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Onan RV generator engine starts and runs but surges will run…

Customer Question
Onan RV generator engine...

Onan RV generator engine starts and runs but surges will run 15 seconds or so and then rpm falls, recovers and does it again.

Mechanic's Assistant: Do you have fuel in the tank? If so, is the shut-off valve open and the fuel clean?

As far as i know just made 300 mile trip fuel level is good and fresh.

Mechanic's Assistant: Do you plan on doing the work yourself?

Yes

Mechanic's Assistant: Anything else we should know to help you best?

Removed fuel shut off from bottom of carb bowl the plunger was free and i had about 40oms through to coil

Submitted: 10 months ago.Category: Small Engine
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Customer reply replied 10 months ago
Just went to check the hours on unit. Hit start button and engint started rite off released start button and engine died. Start button again and this time stayed running bbut starting surging rite away. This unit has 260 hours of run time. This has never given any sort of trouble before.
Customer reply replied 10 months ago
The only service this unit has recieved is oil and air cleaner changes.
Answered in 4 hours by:
10/6/2017
Technician: Hank F., Technician replied 10 months ago
Hank F.
Hank F., Technician
Category: Small Engine
Satisfied Customers: 15,995
Experience: Certified on Onan and Generac generators
Verified

Hello, my name is ***** ***** I am going to assist you with this.

Surging is a sign that not enough fuel is getting to the engine - you have a bad 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.
If this is the case, you will need to change the oil and filter, if it has a filter.

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. It takes months or years for this accumulation to build up – it just can not be removed in minutes.
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.

Personally, I do not recommend trying to clean this carb. I recommend replacement.

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Technician: Hank F., Technician replied 10 months ago

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