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Old 10-23-2021, 09:25 PM   #1
Join Date: Sep 2016
Location: Texas
Posts: 58
Onan Gen. Carb Replacement

Reporting good luck with a carburetor replacement on my Onan QG 4000 Generator. It took two tries, but I got it right the second time.

The generator had been getting harder and harder to crank. However, once running, the generator would run just fine. I had been cranking, and running the generator under load for one hour each month. And I used Stabil in the fuel tank, and Seafoam also. But the hard starting got steadily worse, until one day, the generator would no longer start. I had already replaced the fuel filter and the fuel pump. The fuel pump would squirt gas out the opened carb fuel line, so I knew the carb was getting fuel. So pulled the spark plug and it checked OK. Figured it must be a clogged carb.

I found several youtube videos on how to replace the Onan 4000 carburetor and they gave me encouragement. I thought I should be able to do it. I researched the part numbers and got on Ebay. There were a good number of sellers of replacement carbs. I found one that seemed a good match and at a very good price (less than $29) so I ordered it. Before installing the carb completely, I saw other videos that claimed that the shut-off solenoid at the bottom of the carb might likely be the problem, So I disassembled my new carburetor to remove the fuel shut-off solenoid and I swapped that out with my old one on the generator. Was not too hard of a job and way easier than trying to replace the whole carb.

Except, it did not fix the hard starting problem. Generator still would not start. And now I was afraid to try and use the new carb because when I had opened it to get the shut-off solenoid, I accidentally had spilled out some small parts and gaskets and rings, etc. And I had no idea how they all fit back inside the carb. So, I just ordered another carb from Ebay ($29).

In the meantime, I had seen another video about fuel and ethanol and hard cranking on small engines. Noteworthy was a test of year old Ethanol based fuel. The test showed that the 'Volatility' suffered greatly in a years time. The year old gas would not ignite when exposed to an open flame. Altho it would burn, it just would not catch fire from a nearby match or flame. Thinking - small engine with carb not starting, but Coach engine with fuel injectors still would crank and run just fine. My problem might be just gas more than a year old. Altho I had topped off the tank once or twice, most of the fuel in the tank was still likely more than a year old. And Stabil and Seafoam would not do a thing to improve volatility IMHO.

So my next step was to get the motorhome out on the road and drive until the tank was almost empty, then fill up with fresh fuel. Got back home and removed the old carburetor. I ran the fuel pump until the new fuel came out (the old fuel was stained Yellow from the Stabil additive). Then, I installed the new carb from Ebay. Got her all put back together and put a bit of starting fluid in the carb throat,and gave her a crank. It was alive. acted like it wanted to run. then I put the air cleaner back on and buttoned everything back up. Gave her a quick couple of cranks and by golly, the generator fired up, smokey at first, but soon settled down.

The generator now cranks right up with no hesitation. Hooray! I did it.

PS. .............. These Onan generators want to backfire at times, best to keep a little fire extinguisher handy when using starting fluid.
Teaman is offline   Reply With Quote

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