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Hank F.
Hank F., Technician
Category: Small Engine
Satisfied Customers: 15275
Experience:  Certified on Onan and Generac generators
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I have a Generac 15 KW portable generator with a 992 cc v-twin

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I have a Generac 15 KW portable generator with a 992 cc v-twin engine. The generator is almost new with only a few hours on the unit. The engine seems to be a little rich and surges a small amount. The electronic governor seems to make this problem a little worse by trying to compensate. I see no adjustments on the carburetor but do notice when I shut the fuel off that it runs very smooth and stable just before going lean and stopping. Is there anything I can do to lean this engine?

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The problem is a dirty carburetor.


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.

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.


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.

This is an EPA compliant carburetor, and as such, is not supposed to be taken apart or adjusted.

When you have an issue with it, you are supposed to replace the entire carb.

But, sometimes it is possible to clean them. Just remember, that as an EPA compliant carb, there are no parts available for it, so if you lose or break anything, you will definitely need to get a new one.


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.



But if this set is still under warranty, you should let an authorized Generac technician repair it, so you do not void your warranty.


Customer: replied 5 years ago.



Thank you for your response, but the problem I described is opposite to the information you provided. Please re-read my symptoms. It appears that my engine is getting to much gas. I want to reduce some of the fuel flow. I realize there is not an adjustment any longer that allows this to be done. I what to know if you know of a way to do this, example modify the main jet. I could have the same problem with a new carburetor as this unit has done this since new. This unit is operated at an altitude of about three thousand feet. Also when I shut off the fuel and run the engine till it stops, I use the choke to get most of the fuel out of the carburetor. Before the engine stops I can adjust the choke and make the engine run smooth when there is little fuel in the carburetor. This is why I know the engine is getting to much gas and I need to make the mixture leaner. Can you help me make this happen?


Don Sebold

The first thing you need to do is to clean the carburetor.


Please scroll back up and read the part about varnish buildup not allowing the float to shut off the flow of fuel when the bowl is full. This leads to too much gas being pushed into the engine.


You do not want to modify the jet - it is a precisely drilled component. It is possible to enlarge the jet, but impossible to make it smaller.


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