Slight surging.

[Ike 75]

I have a low hour 520h that runs excellent when I am using it under load (with my rear bagger setup) and does great on all types of crazy hills on my property. When I pull up to the area I empty the grass catcher and idle down it isles smooth.  If I start driving it across the yard without engaging the pto, it will start to surge.  The vacuum gauge goes up and down but the rpm seem to remain constant.  Very strange. Anybody have any ideas what could cause this?  

[lynnmor 7,817]

The best place to start is clean the carburetor.  

[Ike 75]

The carburetor is brand new from ebay.  I guess I should try that.  I was going to put the old one back on after soaking it.  Both look very clean. And the machine acts the same way with both carburetors.  

[lynnmor 7,817]

Did you check for leaks between the intake manifold halves, intake gaskets or vacuum hose?  What year is your tractor and does the carburetor have an accelerator pump?

 

I am not a fan of eBay carburetors, did you completely disassemble the old one and clean it properly?  Why did you replace it?

 

On page 7-6 of the Onan manual, there are instructions on adjusting the governor.  I would avoid doing adjustments till other faults are ruled out.

[Ike 75]

19 hours ago, lynnmor said:

Did you check for leaks between the intake manifold halves, intake gaskets or vacuum hose?  What year is your tractor (1993) and does the carburetor have an accelerator pump? Not sure

I checked for leaks on the half’s when I had it apart and there did not appear to be any leaks, intake gaskets are brand new, not sure on vacuum hoses.  

I am not a fan of eBay carburetors, did you completely disassemble the old one and clean it properly?  I cleaned it the exact same way I did my 1992 520 and 1997 520 (which both run awesome) Why did you replace it?  I replaced it because I wanted to try to smooth out the operation.  My 1997 and 1992 520’s run smooth with no surging after I cleaned out both carbs.

 

On page 7-6 of the Onan manual, there are instructions on adjusting the governor.  I would avoid doing adjustments till other faults are ruled out.

 

[lynnmor 7,817]

The later models, maybe only 1997, have this.  A 1993 shouldn't have it unless the carb has been replaced.  Look for the protrusion with that shape on the carb side as seen in the photo.

 

There is a welch plug just above the idle mixture screw that can work loose or fall out, also there can be dirt behind it that is difficult to clean requiring the plug to be removed.  Since you have a part throttle problem, I am guessing this could be the issue.  You can spray carb cleaner into every orifice to dislodge debris in there, but the best is to remove the welch plug.  You will need a 3/8" aluminum plug to replace the old one.

 

[richmondred01 2,307]

Is it a Nikki Carburetor?