A Remedy For Stopping A Leaking Carburetor Float Needle Valve And Seat

[greaseyfingers]

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.

[Alizard]

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.

[coppertales]

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

[86klf300guy]

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

[coppertales]

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

[86klf300guy]

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-

[coppertales]

I have never seen a rubber tipped float valve..............chris3

[AndrewWoodard]

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....