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Hank F.
Hank F., Technician
Category: Small Engine
Satisfied Customers: 14630
Experience:  Certified on Onan and Generac generators
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I have a Briggs and Stratton 13.5 hp engine on my riding lawnmower.

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I have a Briggs and Stratton 13.5 hp engine on my riding lawnmower. I purchased it used and when I bought it, it was difficult to start including having gas coming out of the muffler when we were cranking it to turn over. I replaced the battery and it started well for a while and had no problems. After I did that, i bumped something and broke the flywheel key. Before I realized this was the problem, I replaced the fuel lines, filter, the spark plug, and the oil. Once I replaced the flywheel key it was working great but only at about half throttle. Recently, I pulled it out of the garage and it sounded weak when at even full throttle. If I throttled it down, it would sputter. So I left it at full throttle. When I dropped the blades, it died and I could not restart it and had to roll it into the garage. After this happened, I lapped the valves assuming that it was a compression problem as it seemed to get gradually worse as I used the mower. This got it running again, and I mowed a small section of my grass. It still must be at almost full throttle to get rolling. One item I thought was strange, is that when I took the carb off of it, gas poured out. I am thinking that it is a problem with the carb being dirty, but it looks spotless when I look down where the "butterfly" is. Also, when I turn the engine over without the air filter, it sprays a fine mist of fuel up into the area where the filter would be at what seems to be random intervals. The engine seems to knock and rattle a bit when running. What do you think the issue is? Thanks so much in advance for your help.
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Your problem is a bad carburetor.

The mistake that you are making, and the one that 99% of the people do, is that you are looking at the throat of the carburetor.
Gas does not flow through the throat, it flows through internal passages that are drilled inside the carb, where you can not see.
Even the bowl has little to do with this. All a bowl does is hold gas for the engine to draw from. A bowl can be completely crudded up and nasty, but still work just fine.
Conversely, a bowl can be absolutely spotless, and the carb still will not work, as it is those internal passages that are the problem.

As gas gets old, it turns to varnish and clogs up the passageways inside the carburetor, not allowing enough gas to get to the engine.

Today’s gasoline formulation goes bad in as little as 30 days.

This condition is cumulative. Every time gas sits, the varnish builds up just a little more, like coats of paint, until eventually gas can not flow. It will not happen overnight, but the symptoms can show up all of a sudden, even while simply stopping to refill with gas.

The use of fuel additives, such as Sta-Bil or Sea Foam will not stop this process from happening. They will greatly slow it down, but the gas will still go bad.

When this happens, either the engine simply will not start, or it will not run without the choke on (this reduces the amount of air getting pulled into the engine, changing the fuel/air mixture), or it will run but surges.

Another issue that varnish in the carb can cause is that the varnish may not allow the float needle to seal properly against the seat, causing the flow of gas to not shut off when the bowl is full. The result will be gas overflowing the carb and running into the cylinder, and possibly out the air intake. If the gas gets into the cylinder, it will seep past the rings and down into the crankcase. This will be evidenced by your oil level being over-full and/or the oil smelling like gas.

If this is the case, you will need to change the oil and filter, if it has a filter.

Varnish can also cause the float needle to stick shut, and not allow any gas to flow from the carb bowl into the engine.

The only 2 solutions are to either replace the carburetor or give it a good, thorough cleaning.

When removing the carb, make sure to take a good picture, or make a good drawing of where all springs and linkages are attached. This will make reassembly much easier.

Most people believe that cleaning a carb involves removing the bowl and wiping it out, then spraying some carb cleaner through it.

This is simply insufficient.

To properly clean the carb, you must remove it, disassemble it (making sure to remove all non-metal parts), and soak it in a commercial solvent for several hours. Soaking it overnight is even better.

Then clean all solvent off with a spray type carb cleaner, making sure to get lots of cleaner into every hole and passage there is. Pay special attention to the tiny holes in the bore of the carb, under the throttle plate for the carbs that have these holes. Use lots of cleaner. And make sure to wear safety goggles to avoid getting the over spray into your eyes. There will be over spray.

Dry the carb with low pressure compressed air.

When reassembling the carb, make sure to use a carb kit, when one is available for your carb.

Occasionally, even a good cleaning is not going to be sufficient, and you may end up having to replace the carb anyhow. Be prepared for this.

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