520 Surge Issues

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13 Oct 2015 22:26 #8741 by FewMoreMiles
Anyone experienced with the steps to diagnose and repair the dreaded Onan surge?

490 Hours
125PSI both cylinders
new plugs
new fuel filter
one can carb cleaner run through

*Was mowing yesterday, having literally cleaned the flywheel screen at least 40 times to prevent overheat. Regardless, I got an engine temp light and immediately cooled the motor and shut it down for an hour or so. Cleaned all fins, etc and started to mow. About a half acre later and 10 more flywheel cleanings, the tractor started that radical surge. All RPM ranges, all the time. One note is that it doesn't do it quite as badly when the deck is engaged at WOT? Governor issue, carb issue or perhaps something else? Assumed it was valve seat but have good compression across both cylinders? Thermal breakdown of a sensor perhaps? Any advice or help is most appreciated. Not sure I want any Onans around here anymore if this is how they behave. Never a single problem from my Kohlers, but just HAD to have a 520. Sorry Geno...should have listened when you told me they were headaches at times!

Thanks!

Jeff

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14 Oct 2015 07:49 #8743 by Geno
Replied by Geno on topic 520 Surge Issues
Did this start right after gassing up?

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14 Oct 2015 09:45 #8746 by FewMoreMiles
Replied by FewMoreMiles on topic 520 Surge Issues
No Geno I was at about 1/2 tank when the over temp light came on. I cooked the unit off as quickly and thoroughly as possible and the surge started when I started the mower up about an hour later. I did take the top off the carb last night and there was nothing in the bow but clean fuel and a freely moving float. The long brass tube with (5?) holes was clean and I could see through all. Sprayed carb cleaner into jets and nothing seems to have changed. The surge is very dramatic. The vacuum Guage always seemed to work well on this one and it now runs from zero to about 80 when the motor surges. One thing that I don't have is the "wad" that goes in that tube from the flywheel to the air cleaner. Can you advise as to what this material is, or if it can be purchased somewhere? Thanks for your help!

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14 Oct 2015 10:26 #8747 by M Bailey
Replied by M Bailey on topic 520 Surge Issues
Jeff, surging + overheating usually means a lean fuel-air condition. Could be anything from a plugged fuel filter or bad fuel pump to a plugged main jet in the carb. Does pulling the choke out help with the surging? If so, that's a good way to tell if it's starving for fuel.

:2cents

Major


"I refuse to join any club that would have me as a member." Groucho Marx

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14 Oct 2015 10:43 #8748 by FewMoreMiles
Replied by FewMoreMiles on topic 520 Surge Issues
Major, choking just kills the engine as it always has. I drained the fuel tank last night, along with the fuel lines all the way to the motor. Took the top off the carb to find nothing but clean fuel in the bowl. Cleaned the carb without taking apart further as it was getting late...installed a new fuel filter with new high octane fuel and the severe surge continues. I did verify that the fuel pump is pumping and it is. Not sure how to verify precisely. I'm wondering if there may be some sort of thermal breakdown or something electrical but this is my first problem like this. Thanks again for any advice.

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14 Oct 2015 11:02 #8749 by Geno
Replied by Geno on topic 520 Surge Issues
The choking was the next logical step. If that doesn't help this is pretty weird. Is the engine running smooth even during the surging without any type of miss?

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14 Oct 2015 11:08 #8750 by FewMoreMiles
Replied by FewMoreMiles on topic 520 Surge Issues
It really is weird. I thought things would clear up once I cleaned things out last night and so forth. There is a miss as well, especially noticeable at high rpm. If I hold the governor at a consistent speed regardless of throttle, I've noticed that the surge stops. I know nothing about these automatic governors (same with the RJ) and I think they are the thorn in my back side. If I hold governor and surge stops, what triggers governor to make the radical adjustments? Fuel pump not actually filling the bowl despite how it appears to be? Taking carb apart again shortly and will see if I can verify/clean out fuel pump as well. Only have a couple of hours today anyway :). Thanks guys

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14 Oct 2015 11:10 #8751 by GlenPettit
Replied by GlenPettit on topic 520 Surge Issues
2¢ from here:
I've heard people comment that the Onan engines don't do well on side slops, even low-angle ones, something about the gravity flow-thing, both for the fuel and for the oil lubrication.

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14 Oct 2015 11:14 #8752 by Geno
Replied by Geno on topic 520 Surge Issues
When you did the compression check the engine never started right?

I would check the spark at both plugs, and the plugs paying attention to the rear. If it has a miss, and spark and plugs are good, it more than likely lost the valve seating. I've seen several need a motor including one I use all the time that lost it at 450 hours. This is why I hate Onans in the Horses...

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14 Oct 2015 14:07 #8756 by FewMoreMiles
Replied by FewMoreMiles on topic 520 Surge Issues
Found it guys. Was a clog in the jet after all. I didn't get into it effectively enough to get to the clog the first time around. Runs smoothly as it did before, but I think this thing is going to become snow queen and we will just leave the mowing to the Kohlers and Briggs on hand

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