Hunting Problem

[lakeradio 6]

I have made three repairs for what Onan calls "Hunting Problem"
on 2 Wheel Horse model 520H tractors.

I believe the engine is hunting due to carburetor fuel to engine is very lean.
This causes the engine to slow down and before stopping the governor
kicks in and speeds up engine. Now the engine is running faster that 
throttle setting and releases slowing engine down.
Now the engine slows down and is about to stop again and then the cycle
repeats.

By adding full choke and making fuel mixture richer the hunting problem
was reduced in my 3 cases.

The Onan manual gives adjustment fixes for hunting. I have not tried these
fixes because I was sure problem was in the carburetor.

Fix number 1:

The two 520H tractors I did this fix on both were 1996 models.
The fix is to remove the idle mixture needle valve, clean and reinstall.

On side of carburetor facing muffler there is 2 round metal disk that has
been used to cover openings on carburetor side. A large one above a smaller
one. DO NOT REMOVE UPPER LARGE ONE. 
Using a small nail or sharp object make a hole in middle of small disk and
pry disk out.

Using a small flat blade screwdriver remove idle valve, turn counter clockwise.
I used carburetor cleaner with small red tube and sprayed valve opening several
times to flush and clean opening.
Clean needle valve and reinstall in hole, screw clockwise in all the way.
Now turn valve out, counter clockwise, 2 turns. In my case 2 turns out gave
idle about 1500 rpm. No need to replace small disk cover.

This fixed hunting problem on 2 model 520H 1996 tractor.

Fix number 2:

After about 10 years and many mowing one of the 520H's the hunting
problem returned.
This time Fix number 1 did not fix problem.
The following is how I fixed problem the second time.

Leaving carburetor on intake manifold remove 4 screws on top
of carburetor and 1 screw on front of carb holding choke linkage.
Remove carb top casting with float assembly and needel valve.
On front of carb, side facing headlights, there is a large nut that covers
a carburetor jet. Remove large nut and jet. Use large flat blade screwdriver
to remove jet.

Using carburetor cleaner with spray tube spout place tube over each hole
in carb body and spray several times. Most holes you will see spray exit
somewhere else on carb. Do same for hole the jet was removed from.

Remove all dirt in float gas well.

Now using 40 psi air do the same for all holes.

Reassemble float assembly. Note the top of needle valve
fits in slot on float assembly. Replace top casting and 4 screws plus
screw for choke linkage.

Replace front jet. I used a round toothpick in jet hole to get jet started
in threaded hole. Replace larged nut covering jet hole.

When all back together hunting problem was fixed. My 520H had been over time
harder to start. After fix starting was much better.
 

[Pullstart 62,933]

  I am not familiar with 520’s or Onans for that matter, but great efforts to document the fix!

[Achto 27,635]

[lynnmor 7,318]

Dirt in a carburetor is common and ethanol corroding things can add to the problem.  I pull the carburetor to do a proper cleaning.  That other disc (welch plug) can have dirt hiding behind it, if the first run after cleaning is poor, pull that plug.

[lakeradio 6]

On 11/26/2020 at 6:51 PM, pullstart said:

  I am not familiar with 520’s or Onans for that matter, but great efforts to document the fix!

Trying to help, Thanks