C120 Bogs Down Under Load When Hot

[Eli 8]

Hello!

I have a mysterious issue with my 1977 C120 Auto. K301 engine. I have searched around but haven't found another post with the exact behavior.

It will run like a champ mowing for about 10-15 minutes.

But once it gets hot the motor will start bogging down at full throttle and under load. It sounds like it's fighting itself trying to run and I hear a loud clack every now and then before I pull the throttle back.

Load can be either the deck or just the hydro. Smoke is blown out of the breather vent and it doesn't recover when I remove the loads. It only recovers when I throttle down all the way, let it idle for a minute, then I can slowly ease it back up to full throttle and it's fine until I apply a load. If I let it cool down fully then it's fine again until it gets hot.

 

I thought maybe the exhaust was clogged so I checked that and it's fine. Other things I have tried:

-Carb adjustment and cleaning, new gasket

-Head Gasket and bolt torque

-Air Filter

-Fuel Filter

-New Spark plug

-New Condenser

-New contact points

 

I'm stumped and hoping a wheel horse expert can point me in the right direction.

 

Thank you!

Edited April 17, 2023 by Eli

[lynnmor 7,307]

My first thought is that the ignition system is breaking down when hot.  Try closing the spark plug gap by half the specification and see what happens.

[Bill D 1,922]

New points, condenser and spark plug.  Set timing with a timing light. Clean up the grounds and install an extra ground strap if needed.   

[oldlineman 1,481]

Sounds to me like running lean maybe clean the carb again.

[Achto 27,594]

There is a chance that the internal resister has failed in your ignition coil. When the charging system gets up to 13.5+vdc it would make the coil collapse. To diagnose this -Try unplugging your voltage regulator and running the tractor. If the issue goes away then your ignition coil is bad.

Edited April 17, 2023 by Achto

[953 nut 55,280]

11 hours ago, oldlineman said:

Sounds to me like running lean

           Running hot and needing to recover proper operation by going back to low idle and occasional application of the choke are all classic symptoms of operating too lean. Cleaning the carburetor would be a good start.

Your carburetor has two adjustment screws that should be reset, for a K-301 the Main fuel needle should be gently screwed in all the way and then backed out 2 1/2 full turns. The Idle fuel needle should be bottomed and then backed out 2 full turns. 

[alvinabish 3]

You can try this too:

1. Check the fuel line and fuel tank for any obstructions or debris that could be restricting fuel flow. Make sure the fuel cap is venting properly.
2. Check the ignition timing. If it is off, it could cause the engine to run poorly when hot.
3. Check the governor and governor springs. If the governor is not functioning properly, it could cause the engine to bog down under load.
4. Check the cooling system. Make sure the radiator and cooling fins are clean and free of debris. Make sure the fan is working properly and the thermostat is not stuck closed.

[953 nut 55,280]

11 hours ago, alvinabish said:

You can try this too:

1. Check the fuel line and fuel tank for any obstructions or debris that could be restricting fuel flow. Make sure the fuel cap is venting properly.
2. Check the ignition timing. If it is off, it could cause the engine to run poorly when hot.
3. Check the governor and governor springs. If the governor is not functioning properly, it could cause the engine to bog down under load.
4. Check the cooling system. Make sure the radiator and cooling fins are clean and free of debris. Make sure the fan is working properly and the thermostat is not stuck closed.

Good things to check but Kohler engines are air cooled. The cooling fins around the cylinder could become plugged so well worth taking a look at.

[Eli 8]

All good stuff that I will check on and try out and report back what I find! I greatly appreciate all of the replies!

[Eli 8]

So in the process of cleaning the carb and checking the fuel system I found two things:

1. I have an aftermarket Walbro WHG carb I believe and it is adjustable. Plastic float. Are these good carbs that are worth maintaining or should I try to source an OEM carb? Gonna finish cleaning it anyways.
2. I have plastic fuel pump. Can't find a brand name but I think I see "Made in PRK". Do these cheap aftermarkets typically have issues?

Thanks!

Edited April 24, 2023 by Eli

[tunahead72 2,423]

22 hours ago, Eli said:

... I have an aftermarket Walbro WHG carb I believe and it is adjustable. Plastic float. Are these good carbs that are worth maintaining or should I try to source an OEM carb? Gonna finish cleaning it anyways...

 

Just my own opinion...  I have a Walbro fully adjustable carb on my '86 310-8 (not original, it was installed as part of an engine rebuild back in 2009).  It works well, fully rebuildable, kits are available, you can even fix a loose throttle shaft if yours has that issue.  I would keep it if you can.

 

They do adjust a little differently than the Kohler/Carter models, make sure you're looking at the correct set of instructions in the owner's manual.  There's also a comprehensive carburetor reference manual here somewhere if you need to dig into it more fully.

 

[peter lena 8,632]

@Eli  how old is your battery ? recently had a engine response , glitch , after many  look arounds , it was the battery , almost 7 years old !  be heading over to wally world , for another 300 cca  U1 series , have battery tenders  on all my stuff .  pete 

[Eli 8]

Just now, peter lena said:

@Eli  how old is your battery ? recently had a engine response , glitch , after many  look arounds , it was the battery , almost 7 years old !  be heading over to wally world , for another 300 cca  U1 series , have battery tenders  on all my stuff .  pete 

 

Hi Pete! Its brand new, I put it in right after I purchased the tractor as the old one wouldn't hold a charge.

[Eli 8]

15 hours ago, tunahead72 said:

 

Just my own opinion...  I have a Walbro fully adjustable carb on my '86 310-8 (not original, it was installed as part of an engine rebuild back in 2009).  It works well, fully rebuildable, kits are available, you can even fix a loose throttle shaft if yours has that issue.  I would keep it if you can.

 

They do adjust a little differently than the Kohler/Carter models, make sure you're looking at the correct set of instructions in the owner's manual.  There's also a comprehensive carburetor reference manual here somewhere if you need to dig into it more fully.

 

 

Good to know. Thank you!

[David Blanchard 9]

Shot in the dark but is there alot of carbon in the motor? Valve guides in good condition and valve springs have not been overheated & lost tension? Could be valves not seating properly when hot?

 

[Eli 8]

Hey everyone! It's been a while but I thought I would write back and let you all know that the tractor is up and running like champ!

I did dig into the carburetor again and cleaned everything I possibly could. Then spent a good amount of time tuning it.

I also did the timing (before I further tuned the carb) and also found that the fuel pump had not been reassembled properly at some point. One of the inner seals had been put back in the wrong place.

 

Once I did all those, it fired right up, and worked hard, flawlessly. And wow can it throw some snow too!

 

Thanks again to everyone! Happy to be a horse owner!