Why won't it fire?

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Eugene
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Re: Why won't it fire?

Postby Eugene » Fri Oct 15, 2021 11:17 pm

Moray wrote:It looks like the vent hole simply hasn't been drilled, which will be causing the fuel level in the float chamber to not rise to close the valve, and tank gravity pressure is simply forcing fuel out the drain plug.
Not quite. Any air trapped in the carburetor, with needle valve open, would return to the gas tank via the fuel line.

Needle valve not closing for some reason, but not from weight of fuel in fuel line.
I have an excuse. CRS.

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Re: Why won't it fire?

Postby Jim Becker » Sat Oct 16, 2021 12:40 am

Eugene wrote:Not quite. Any air trapped in the carburetor, with needle valve open, would return to the gas tank via the fuel line.

Probably not as long as gas is flowing down through the needle valve.

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Rick Spivey
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Re: Why won't it fire?

Postby Rick Spivey » Sat Oct 16, 2021 5:47 am

Suggestions. Drill the hole out, that should be open. Then check CAREFULLY that the float doesn't rub any surface of the bowl, and that the float height and drop are correct. With the carb assembled and dry, prior to putting it back on the tractor, you can turn the carb upside down, to and fro, and hear the float dropping and raising. Lastly check that the float stops off the flow when shut, by blowing air through (low pressure, I sometimes blow through them by mouth if the gas is wiped off). As mentioned. sometimes the viton tips don't work as well in these carbs, even though virtually every other float on a smaller engine uses them.
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Re: Why won't it fire?

Postby Moray » Sun Oct 17, 2021 4:00 pm

Drilled the hole out this evening, fitted everything back together, checked the fuel level/float was good, and the tractor now runs :{_}:

Just waiting for some UNC bolts/nuts to be delivered to make some studs to fit the carb on properly, as the old carb had been drilled out and fitted on with M6 bolts/nuts, so I used a couple M5 bolts to avoid drilling out the new carb to test things.

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Re: Why won't it fire?

Postby Bob McCarty » Sun Oct 17, 2021 5:25 pm

The original studs were 1/4-20 into the carb, and 1/4-28 thru the manifold with a nut. Probably unavailable to you.
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Glen
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Re: Why won't it fire?

Postby Glen » Sun Oct 17, 2021 5:40 pm

Hi,
Good news that the engine runs. :)

Below is a listing from TM Tractor, showing the studs Bob was talking about above.

http://www.tmtractor.com/new/fl/405fp.htm

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Don McCombs
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Re: Why won't it fire?

Postby Don McCombs » Mon Oct 18, 2021 4:18 pm

Checked a repro carb that I recently purchased on eBay. It, too, was not drilled. The hole on an original carb appears to be 11/64. What size bit did you use?

Undrilled Chinese Carb Bottom.jpeg
Last edited by Don McCombs on Mon Oct 18, 2021 10:04 pm, edited 1 time in total.
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Moray
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Re: Why won't it fire?

Postby Moray » Mon Oct 18, 2021 5:02 pm

I never thought about the original studs being different threads, but you're correct in that that kind of part won't be easily found on this side of the pond.
I could have machined a couple up, but some 1/4-20 bolts cut to length with a couple nuts have done the job.

And proof it runs-
https://www.youtube.com/watch?v=p6l4Vnks48s


Don McCombs wrote:Checked a repro carb that I recently purchased on eBay. It, too, was not drilled. The hole on an original carb appears to be 11/64. What size bit did you use?


I just picked whatever drill bit went through the old carb with a bit clearance to allow for it drilling slightly oversize.
It's just a vent hole, so size won't be too critical.

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Don McCombs
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Re: Why won't it fire?

Postby Don McCombs » Mon Oct 18, 2021 10:08 pm

Moray wrote:...size won't be too critical.

Agreed.
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k hutchins
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Re: Why won't it fire?

Postby k hutchins » Wed Oct 20, 2021 6:33 pm

Congratulations on a long tedious and frustrating journey.

Well done sir
Why is there never enough time to do the job right, but always enough time to do it over. :?:

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Don McCombs
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Re: Why won't it fire?

Postby Don McCombs » Sat Dec 04, 2021 9:25 am

Don McCombs wrote:Checked a repro carb that I recently purchased on eBay. It, too, was not drilled. The hole on an original carb appears to be 11/64. What size bit did you use?

Undrilled Chinese Carb Bottom.jpeg


There has recently been a little confusion about which hole needs drilled. It is the dimple in the top of the channel over the choke shaft. Be careful when drilling, so that you don’t damage any of the choke parts.

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Don McCombs
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