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J.R., Small Engine Technician
Category: Small Engine
Satisfied Customers: 7445
Experience:  Owner of outdoor power equipment business, repairing all makes and models.
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Have a 16 hp Kohler engine on a Craftman Mower - Runs fine,

Customer Question

Have a 16 hp Kohler engine on a Craftman Mower - Runs fine, but then sputters going up slight hill with loss of power and eventual stall. Have cleaned carb, replaced oil, fuel filter, and have only fresh gas. What I notice is that there is a lot of air in the fuel line - the filter is clear plastic and mostly full of air. When it's running, gas trickles into the filter at about the rate it is being used, but the filter is mostly full of air. It is as though the engine is starving for gas. If I disconnect the fuel line from the carb, gas runs freely through the line into a can. However, when the line is attached to the carb, no fuel runs in to replace the air that I can see in the filter. I have blown out the fuel line, between the filter and tank, and it appears to be clear, but I still can't get gas into the line to replace the air. Should there be this much air in the fuel line ? No change if I loosen the fuel tank cap ? Runs fine with little air in filter. Any suggestions?
Submitted: 8 years ago.
Category: Small Engine
Expert:  J.R. replied 8 years ago.
The problem is likely a bad fuel pump or the fuel lines are bad. Check the fuel lines for cracks and replace them if needed. The fuel lines can suck air, but not leak fuel.
Customer: replied 8 years ago.
The fuel lines appear to be fine. Plus, wouldn't small leaks allow air to escape and let the line fill with gas, which it doesn't ? I can jiggle the filter for several minutes and eventually gets all the air out of the filter, but then as the engine runs, I can see the air gradually (over maybe 15 minutes) replace the fuel. The problem has to that there is problem causing some sort of vacuum that is not letting the air go up the line into the tank ... otherwise, why does the fuel run freely through the line when it is disconnected from the carb ? The isn't a screen where the line comes out of the tank ? Lastly, I didn't/don't think it has a fuel pump.
Expert:  J.R. replied 8 years ago.

Does the fuel line go straight from the tank to the carb with only the fuel filter between it?

Customer: replied 8 years ago.
Yes. And I have disconnected it, blown it out, and replaced the filter but it is still mostly full of air. I thought maybe the fuel tank cap is vented and may be clogged, thereby creating a vacuum, but I loosen it an d no change. The only change is when I disconnect the line at the carb, the gas then flows freely through the line and out into a can.
Expert:  J.R. replied 8 years ago.

The gas will only flow for so long until the carb bowl fills up and then it will stop until the fuel level drops and the float opens the needle valve and allows more fuel in it.

Customer: replied 8 years ago.
But shouldn't gas flow down the line towards the filter to replace the air that is in the line ?
Expert:  J.R. replied 8 years ago.

It should remain full. I think I would replace the fuel line to eliminate a line that is sucking air to start with.

Customer: replied 8 years ago.
I'll give it a shot and see if it works. I'll get back to you later.
Expert:  J.R. replied 8 years ago.
Let me know.
Customer: replied 8 years ago.
Good morning.

Replaced the fuel lines and it didn't help.

Is there any kind of vent in the carb that might be plugged up, thereby creating this vacuum that isn't letting the fuel flow down the line ?
Expert:  J.R. replied 8 years ago.
The carb has a air bleed, but that is for the engine when pulling fuel out of the carb. The fuel line goes into the catb and the fuel passes the inlet needle and into the bowl. Once the bowl fills up and the float rises the fuel shuts off so at that point no more fuel can enter the carb. Pull the bowl off of the carb and work the float/needle valve by hand and see if you can make it fill up.

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