Won't start, Gaso backflows out of carb after cranking

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chashlow
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Won't start, Gaso backflows out of carb after cranking

Postby chashlow » Sun Feb 21, 2021 10:19 pm

1947 Cub--When cranking with starter, won't start. When stop cranking, gas flows/drips (bigtime) out of carb intake. Seems like carb feeds gas into venturi while cranking, but when cranking stops, gas vapor in manifold condenses and falls back down to carb. 2 carbs tried, both same results. Both carbs recently rebuilt. Tractor valves recently reseated.

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Re: Won't start, Gaso backflows out of carb after cranking

Postby tinnerjohn » Sun Feb 21, 2021 10:35 pm

Have you checked to be sure you have spark? Also, is the ignition switch on? Sounds dumb, but I will admit I've tried in vain to start engines only to find I had forgot to pull the switch out. You might want to pull the spark plugs and be sure they are clean and not soaked with raw gas, proper gap, etc. Good Luck. John

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Don McCombs
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Re: Won't start, Gaso backflows out of carb after cranking

Postby Don McCombs » Sun Feb 21, 2021 10:43 pm

chashlow wrote:Seems like carb feeds gas into venturi while cranking, but when cranking stops, gas vapor in manifold condenses and falls back down to carb.

That is exactly what's happening and is perfectly normal on a no-start situation.
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Re: Won't start, Gaso backflows out of carb after cranking

Postby Magnum » Sun Feb 21, 2021 11:24 pm

chashlow wrote:1947 Cub--When cranking with starter, won't start. When stop cranking, gas flows/drips (bigtime) out of carb intake. Seems like carb feeds gas into venturi while cranking, but when cranking stops, gas vapor in manifold condenses and falls back down to carb. 2 carbs tried, both same results. Both carbs recently rebuilt. Tractor valves recently reseated.


Too much choke ?

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Re: Won't start, Gaso backflows out of carb after cranking

Postby Gary Dotson » Mon Feb 22, 2021 8:21 am

Make sure your ignition system is in shape and providing a good spark. It should be a strong blue spark that will jump a 3/8" gap.

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Rick Spivey
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Re: Won't start, Gaso backflows out of carb after cranking

Postby Rick Spivey » Mon Feb 22, 2021 8:24 am

There is in fact a weep hole in the bottom of the carburetor to allow that gas to flow out, trying to avoid a flooded situation. This is normal with an updraft carb. Your problem is spark.
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Re: Won't start, Gaso backflows out of carb after cranking

Postby Magnum » Mon Feb 22, 2021 9:18 pm

How about timing ? One cub I bought the timing was out and the dang thing was impossible to start. It would start if pulled , but really hard if the starter was doing the job

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John *.?-!.* cub owner
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Re: Won't start, Gaso backflows out of carb after cranking

Postby John *.?-!.* cub owner » Mon Feb 22, 2021 9:42 pm

Most cubs unless it is pretty cold do not need to be choked to start, but frequently need a little (1/2 to 1/3 way out) to keep running. You may be over choking it or you may have an ignition problem.
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Re: Won't start, Gaso backflows out of carb after cranking

Postby Waif » Mon Feb 22, 2021 11:31 pm

It's been mentioned before. Ideally a Cub fires up in several seconds or less. It's nice when you hardly bump one and it starts. (!)

You have more going on than the carb.( I can say that after you've tried two that are good..L.o.l..)

When you have gas in the carb, shut the gas off if you're going to keep cranking during a no start.. You have the gravity and fuel above it still. So if it's going to start reasonably soon , it will have the fuel to do so. And before it starves if you keep cranking (don't over heat the starter) it will be leaned out when less fuel and more air are being fed.

Look into other potential issues first. Like spark as mentioned. Plug gaps. And your kill /on/off switch.
Clean points adjusted properly ect. (Members have mentioned the insulating material the points rest on breaking and the points shorting/grounding out there).

Should have gone through full tune up procedure if it's a new to you Cub. That covers timing too.
Make sure your air intake system is clear and it's oil at proper level too. I found gobs and gobs of drowned insects in one. How they got in there I don't know. They looked much bigger than the intake screen.

Double check your throttle linkage on the back of the carb that operates your choke plate. When it is loose or installed wrong and you're closing the choke when you think you are opening it...You're doing what I did once. (Or twice!) The rectangular piece on the carb you use (or , can use) to adjust the choke with your fingers is a good indicator of choke plate position.

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Re: Won't start, Gaso backflows out of carb after cranking

Postby Glen » Tue Feb 23, 2021 12:57 am

Hi,
Below is a pic from TM Tractor of the engine side of the IH carburetor, showing the choke lever in the fully open position, at the bottom right of the pic.
It faces about straight down, then the choke rod is in almost against the dash when the choke is fully open, if the rod is the original length.
You pull the rod out to put the choke on.

You can remove the air cleaner intake hose from the carb to see if the choke butterfly valve is open or closed.

Sometimes the lever is upside down, then the rod works backwards.
Use a light, and you can probably see if the lever is on the right way.

The screw holding the lever can loosen from use, then the lever can turn in the shaft, without closing the choke.
Check the screw and be sure it is tight, and the lever is closing the choke.

The throttle rod from the governor connects to the linkage at the top of the carb. :)
Attachments
Cub carb.jpg


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