Engine Running Lean

[1987_WHEEHORSE312 78]

So my Wheelhorse 312 with the 12 HP Kohler Magnum Seems to be running lean. I just cleaned the carb and it idles fine, but after 45 min or so at full throttle it starts running horrible and needs the choke cracked open to run smoothly again, but it idles fine with the choke off. any adjustment to the carb doesn't improve how it runs. Gas cap has been replaced, plugs and air filter are new. Starting to think it is the pump. any other ideas? I don't want to keep throwing money at it.

[pfrederi 19,276]

Many Magnums...but not all came with a Walbro fixed main jet car.  (no high speed adjustment)  and a bit of dirt tin that fixed jet will cause issues...What kind of carb do you have/

[953 nut 63,047]

19 minutes ago, 1987_WHEEHORSE312 said:

full throttle it starts running horrible and needs the choke cracked open to run smoothly again, but it idles fine with the choke off

You may want to remove the main needle and reclean it. There are very small passages that allow the correct amount of fuel to enter the venturi, if they are partly plugged that would be the cause.

[1987_WHEEHORSE312 78]

11 minutes ago, 953 nut said:

You may want to remove the main needle and reclean it. There are very small passages that allow the correct amount of fuel to enter the venturi, if they are partly plugged that would be the cause.

Ill try that again, should be fine to do on the tractor without removing the whole carb again? 

[1987_WHEEHORSE312 78]

13 minutes ago, pfrederi said:

Many Magnums...but not all came with a Walbro fixed main jet car.  (no high speed adjustment)  and a bit of dirt tin that fixed jet will cause issues...What kind of carb do you have/

Not sure of the Brand of carb, but there is adjustment for the main and idle jets. The main Jet is the one that when the motor is choking out, I get no improvement when I adjust it. 

[ri702bill 10,581]

Check to see that your carb to engine gasket is good and sealing all around.

Not like this!!  On a C81 I bought - that ran!!

 

 

[1987_WHEEHORSE312 78]

Hmm ill have to reinspect that 

[1987_WHEEHORSE312 78]

On 12/15/2025 at 9:51 AM, pfrederi said:

Many Magnums...but not all came with a Walbro fixed main jet car.  (no high speed adjustment)  and a bit of dirt tin that fixed jet will cause issues...What kind of carb do you have/

Just looked at the carb last night again and noticed that the rod the throttle is on, is super super loose and on idle allows enough of a gap you can get the tip of a flatbed into it. I'm gonna do a replacement carb and see what happens. 

[953 nut 63,047]

Bronze bushings can close up the gap and save your carb. They can be found in the Hillman Cabinets, look for # 58087 at hardware or home improvement stores.

 

[1987_WHEEHORSE312 78]

1 minute ago, 953 nut said:

Bronze bushings can close up the gap and save your carb. They can be found in the Hillman Cabinets, look for # 58087 at hardware or home improvement stores.

 

How hard is it to install them? 

 

[ebinmaine 75,775]

3 minutes ago, 1987_WHEEHORSE312 said:

How hard is it to install them? 

 

 

The tricky part is CAREFULLY getting the throttle shaft screws out without damage. 

They're very prone to breakage.   

I usually grind the back of the screw down right to the shaft using a dremel.  Care is needed not to scar the shaft.  

[8ntruck 8,347]

  the screws holding the butterfly plate to the shaft are staked.  They were installed, then the threaded end were squeezed to lock them in place.  Grinding the screws loosens them.

 

When reinstalled, the screws will need loctite to prevent them from vibrating loose and getting tucked into the engine.

[ebinmaine 75,775]

9 hours ago, 1987_WHEEHORSE312 said:

Just looked at the carb last night again and noticed that the rod the throttle is on, is super super loose and on idle allows enough of a gap you can get the tip of a flatbed into it. I'm gonna do a replacement carb and see what happens. 

 

 

Watch this...

 

 

 

 

 

 

 

 

[8ntruck 8,347]

Those are socket headed screws holding that butterfly plate on.  Somebody else has had that carb apart.

 

The bottom shaft/bore looks really worn as well.  If it were mine, I'd go ahead and take the butterfly plate off  so I could get the shaft out for inspection.  It would not suprise me if the bottom end of the shaft is severely worn.  A new shaft might be needed here.

[ebinmaine 75,775]

Just now, 8ntruck said:

Those are socket headed screws holding that butterfly plate on.  Somebody else has had that carb apart.

 

The bottom shaft/bore looks really worn as well.  If it were mine, I'd go ahead and take the butterfly plate off  so I could get the shaft out for inspection.  It would not suprise me if the bottom end of the shaft is severely worn.  A new shaft might be needed here.

 

 

That one's junk. It was here several years ago for repairs. 

 

Because the bottom bore was so loose I couldn't rebuild it.  

 

 

[1987_WHEEHORSE312 78]

49 minutes ago, ebinmaine said:

 

 

Watch this...

 

 

 

 

 

 

 

 

Its pretty bad but idk if its that bad. If you are on Wheelhorse Fanatics on Facebook I made a post showing the video of how it is. My game plan right now is get a cheap carb off Amazon to throw in the tractor as it is my only way of snow removal so I need it to be ready at a moments notice. Then once it warms up and isn't 30 degrees in the shop im gonna pull the carb apart again and play around with it. 

6 minutes ago, 8ntruck said:

Those are socket headed screws holding that butterfly plate on.  Somebody else has had that carb apart.

 

The bottom shaft/bore looks really worn as well.  If it were mine, I'd go ahead and take the butterfly plate off  so I could get the shaft out for inspection.  It would not suprise me if the bottom end of the shaft is severely worn.  A new shaft might be needed here.

I assume you saw my video on Facebook or are you replying to the YouTube video? 

Edited 21 hours ago by 1987_WHEEHORSE312

[8ntruck 8,347]

5 minutes ago, 1987_WHEEHORSE312 said:

Its pretty bad but idk if its that bad. If you are on Wheelhorse Fanatics on Facebook I made a post showing the video of how it is. My game plan right now is get a cheap carb off Amazon to throw in the tractor as it is my only way of snow removal so I need it to be ready at a moments notice. Then once it warms up and isn't 30 degrees in the shop im gonna pull the carb apart again and play around with it. 

I assume you saw my video on Facebook or are you replying to the YouTube video? 

The YouTotube video that I just realized @ebinmaine posted.

[1987_WHEEHORSE312 78]

Ahh yeah!

[sqrlgtr 1,342]

26 minutes ago, 8ntruck said:

The YouTotube video that I just realized @ebinmaine posted.

showing a double feature got our moneys worth. 

[oliver2-44 11,065]

After you grind the back of the screws off work the screws back and ford as you remove them. Out a little, then back in, repeat, repeat.