WH 520 Onan surging

[pgiro 36]

Out of nowhere my 88 520H starting surging as if it were starving for fuel.

I replaced the fuel filter and fuel pump and neither helped. I can see that fuel is flowing properly.

Any idea what the problem might be? Short video is attached.

 

 

[lynnmor 7,321]

On the muffler side of the carburetor you will find the idle mixture screw, just above it is a welch plug, see if it fell out.  Failing that, you may have debris inside the carburetor that needs to be cleaned out.

[Handy Don 12,281]

13 hours ago, lynnmor said:

On the muffler side of the carburetor you will find the idle mixture screw, just above it is a welch plug, see if it fell out.  Failing that, you may have debris inside the carburetor that needs to be cleaned out.

Yep. Either blocked idle fuel passages or the Welch plug fell out.

[pgiro 36]

Thanks for the replies. I took a look at it tonight and I don't see anything obvious unless this one hole on the side of the carb which I've circled is supposed to have a screw. Otherwise, I don't see anything loose or missing that would cause it to keep surging. If I leave the choke up almost all the way, it will stop surging and run ok, which seems to indicate a fuel/air mixture issue.

[lynnmor 7,321]

 

You still need to remove the carburetor and give it a proper cleaning, the need for the choke says it all.  You will need to pull the intake with the carburetor, so new genuine Onan intake gaskets will be needed.  Since you will have it apart it would be a great time to adjust the valves.

Here is the location of the welch plug I mentioned:

 

[Handy Don 12,281]

48 minutes ago, lynnmor said:

Here is the location of the welch plug I mentioned:

 

And yes, in this image the welch plug IS missing 😁

The plug is a metal disk that wedges in tightly to seal off that hole.

[pgiro 36]

I pulled back the muffler shield and I was able to get a look at the side of the carb.

The welch plug is intact so that's not the problem.

The idle mixture screw has a plastic cap on it with a tab that prevents it from moving much in either direction

Is it worth removing the cap and messing with the idle mixture screw or removing and cleaning that port?  

[Handy Don 12,281]

7 minutes ago, pgiro said:

I pulled back the muffler shield and I was able to get a look at the side of the carb.

The welch plug is intact so that's not the problem.

The idle mixture screw has a plastic cap on it with a tab that prevents it from moving much in either direction

Is it worth removing the cap and messing with the idle mixture screw or removing and cleaning that port?  

The cap is to keep that carb within factory settings that meet emission requirements. Its the same reason Onan went to fixed high-speed jets. Eventually, these weren’t enough to meet the stringent rules, they shifted production to Canada for a while and then finally gave up on the P-series and, like all the other makers, went to overhead valves and more efficient combustion chambers

If it comes down to needing a really thorough carb clean, you’ll have to get behind there.

To do a really, really thorough clean, you first make sure you have the replacement welch plug in hand before you remove that too!

[RED-Z06 2,221]

The pilot jet is probably clogged, its very small and even the tiniest debris will clog it.  Remove the 4 screws on top of the carb, the top section will lift off, remove the choke cable from the little lever.  Lift the float out by the hinge pin, the needle is under it..dont lose it.  Use a rag to soak up the fuel in the base/bowl.  Blow the bowl out with air to dislodge any scale or trash.  Now look at the holes in the top of the bowl/base..one will have a little brass screw in it, flathead.  Turn it out...now find a tiny bit of stiff wire...ill pull a wire out my bench grinder wheel for this, and work the wire into rhe hole in the bottom of the jet to clear it.  Once clear, hit the hole with some cleaner...but not high pressure air.  Blow the jet out..put it back in.  You can put some fresh clean gas in the bowl if you like..put the needle, float back in, put the choke on, put the gasket (these usually are fine to reuse) and put the cap on.

 

This fixes it nearly every time for me...if it doesn't, you may have a splitting intake manifold.

 

[pgiro 36]

On 6/10/2023 at 1:40 AM, RED-Z06 said:

The pilot jet is probably clogged, its very small and even the tiniest debris will clog it.  Remove the 4 screws on top of the carb, the top section will lift off, remove the choke cable from the little lever.  Lift the float out by the hinge pin, the needle is under it..dont lose it.  Use a rag to soak up the fuel in the base/bowl.  Blow the bowl out with air to dislodge any scale or trash.  Now look at the holes in the top of the bowl/base..one will have a little brass screw in it, flathead.  Turn it out...now find a tiny bit of stiff wire...ill pull a wire out my bench grinder wheel for this, and work the wire into rhe hole in the bottom of the jet to clear it.  Once clear, hit the hole with some cleaner...but not high pressure air.  Blow the jet out..put it back in.  You can put some fresh clean gas in the bowl if you like..put the needle, float back in, put the choke on, put the gasket (these usually are fine to reuse) and put the cap on.

 

This fixes it nearly every time for me...if it doesn't, you may have a splitting intake manifold.

 

Thanks for the detailed instructions. I will try this as soon as I get some time and will let you know if it resolves the problem.

[cleat 6,098]

Yes, I find cleaning this cleared up the surge nearly all the time.

 

I run a bit of seafoam occasionally in the tractor if it is showing a sign that it might start to act up and usually it clears up after one grass cutting.

 

However, if there is a totally plugged hole then the carb needs to come apart for cleaning as the seafoam can't clean what it can't get through.

 

On another note, all of my 520's seem to have slightly different governor sensitivity. They all work properly but each has it's own personality.

 

Edited June 12, 2023 by cleat

[pgiro 36]

Thanks for the very good advise on taking of the top of the carb and cleaning the ports.

That seems to have fixed the problem. It's running normal now.

One last question.. there is a thin vacuum type line that is connected to the air cleaner and I can't figure out where it was connected, if someone could tell me where it is supposed to be connected..

 

[RED-Z06 2,221]

2 hours ago, pgiro said:

Thanks for the very good advise on taking of the top of the carb and cleaning the ports.

That seems to have fixed the problem. It's running normal now.

One last question.. there is a thin vacuum type line that is connected to the air cleaner and I can't figure out where it was connected, if someone could tell me where it is supposed to be connected..

 

Im not explicitly familiar but usually any vacuum fitting inside a filter housing is for a filter minder, as a filter gets dirty it pulls a slight vacuum.

[lynnmor 7,321]

A small vacuum hose connects to the fitting on the bottom of the air filter base.  The other end connects to a vacuum switch to turn on the air filter lamp when the vacuum gets too high.  That vacuum switch is located on the left side of the dash, if the hose is not connected it needs to be plugged to keep dirt out. .  It is common for those to have a broken nipple and the wiring connections fail as well.  It will look something like this:

[MAD MARK 74]

Where is the best place to pick up a new welch plug? 

 

Mine is missing. Is it just a common 3/8" concave?

[Handy Don 12,281]

2 hours ago, MAD MARK said:

Where is the best place to pick up a new welch plug? 

 

Mine is missing. Is it just a common 3/8" concave?

Mine was a standard ⅜ concave aluminum. It came in the rebuild kit. Almost lost the little bitty thing opening the package!