Float Valve Operation

C

Colonel Biggs

Senior Member
I have two original float valves that have performed well in my Zenith carb over the last few years. Recently, one of them appears to be failing to close fully when the engine is shut down, allowing gasoline to leak from the carb.

When I blow into the top of the valve with the valve open, the valve gradually closes up after about 10-15 seconds and the valve still in the open position. This occurs with both valves. Is this normal? I would think that air should pass through as long as the valve is in the open position.

I chatted with Allen LePore the other day, and he told me that vendors have been having problems with the viton-tip valves (i.e., many returns because of leakage), and he recommended that I go the stainless steel tip route.

Are there any reliable tests that can be applied to a float valve before installing it in the carb?
 
Colonel, connect a hand pump vacuum gauge to the valve with a bit of plastic hose, put your finger on the valve to close it and pull a vacuum on the gauge dial with the pump. It should hold for a good time. Many brand new float valves leak from the word go. Tapping the needle lightly onto the seat often gets new leaky ones to seal.
Mityvac is the good American pump. The cheap plastic Chinese ones also work for this job if you make sure they hold a vacuum with your finger tight over the inlet before doing the test.
SAJ in NZ
 
Hello, New Zealand --

I have a Mighty Vac; the valves seal well. But, I don't understand that when blowing through the valve, and the valve stays open, why it blocks up after 10-15 seconds.

Thanks for the response.
 
Colonel, Hello Houston!
Are you blowing through the valve when removed from the carby, or blowing into it while still installed and mounted on the float bowl?. If the former, like you, I cannot explain that.
But if the latter I wonder if air pressure in the float bowl raises the float and shuts the valve off?
One would think that the bowl vent would prevent that, but maybe it is blocked. And your lungs are overcoming the various jet vents?
SAJ in NZ
 
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If I understand... valve open, high velocity air flow through the valve and around the needle... then the needle closes, correct?
I think that you are experiencing a phenomenon of fluid flow, fluidics. Somehow the positive pressure air flow is creating a low-pressure area that pulls the needle back into its seat.
An everyday example of this phenomenon is when you blow air across the edge of two sheets of paper in order to separate them. The air flow is NOT pushing the sheets apart, rather the air flow creates a low pressure that pulls the sheets apart.
We need a fluid dynamics engineer to explain this better.
Joe B
 
Again, thanks for the responses. No, I am blowing through the top of the valve (off the carb) in the direction that the fuel would be flowing, and I am holding the needle in the open position. This morning, one of the original valves resumed its flow through, but the other did not. I had used some Silicroil sprayed into the valves yesterday to clean them out, overcome any internal rust, etc. Maybe that has caused some stickiness in the one not operating. I've also placed the needle ends in my drill chuck and tried some spinning to clear the seat. Time will tell. I may have to lay out the coins to run these tests on a new valve.
 
A number of years ago when the Viton tip float valves came on the market, I purchased one from Bratton's and it would not hold at all. I disassembled it and found it full of brass machine chips. I called Walt Bratton and advised him or the situation. He contacted his supplier and the situation improved somewhat. Since then I have purchased any number of them. I always take them apart and inspect for chips. Sometimes I will find chips imbedded in the Viton chip. It is easy enough to clean them up. After that they work extremely well and are all I will use. They are easy to take apart, just pry out the plastic washer holding the tip in place.

Another important issue is the float. Quite often I find the float has been molested by someone trying to set the float level by bending the float. This can cause it to come up crooked and offset the float valve. The only way the float level should be set is with the number of gasket shims under the float valve.

Tom Endy
 
Sir Thomas!

Good to hear from you. There are times in the winter when working indoors on a float valve is much more appealing than rebuilding rear ends in a cold garage, and your write-up on same is the best on the market!

I was toying with disassembling a valve out of curiosity, even if it meant creating a throw-away, but your comments mean that this is the next step, for sure. The valve itself must be a relatively simple engineering item, and by getting to the valve mechanism itself should be enlightening.

I understand your comments about the float. The only test I ran here was to ascertain whether it had developed a small leak.

Thanks for your comments.
 
Also, you might take a look at the float where the valve stem contacts it. In most cases there will be a dimple worn into the float. I like to polish this area with 370-400 paper so there are no hangups! Good luck, Ed
 
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