516-H won't stop surging

[1990Onan520-H 15]

Ok all I cave... need some help. I've got a 516-H bought brand new in 87 by my father and used every year as his main workhouse for mowing and general yard work to this day. The past couple years it has been surging and getting progressively worse. Would run better choked but now it's at the point where it's so bad it's parked. Fires right up damn near instantly but starts surging right away. Every once in a blue moon it will run perfect out of nowhere, but that's very rare. 

 

Before the comments come I've been on him for years to get it fixed before it blew apart. So that being said we have been trying to get it fixed. Carb has been cleaned 3 times and dunked in an ultrasonic tank as well. All fuel lines have been replaced, new in tank fuel screen, new fuel filter, new fuel pump, new air cleaner, new spark plugs and a carb rebuild kit. Ive even smoke tested it and see zero vacuum leaks. This mower has never sat since new so I don't think it has anything to do with fuel breaking down, although nothing is impossible. Always run shell premium fuel, at least for 5 or 10 years, probably regular though years before I got on him about it.

 

Now he has never done any major maintenance, so it never had a decarbon or valve adjustment,  just regular fluid changes. There is no hour gauge but were guessing it has around 1000 hours on it. Talked to a wheel horse mechanic and he said at those hours don't open it up if it runs ok. I don't personally agree, I think it should have at least a valve adjustment but either way I don't think that would cause surging. But feel free to disagree because I'm not 100% sure. Any thoughts? 

[ericj 1,576]

you've cleaned the carb, did you look to see if any of the welch plugs are missing ? double check the governor linkage. Check the intake for leaks. take card and choke cleaner and spray around carb and intake when running to see if surging gets any better. Did you take the carb apart when you cleaned it ? Most of the time when I have a surging onan it's the main jet that has a spec of dirt in it. good luck.

 

Last couple of years my main mower has been a 516H

 

 

 

 

eric j

 

 

 

Edited April 15, 2019 by ericj

[1990Onan520-H 15]

20 minutes ago, ericj said:

you've cleaned the carb, did you look to see if any of the welch plugs are missing ? double check the governor linkage. Check the intake for leaks. take card and choke cleaner and spray around carb and intake when running to see if surging gets any better. Did you take the carb apart when you cleaned it ? Most of the time when I have a surging onan it's the main jet that has a spec of dirt in it. good luck.

 

Last couple of years my main mower has been a 516H

 

 

 

 

eric j

 

 

 

Welch plug... what's that? Haha. Thanks for the other tips too.

Edited April 15, 2019 by 1990Onan520-H

[WildHorses 378]

(Preface for others with a surging Onan or looking to buy one - I'm  assuming the carb is clean and the fuel line is new and fuel pump is new, which are all good first things to check. I've had at least two Onan powered tractors surge from dirty carburetors. You really need to make sure you know what you're doing to clean them properly and get all passages but it sounds like you do.)

 

Onan's are known for having bad valve seats, specifically on the rear cylinder. It seems common to occur around 1200 hours in my experience but that varies with care and use. I would take the head off of the rear cylinder and inspect the valve and the valve seat. You're only out some time and a new head gasket if that's not the issue. You will need to remove the bolts holding the engine to the frame and turn the engine sideways or take it off completely to check the rear valves. 

Edited April 15, 2019 by WildHorses

[1990Onan520-H 15]

6 minutes ago, WildHorses said:

(Preface for others with a surging Onan or looking to buy one - I'm  assuming the carb is clean and the fuel line is new and fuel pump is new, which are all good first things to check. I've had at least two Onan powered tractors surge from dirty carburetors. You really need to make sure you know what you're doing to clean them properly and get all passages but it sounds like you do.)

 

Onan's are known for having bad valve seats, specifically on the rear cylinder. It seems common to occur around 1200 hours in my experience but that varies with care and use. I would take the head off of the rear cylinder and inspect the valve and the valve seat. You're only out some time and a new head gasket if that's not the issue. You will need to remove the bolts holding the engine to the frame and turn the engine sideways or take it off completely to check the rear valves. 

The engine runs very quite. No tapping noise or anything. Would a valve seat really cause a surging?

 

Also feels very even. Both cylinders seem to hit hard and the same.

Edited April 15, 2019 by 1990Onan520-H

[ericj 1,576]

1 minute ago, WildHorses said:

(Preface for others with a surging Onan or looking to buy one - I'm  assuming the carb is clean and the fuel line is new and fuel pump is new, which are all good first things to check. I've had at least two Onan powered tractors surge from dirty carburetors. You really need to make sure you know what you're doing to clean them properly and get all passages but it sounds like you do.)

 

Onan's are known for having bad valve seats, specifically on the rear cylinders. It seems common to occur around 1200 hours in my experience but that varies with care and use. I would take the head off of the rear cylinder and inspect the valve and the valve seat. You're only out some time and a new head gasket if that's not the issue. You will need to remove the bolts holding the engine to the frame and turn the engine sideways or take it off completely to check the rear valves. 

valve seats will make it miss or spit back through the carb, not sure it would make it surge though, I've been wrong before and will be wrong again,  so I've learned to never say never....  do a compression check. I have enough onan parts laying around, that why I have them for an emergency, I swap the carb to see if that takes care of the problem. I have had several over the years that I have had to do that to to get them to run right

 

 

 

 

eric j 

[1990Onan520-H 15]

3 minutes ago, ericj said:

valve seats will make it miss or spit back through the carb, not sure it would make it surge though, I've been wrong before and will be wrong again,  so I've learned to never say never....  do a compression check. I have enough onan parts laying around, that why I have them for an emergency, I swap the carb to see if that takes care of the problem. I have had several over the years that I have had to do that to to get them to run right

 

 

 

 

eric j 

I have 2 520's I can pull parts off just to test with. A 90 and a 91. Can I use a 520 carb or was the 516 different?

[ericj 1,576]

5 minutes ago, 1990Onan520-H said:

I have 2 520's I can pull parts off just to test with. A 90 and a 91. Can I use a 520 carb or was the 516 different?

 

the 520 runs a bigger carb, but it might work for a trail to test and see if it's a carb problem. The 18 runs the same carb as the 16. I would guess the worst case would be it would run rich, be careful not to wash the cylinder down running too rich

 

 

that's a lot of work to pull a running motor apart to test, but sometimes you have to do what you have to do before shelling out a lot of cash on a part that's not needed

 

 

 

 

eric j

Edited April 15, 2019 by ericj

[WildHorses 378]

I believe the main jet is the only difference in the carburetors on at least the 18 and 20, I’m not certain on the 16 but it would be worth trying to swap them for short time.

 

Loose valves seats can cause a variety of symptoms. With the work he’s done it would certainly be worth examining them imo. 

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Garry