sorry you are mistaken and do not understand the system. The vacuum petcock is a secondary backup device in case the carb fails to work properly. As noted in another thread no manufacturer designs and builds carbs that leak. They leak do to some failure. You are way off on your assumption. I was a professional motorcycle mechanic for many years. Have a top technician award for the eastern third of the US from Suzuki. These statements though well meaning are completely false. We can argue forever but you simply do not fully understand the design and function of the system.This is a prime example of misleading advice. I do not question your intent and I certainly understand you believe what you are saying but it is nerveless false. A point of logic, if the inlet valve leaks, how do we control or know how much it leaks? Under static conditions this would cause a discharge from the over flow. But what about when the engine is running? Where does this excess fuel go? The basic premise that the inlet valve can not seal under logical scrutiny makes no sense. I can not tell you how many carbs I have had apart, serviced, tuned. I would never accept a leaking carb as acceptible. Dont know how oild you are but for decades motorcycles did not have vacuum petcocks. carb leaks were no more acceptable in that era as with vac. petcocks. It is an endless misconception that petcocks or in line shut offs are the cure for leaking carbs, this is simply not the case. The petcock is a back up protection in case the carb fails.
 

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GPZ550 I think you misunderstood Martin-CSR's point. What he said was "In real world conditions a perfect metal-to-metal contact seal isn't always attainable and lessens the older a motorcycle is, for various reasons." I totally agree with this statement and I don't think anybody who has ever worked on motorcycles could disagree. The float needle and needle seat interface is not perfect. It is subject to wear and to dirt thus leading to leakage. Although we can never truly know why Kawasaki engineers designed vacuum petcocks, it stands to reason that they wanted a backup safety device for an overflowing carb. They too, probably recognized that with age or dirt, carbs will overflow.

In a thread about clutch design you said there was no perfect clutch and everything is a compromise. I agree. But also the metal to metal float needle interface is also not perfect and is a compromise.
 

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Yes it is, but there is no mechanical device ever devised that is not subject to wear. When it wears and fails to perform as designed are we just to ignore the failure or do we repair it. As I said I would never let a motorcycle with leaking carbs leave the shop. Would a vac. petcock mask this? absolutely. Is it right? Absolutely not. Any time a carb has a static leak it can be expected to have a negative effect on dynamic operation. Bottom line, if the carb leaks weather metal to metal or neoprene to metal there is a problem that needs to be addressed. There are countless motor vehicles on our roads with half assed repairs that function with some level of acceptance, does that make those solutions correct? If one wants to cut corners, so be it, I am referring to theory and accepted practices.
 

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Let me add this. I worked for dealers who sold Suzuki, Kawasaki and Yamaha motorcycles. Should I un crate assemble, and do a PDI on any motorcycle only to find the carbs were leaking on the prime position I would never release it for delivery. What is right and what we can get away with are two entirely different things. I consider myself incredibly lucky that the first dealer I worked for wanted everything right. Actually the second dealer was even more intent on having everything right. Imagine my dismay when the third, and very successful dealer I worked for was only interested in numbers and production. My conscience will simply not let me cut corners on motorcycle maintenance. A loose drain plug on an auto might result in the loss of an engine, a loose drain plug on a motorcycle may result in the loss of a rider. Through all my years a s professional motorcycle snowmobile mechanic this never left my mind. Very unfortunate that so much mis information exists.
 

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And I don't think anyone is suggesting to ignore the leaking carb. In fact this thread was revived by 84kaw who wanted to know how to fix his leaking carbs. I think we are all on the same page but sometimes we can misinterpret what others are saying. It's all good.
 

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WFO-KZ, I have read many of your posts and they have always been both knowledgeable and respectful. So though I still somewhat disagree with the intent of the original question I will defer to your interpretation. Perhaps I am indeed missing something.
 

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I would like to start by making a general statement. I am a retired professional mechanic and I follow multiple forums. Auto, motorcycle and aircraft. There is much valuable information from very competent posters. However there are also posts from those who have perhaps rebuilt a couple of sets of carbs, adjusted valves and perhaps rebuilt their clutch. These people make all encompassing statements of fact, in which I believe are all made in complete sincerity, but are simply false. One of the above posts states that no metal to metal valve can produce a complete seal. Wrong. For decades one of the final tests for a valve job is to fill the port with solvent, if it leaks past the valve you have more work to do. no leakage, good to go. The valve to valve seat is a one hundred percent metal to metal seal. Very hard for those who are just learning to separate accurate advice from that given by those who think they know but do not truly understand the systems they are commenting on. 84 Kaw, at this point I would suggest you purchase new inlet valves and seats. Install, set float level and go from there. Also when cleaning carbs it is important to always blow compressed air in tn the opposite direction of normal flow.
Thanks for reply 1981GPZ550
Unfortunately Kawasaki have really made it difficult because you can't replace the seat, it's been pressed in to the carburetor at the factory, and there aren't any available. Don't know why they did this, I had an old CB750K2 and you could just unscrew the seat and throw it away and replace it. Not an option here. Also the needles are rubber tipped, so they should be easier to seal but don't. I've turned carbies upside down and blew into fuel inlet, but there not holding pressure. I figure I can't check float levels if bowls are overflowing.
Thanks all the same
 

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You are very welcome. Hope you find a solution. I have been a motorcyclist and mechanic for fifty years. New bikes long ago surpassed the capabilities of the average street rider. I prefer the simplicity of the older machines though as you are discovering parts can be a problem. I have found that if I monitor ebay long enough most of what I need will eventually show up, In fact I was able to purchase an nos head for my 81 GPZ for only eighty dollars.
 

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Unfortunately Kawasaki have really made it difficult because you can't replace the seat, it's been pressed in to the carburetor at the factory, and there aren't any available... I've turned carbies upside down and blew into fuel inlet, but there not holding pressure.
Thanks all the same
I did not realize the seats are non-replaceable. That sucks. But how much pressure did you use when you blew into the fuel inlet when testing the valve? In normal use, the valve has to be able to close off the gravity-fed fuel which would be at a very low pressure of less than 1 PSI. Even your own lung capacity can far exceed 1 PSI so your test may not have been valid. Why not bench test the carb by clamping it in its normal upright position and attach a hose that is at least as long as the distance from the carb on the bike to the top of the gas tank. Hold the hose vertical and add gasoline and see if the carb leaks. If you use a clear hose you can watch the level go down if there is any leakage.

To stress test the valve, you can double or triple the length of hose. Let it sit for several hours to make sure you have no leakage.
 
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