Why won't it fire?

[Moray]

Do you ever wish you said no to certain jobs?

With the new carb on, the tractor started but wouldn't run without the choke. Any reduction in the choke, resulted in it stopping within a couple of seconds.

Tonight, I found a bit clear hose, and bodged it into the drain plug.
Turning the fuel on, resulted in the fuel level initially stopping about 1/2" from the bottom of the carb, which would have explained it running extremely lean, but then after a couple seconds, it started rising again and didn't stop, even with the pipe an inch or so above the top of the carb.
Removed the hose, and pushing the float up resulted in the fuel stopping, so I split the carb to have a look.

Anybody want to play spot the difference?

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.
I'll drill the hole out over the weekend, and see if I can get this tractor to actually run for more than a few seconds without manual interference.

[indy61]

I'm not fully understanding but the drain plug has nothing to with the float. It will drain the tank when removed.

[Nelson 634]

Thanks for the information. This is helpful to everyone. I have used these "Chinese carburetors" several times with no problems. But this verifies what others have said you have to check them over thoroughly. Quality control seems to be a problem.

[Eugene]

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.

[Jim Becker]

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.

[Rick Spivey]

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.

[Moray]

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.

[Bob McCarty]

The original studs were 1/4-20 into the carb, and 1/4-28 thru the manifold with a nut. Probably unavailable to you.

[Glen]

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

[Don McCombs]

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?

[Moray]

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

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.

[Don McCombs]

...size won't be too critical.

Agreed.

[k hutchins]

Congratulations on a long tedious and frustrating journey.

Well done sir

[Don McCombs]

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.