Engine fluctuation

[BrotherTobias 12]

I have an early 90s 520h with a 20hp onan, I think. Just starting to get her back in running order. Sorted out some electrical issues, but I have a long way to go. I believe the automatic choke is misbehaving. The engine at an idle or idled up fluctuates about 200rpms. I've cleaned up the carb, but I'm fairly unfamiliar with how the automatic choke functions. Is it most likely a vacuum issue or is there a spring that needs addressed? Any help is appreciated. 

[davem1111 2,030]

8 minutes ago, BrotherTobias said:

I have an early 90s 520h with a 20hp onan, I think. Just starting to get her back in running order. Sorted out some electrical issues, but I have a long way to go. I believe the automatic choke is misbehaving. The engine at an idle or idled up fluctuates about 200rpms. I've cleaned up the carb, but I'm fairly unfamiliar with how the automatic choke functions. Is it most likely a vacuum issue or is there a spring that needs addressed? Any help is appreciated. 

 

Welcome to Red Square! 

 

I'm sure you'll get some better ideas when the experts  come along, but since I'm having a somewhat similar issue with a Honda GX610 18hp on a GT-1848 and I noticed your post, I thought I'd chime in.  A few things that I'm going to check out are the fuel pump and fuel filter (and carb, but you already cleaned yours).  I don't have auto choke so that isn't the problem with mine, but it could be with yours.  I'm sure by tomorrow at least you'll have some very useful tips.

[BrotherTobias 12]

Thank you. 

[kpinnc 12,083]

Common issue on Onan engines. Carb cleaning may help. 

 

On mine, putting some silicone on the seam of the two piece intake manifold cured the surging. You don't need a bead, just a thin film smeared on the seam with your finger.

 

If adding some choke on yours calms down the surge after cleaning the carb- then the engine is sucking too much air at or around the intake. 

[Ed Kennell 38,209]

I cleaned the carb three times.  The surging finally stopped after the third time.

[roadapples 6,983]

[SylvanLakeWH 25,598]

 

In addition to carb clean, I’d replace the fuel line, filter etc. to get residual gunk out…

Edited May 19, 2022 by SylvanLakeWH

[953 nut 55,297]

Sounds like you have gotten some good suggestions, please let us know how you make out on this.

[peter lena 8,634]

@BrotherTobias   plan on a tank valve replacement ,https://www.ebay.com/itm/172159421400?c41348861725295&rlsatarget=pla- , this only for reference , if you can get it locally , support them. another detail on installation , and cleaning  is to use DIELECTRIC GREASE  on the grommet and valve , installation, makes it much easier , and its made for rubber / plastic  fitting,  also add a first filter  close to tank , and another close to carb . that has worked for me with zero debris in carb bowls . might even consider adding a vertical fuel check valve . to hold fuel charge , for quicker starts , look up EVIL ENERGY 5/16 aluminum  check valve . install VERTICALLY  before carb . they work , just some ideas . pete 

[peter lena 8,634]

@davem1111  , having done all my tanks , for valves and grommets , the grease makes for a very easy slide in fit , regularly used super lube as a millwright , in food grade fitting work . use a large fuel filter in rear seat frame area , and another small size forward  before carb  and check  valve , also used fuel rated clear vinyl hose , dark green squeeze clamps . have lots of hours on this set up , zero leaks , visible fuel flow . anything I  suggest , I  am currently using , so I  know it works consistently, regularly use my 3 horses , they all run and are set up the same way . reliable running , pete   

[lynnmor 7,307]

12 hours ago, BrotherTobias said:

I have an early 90s 520h with a 20hp onan, I think. Just starting to get her back in running order. Sorted out some electrical issues, but I have a long way to go. I believe the automatic choke is misbehaving. The engine at an idle or idled up fluctuates about 200rpms. I've cleaned up the carb, but I'm fairly unfamiliar with how the automatic choke functions. Is it most likely a vacuum issue or is there a spring that needs addressed? Any help is appreciated. 

 

It does not have an automatic choke.

 

The carburetor should be removed and cleaned properly.  Check that the welch plug just above the idle mixture screw is in place.

[BrotherTobias 12]

19 hours ago, kpinnc said:

Common issue on Onan engines. Carb cleaning may help. 

 

On mine, putting some silicone on the seam of the two piece intake manifold cured the surging. You don't need a bead, just a thin film smeared on the seam with your finger.

 

If adding some choke on yours calms down the surge after cleaning the carb- then the engine is sucking too much air at or around the intake. 

Adding choke does calm it down. I'm going to clean the carb out again since I'll be there anyway, but am definitely going to give the silicon a try!

[lynnmor 7,307]

Always do a compression test to get an idea if the engine is healthy, then go after the other issues.

There might be a vacuum hose attached to the intake near the carburetor, make sure it is not leaking at each end.  

The 96 & 97 years might have an accelerator pump, it would be inside a cover held in place with 3 screws, check to see if you might have one, carbs get swapped over the years.

 

If a proper cleaning with the carburetor and intake removed and then reinstalled with new Onan gaskets, does not fix the problem, then pop out the Welch plug previously mentioned and clean in there.  It is a touchy job and finding a 3/8” Welch aluminum plug can be time consuming or I would tell you to do it first thing.

[Horse Newbie 7,074]

22 hours ago, kpinnc said:

Common issue on Onan engines. Carb cleaning may help. 

 

On mine, putting some silicone on the seam of the two piece intake manifold cured the surging. You don't need a bead, just a thin film smeared on the seam with your finger.

 

If adding some choke on yours calms down the surge after cleaning the carb- then the engine is sucking too much air at or around the intake. 

 

22 hours ago, Ed Kennell said:

I cleaned the carb three times.  The surging finally stopped after the third time.

I was gonna say… check the intake manifold to see if the exhaust heat shields have rubbed a “groove “ in the sides where the seam is…

If they have you can take some tin snips a trim a little of the sheet metal back on the heat shields. I had to do that on my 1994 520H…

[Horse Newbie 7,074]

11 hours ago, peter lena said:

@davem1111  , having done all my tanks , for valves and grommets , the grease makes for a very easy slide in fit , regularly used super lube as a millwright , in food grade fitting work . use a large fuel filter in rear seat frame area , and another small size forward  before carb  and check  valve , also used fuel rated clear vinyl hose , dark green squeeze clamps . have lots of hours on this set up , zero leaks , visible fuel flow . anything I  suggest , I  am currently using , so I  know it works consistently, regularly use my 3 horses , they all run and are set up the same way . reliable running , pete   

Didn’t mean to quote Peter… touched the screen, now it’s here…

@BrotherTobias  to

!!!

The best tractor forum in the world !

[BrotherTobias 12]

Thanks for all the great info, y'all. I tried a quick carb clean, but to no avail. Looks like I'll be tearing it down and deep cleaning it as well as most of the other tips received. Now if only there was more hours in a day. Haha!

[kpinnc 12,083]

2 hours ago, BrotherTobias said:

Now if only there was more hours in a day. Haha

 

I will say this: a good running Wheel Horse is worth the effort. Once it gets to work like it was designed for- you'll be happy to look back on time spent getting it back up and running. 

 

Nothing produced in the last decade will ever compare to these machines, and once they are gone- there won't ever be any more. 

 

It's good to see another one being saved and put back to work!

Edited May 20, 2022 by kpinnc

[Fishroe 261]

21 hours ago, kpinnc said:

 

I will say this: a good running Wheel Horse is worth the effort. Once it gets to work like it was designed for- you'll be happy to look back on time spent getting it back up and running. 

 

Nothing produced in the last decade will ever compare to these machines, and once they are gone- there won't ever be any more. 

 

It's good to see another one being saved and put back to work!

You got that right!!

[BrotherTobias 12]

22 hours ago, kpinnc said:

 

I will say this: a good running Wheel Horse is worth the effort. Once it gets to work like it was designed for- you'll be happy to look back on time spent getting it back up and running. 

 

Nothing produced in the last decade will ever compare to these machines, and once they are gone- there won't ever be any more. 

 

It's good to see another one being saved and put back to work!

I must admit, at first I was feeling a bit dejected over the thing. I'm looking forward to getting it back in shape though and seeing the potential more and more!

[lynnmor 7,307]

All engines with carburetors have the potential of deposits causing performance issues.  Tractors are often abused by sitting for long times, dirty gas cans, dirty gas tanks and no maintenance on fuel lines and filters.  Just a little dirt is no reason to dispose of a tractor but it does give us buying opportunities from those that don't know.  The problem isn't an Onan issue, it is just the usual abuse, no engine is designed to run on dirt.