I have recently purchased a used living quarter trailer.  The dealer had recently winterized the trailer and when I came to pick it up, they tried to start the Onan 4000 generator.  It wouldn't start.  They said that it was from the fuel stabilizer that they used to winterize it.  They said they didn't have any gas around to run through it and that it was really simple to start it.  "It was running fine when we winterized it last week".  After trying to start it myself with no success I took it to a much closer dealer to have them look at the generator.  After draining the gas tank, changing the spark plug and carburator bowl, it started and ran.  Although it ran, it ran rough.  It would run smooth for a short period and then surge several times.  The dealer said I should run some STP through it with about 5 gallons of gas.  So I put in the STP and left it running on the 2 hour drive home.  When at home it was surging worse. 

My question, will this be my fix or am I going to have to replace the carburator. 

I agree with both Paul and the plugged carb theory. If the wrong kind of stabilizer was used or too much, it should have improved the running with new gas and a flushing. Because it hasn't, some of the mentioned small orifices could be plugged.

Sea Foam isn't a magical cure all, but it does solve a lot of problems. I would try adding some to the gas and run it for a short while and see if things improve.

If it doesn't, an other fix you can try is remove the mixture adjustment screws, idle and main if equipped with both, and blow some compressed air through them (aprox 30 psi). When they are removed, carefully count the amount of turns, write the numbers down, and reinstall them, using exactly the same number.

These attempts will some times work, and are less expensive than pulling and rebuilding the carb. If they don't work, you haven't wasted a lot of time or money.

The carburators on the small HP motors are very simple, and there are only a few things that can change within them that would cause the engine to run roughly. To have to replace one for $150 seems a bit excessive, considering that it may only need a cleaning or adjustment to make things right. Carburators don't wear out in only a couple hundred hours of usage. They may plug up or go out of adjustment.

The first thing to check is the inlet. Pull the air filter and look at the throat of the carb. The first thing you will see is a butterfly plate blocking the inlet tube. With the engine cold, this should almost completely block the inlet. If the engine will start, run it for a few minutes and this plate should open so that there is no restriction for the air flow. If it doesn't open, the engine will start to hunt in its RPM and may start stalling out. The choke is making too rich a mixture, and causing the plugs to be fuel soaked and the engine will loose power. Usually the linkage controlling the plate's movement is dirty and needs some light lubrication like WD 40. Manually move the linkage to ensure it is free and working. The throttle plate wil be moved by a linkage and spring (S) going to the engine's governor assembly. Be careful not to bend this linkage by moving the plate. A light spray of oil onto these moving parts won't hurt.

While the air filter is removed, replace it with a new one. This should be done every 50 - 100 hours depending on the environment.

Many small HP motors don't have a fuel filter, and if you don't have a funnel with a screen installed, every time the tank is fueled, small particles can enter the fuel system. These will travel into the float bowl assembly and plug the main metering jet. These too, will cause the motor RPM to hunt and eventually stall out. Low power, variable RPM and hard starting are symptoms of this condition.

In addition to the main metering jet, usually you will find two adjustable jets, An idle and a high speed. On some motors the high speed is fixed and is not adjustable. These jets can become clogged or partially blocked by sediment or old fuel that has varnished or gelled. As I previously mentioned, these can be opened up by pressurised air and sometimes cleaned by various automotive products.

To properly clean the carb, it should be removed and partially disassembled. The only part that can fail, especially on a newer carb, is the float assembly with its needle and seat. The replacement costs are generally less than $15. Nothing else on the carb wears out, until many hundreds of hours running time have passed.

Anyone with some simple tools can remove, inspect and clean this very simple carb. It may take an hour or so, but at a replacement cost of $150 which you probably don't need any way, it's very good wages.

If you're not comfortable with this mechanical project, try calling an individual who works on lawn equipement. It does not take an Onan dealer to mess with these carbs. We have a local mechanic who specialises in keeping old lawn machinery running, and most of his "repairs" are cleaning and adjustments.

Draining the carb empties the fuel from the float bowl. It doesn't remove the remaining fuel trapped in the predrilled passages, orifices and jets. As this fuel ages and dries out, it can turn to varnish and a gell consistancy, and can plug these smaller passages.

It's better to use a fuel stabilizer mixed with the gas. If the motor is run a minute or so after the stabilizer is added, it will be circulated throughout the carb, protecting all the passages and orifices from contamination. There is no need to drain anything, and any remaining fuel in the tank will be protected from ageing. You will find the first start after storage much easier and the engine will run smoother.

I run additive through it and then drain it as the Onan dealer instructed me to do.

I did replace the carb as I was told that was what caused mine to surge. It was very labor intensive to get it out to get the cover off to get to the carb. When that didn't work I broke down and took it to the very expensive Onan dealer taht charges 95.00 an hour. It was the fuel pump so all my money and labor was useless on the carb. On a good note the fuel pump was on the bottom of the trailer so it was fairly easy to change