520H Carburetor Cleaning

[Krice58 90]

A little nervous to dive into this carburetor as it seems much more complicated that the Carters and Walbros I’ve taken off of the K series and Magnum. I’ve seen conflicting posts saying that it’s not necessary to remove the carb entirely and others encouraging the removal of the entire unit with the intake. Surging is my main issue, so it’s possible it could be intake related. Would it be better to just be proactive and take out everything? Clean the carb and reseal the intake? Or just clean what I can with the carb still mounted? 

[ri702bill 10,035]

You can tell if the intake is the issue by spraying a low-volatile hydrocarbon spray like WD40 at the various intake sealing surfaces while it is running. Spray & the surge goes away, there's your vacuum leak. No change if the intake is OK.

Do not use spray ether starting fluid... 

Edited October 6 by ri702bill

[Ed Kennell 43,094]

My 520 surged badly when I got it.   I actually cleaned the carb 3 times before I got that spec of dirt that was causing the surging.

[Krice58 90]

21 minutes ago, Ed Kennell said:

My 520 surged badly when I got it.   I actually cleaned the carb 3 times before I got that spec of dirt that was causing the surging.

How would you recommend cleaning it? Just the top portion you can reach from under the filter housing or as an assembly with the intake?

[Handy Don 14,637]

43 minutes ago, Krice58 said:

How would you recommend cleaning it? Just the top portion you can reach from under the filter housing or as an assembly with the intake?

4 hours ago, Krice58 said:

A little nervous to dive into this carburetor as it seems much more complicated that the Carters and Walbros I’ve taken off of the K series and Magnum. I’ve seen conflicting posts saying that it’s not necessary to remove the carb entirely and others encouraging the removal of the entire unit with the intake. Surging is my main issue, so it’s possible it could be intake related. Would it be better to just be proactive and take out everything? Clean the carb and reseal the intake? Or just clean what I can with the carb still mounted? 

 

The Onan carbs are not especially complicated, though they can seem so on initial viewing since the air cleaner housing is so large and the linkages for throttle & choke are a bit more involved. 

 

A LOT depends on the year of your engine, but for all of them (IMHO) the best way to thoroughly clean is via immersion in an ultrasonic bath and, as @Ed Kennell noted, it can take multiple trips--mine needed three, as well, having come to me pretty neglected.

When, or if, I have to do mine again, and given the time and parts being invested, I would just take it out once, clean the dickens out of it, and put it back with fingers crossed. Three removals and installs was just too tedious for me. You’ll need the carb kit, two intake gaskets, and the carb-to-manifold gasket.

 

The image below is my “spare” from one of the later engines which has no after-factory adjustments due to rules introduced to limit small engine emissions in the ‘90s! 

The main jet is fixed. The “idle mixture” adjustment is sealed at the factory behind a plug. The Welsh Plug covers several internal channels. The accelerator pump adjustment is also factory sealed. The pump and the choke balance spring (also preset) help prevent the deliberately lean running engine from stumbling from fuel starvation when the throttle is opened. 

 

To clean this one, I’ll remove the main jet and the Welsh plug as well as opening the upper body. Welsh plugs cannot be reused so you’ll need a new one--0.375”. I will NOT mess with the accelerator pump or the choke spring, temping as that might be. I’ll repeatedly force carb cleaner under pressure through every orifice and channel I can find between tank sessions. 

  

This shows the full intake manifold and one of the bolts holding the carb to the manifold from below. The manifolds are in two parts, factory sealed together. Some have the sealant age and deteriorate allowing air to leak in, this is what @ri702bill’s suggestion above aims to detect. We can explain remedies if you find you need them!

 

Edited October 6 by Handy Don

[Krice58 90]

1 hour ago, Handy Don said:

 

The Onan carbs are not especially complicated, though they can seem so on initial viewing since the air cleaner housing is so large and the linkages for throttle & choke are a bit more involved. 

 

A LOT depends on the year of your engine, but for all of them (IMHO) the best way to thoroughly clean is via immersion in an ultrasonic bath and, as @Ed Kennell noted, it can take multiple trips--mine needed three, as well, having come to me pretty neglected.

When, or if, I have to do mine again, and given the time and parts being invested, I would just take it out once, clean the dickens out of it, and put it back with fingers crossed. Three removals and installs was just too tedious for me. You’ll need the carb kit, two intake gaskets, and the carb-to-manifold gasket.

 

The image below is my “spare” from one of the later engines which has no after-factory adjustments due to rules introduced to limit small engine emissions in the ‘90s! 

The main jet is fixed. The “idle mixture” adjustment is sealed at the factory behind a plug. The Welsh Plug covers several internal channels. The accelerator pump adjustment is also factory sealed. The pump and the choke balance spring (also preset) help prevent the deliberately lean running engine from stumbling from fuel starvation when the throttle is opened. 

 

To clean this one, I’ll remove the main jet and the Welsh plug as well as opening the upper body. Welsh plugs cannot be reused so you’ll need a new one--0.375”. I will NOT mess with the accelerator pump or the choke spring, temping as that might be. I’ll repeatedly force carb cleaner under pressure through every orifice and channel I can find between tank sessions. 

  

This shows the full intake manifold and one of the bolts holding the carb to the manifold from below. The manifolds are in two parts, factory sealed together. Some have the sealant age and deteriorate allowing air to leak in, this is what @ri702bill’s suggestion above aims to detect. We can explain remedies if you find you need them!

 

Great info and pictures, thanks! I’ll start looking into it today. 

[Handy Don 14,637]

6 minutes ago, Krice58 said:

Great info and pictures, thanks! I’ll start looking into it today. 

You're welcome. If you haven’t worked on aluminum blocks before, a torque wrench is your friend to prevent stripping and breaking stuff. I always chase the threaded holes with a tap to clean them and spread a tiny dab of lithium grease on the bolt before inserting it. 

[Chaz54 85]

5 hours ago, Krice58 said:

A little nervous to dive into this carburetor as it seems much more complicated that the Carters and Walbros I’ve taken off of the K series and Magnum. I’ve seen conflicting posts saying that it’s not necessary to remove the carb entirely and others encouraging the removal of the entire unit with the intake. Surging is my main issue, so it’s possible it could be intake related. Would it be better to just be proactive and take out everything? Clean the carb and reseal the intake? Or just clean what I can with the carb still mounted? 

For my own info, what year 520 do you have?

[Krice58 90]

2 minutes ago, Chaz54 said:

For my own info, what year 520 do you have?

‘88. First year. 

[Ed Kennell 43,094]

2 hours ago, Krice58 said:

How would you recommend cleaning it? Just the top portion you can reach from under the filter housing or as an assembly with the intake?

Don has offered some great advice.   My '88 520 with 560 hours had been sitting for several years.  I removed all the tins, intake, and heads to do complete top end cleaning.   After reassembly, it ran better, but still surged.  Two more carb cleanings and the surging stopped.   I could see improvement every time I cleaned the carb.  

[Handy Don 14,637]

On 10/6/2025 at 2:48 PM, Chaz54 said:

For my own info, what year 520 do you have?

We’re running a hybrid. The chassis, steering, transmission, and hydraulics are a ’95 520-H. The topsides, including an 18 hp Onan. are from an ’88 518-H.

The P218 and P220 differ only in the size of the main carb jet and their camshafts. 

Edited October 13 by Handy Don

[lynnmor 8,076]

3 hours ago, Krice58 said:

‘88. First year. 

You don’t have a plug over the idle mixture screw, or an accelerator pump.  Download the Onan Service Manual and you will find the information you want.  Very easy carb and often no parts are needed.  Use only genuine Onan gaskets.

[bo dawg 536]

So where can you get the welsh plug and the idle mixture cover? Thats the only thing I didnt do cleaning my carb.

[lynnmor 8,076]

1 hour ago, bo dawg said:

So where can you get the welsh plug and the idle mixture cover? Thats the only thing I didnt do cleaning my carb.

Just Google 3/8 aluminum Welch plug and choose a vendor.  There is no need to replace the plug over the idle mixture screw.  Be very careful drilling the plugs, a little too deep and damage will occur.