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Kubota Z421 - Hard to get blades spinning.

Oct 6, 2025#1

B

BIGBREW

Bought a used Kubota Z421 60" with 207 hours on it. Great mower, super comfortable. My only complaint is that I have to be 1/2 throttle and up to get the blades to start spinning. Friend of mine has a lawn care business, he came and took a look at it and said that he could not find anything wrong with it. All the spindles are good. It is the smaller motor though, maybe 23.5hp. Anyone else have this issue with their Kubota?

Oct 6, 2025#2

A

Auto Doc's

Hello B,

When you post be sure to add the Product label information along with engine product label. I for one hate to take wild guesses with minimal information.

There were many different Z421 mowers produced, and they are not all the same.

A friend owning a lawncare business is a far cry from knowing how to diagnose if there is a problem or not.

The horsepower rating sounds adequate for the deck size.

In order to get proper blade speed for the deck, the engine has to be run at high RPM. You are not saving any fuel by running at low RPM, it just overloads everything, especially in heavy grass.

WAG: Deck belt is slipping when under load due to age and wear.

Oct 6, 2025#3

StarTech

Checking the engine charging voltage also check clutch air gaps.

3. With feeler gauge check gap at all three
locations (minimum of two).
4. Factory air gap setting is .005” - .023”.
5. If gap doesn’t fall between .005” - .023” then
reset using a .012” feeler gauge.
Changing the air gap is achieved by tightening and/
or loosening the three nuts.

Tuesday at 8:45 AM#4

B

BIGBREW

Auto Doc's said:
Hello B,

When you post be sure to add the Product label information along with engine product label. I for one hate to take wild guesses with minimal information.

There were many different Z421 mowers produced, and they are not all the same.

A friend owning a lawncare business is a far cry from knowing how to diagnose if there is a problem or not.

The horsepower rating sounds adequate for the deck size.

In order to get proper blade speed for the deck, the engine has to be run at high RPM. You are not saving any fuel by running at low RPM, it just overloads everything, especially in heavy grass.

WAG: Deck belt is slipping when under load due to age and wear.

You are correct, I should have put that information in. The motor is the Kawasaki FS730V. If I try and pull the PTO at 1/4 throttle, the motor will stall. I have to put it up to 1/2-2/3 then pull the PTO, they still start a bit rough but once they are spinning it is fine. I am on my third set of blades, I received two sets with the mower, so I bought a brand-new set to take an unbalanced blade out of the equation. When I am pulling the PTO at 1/2-2/3 I get a little belt squeak so that would make me think I am turning the motor too hard at that point.

Tuesday at 8:46 AM#5

B

BIGBREW

StarTech said:
Checking the engine charging voltage also check clutch air gaps.

3. With feeler gauge check gap at all three
locations (minimum of two).
4. Factory air gap setting is .005” - .023”.
5. If gap doesn’t fall between .005” - .023” then
reset using a .012” feeler gauge.
Changing the air gap is achieved by tightening and/
or loosening the three nuts.

Is there some guidance on checking the clutch air gaps?

Thank God for YouTube. I understand what you are saying now. I will have to check this out. Can it be done while the clutch is on the mower, it is obviously probably much easier with it off. I will go out to the garage when I have some free time and take a look at this.

Tuesday at 2:31 PM#6

A

Auto Doc's

Hello B,

First of all, don't engage the PTO at 1/4 throttle. That will cause a "load stall" on many different riders.

The engine requires higher RPM to overcome the deck rotational starting load to get everything moving. Also, the higher engine RPM provides more charging system voltage and amperage to engage the electric PTO clutch quicker.

Of course, the PTO clutch air gap is important to prevent excessive clutch slippage and overheating. So, make sure it is within specs.

Maximum rated horsepower is determined at 3450-3600 RPM, not at idle.

Idle charging voltage on a machine like this is typically at 13.0V and minimal amperage to maintain only the battery.

When RPM is raised, the Voltage and Amperage climb up to be able to handle the additional electrical load of the PTO clutch and any lights that may be required for late evening or night mowing.

I don't have an exact specification chart for your machine, but a few simple tests with a multimeter and Amp clamp will confirm what I'm saying.

Tuesday at 3:14 PM#7

S

slomo

BIGBREW said:
Friend of mine has a lawn care business, he came and took a look at it and said that he could not find anything wrong with it

No surprise.

BIGBREW said:
All the spindles are good

How about the spindle BEARINGS which the lawn crew owner, could not see, unless he gutted the spindles from the deck CSI style. With the belt off, do all the spindles rotate smoothly and silently?

BIGBREW said:
It is the smaller motor though, maybe 23.5hp

Should be more than enough if Kubota made it that way.

Probably running on one dead cylinder.

Wednesday at 8:11 AM#8

B

BIGBREW

Auto Doc's said:
Hello B,

First of all, don't engage the PTO at 1/4 throttle. That will cause a "load stall" on many different riders.

The engine requires higher RPM to overcome the deck rotational starting load to get everything moving. Also, the higher engine RPM provides more charging system voltage and amperage to engage the electric PTO clutch quicker.

Of course, the PTO clutch air gap is important to prevent excessive clutch slippage and overheating. So, make sure it is within specs.

Maximum rated horsepower is determined at 3450-3600 RPM, not at idle.

Idle charging voltage on a machine like this is typically at 13.0V and minimal amperage to maintain only the battery.

When RPM is raised, the Voltage and Amperage climb up to be able to handle the additional electrical load of the PTO clutch and any lights that may be required for late evening or night mowing.

I don't have an exact specification chart for your machine, but a few simple tests with a multimeter and Amp clamp will confirm what I'm saying.

You know, never thought of that. Makes a lot of sense.

Wednesday at 8:35 AM#9

B

BIGBREW

slomo said:
No surprise.

Ya know, just because we are on here looking for information does not mean that we are stupid.

slomo said:
How about the spindle BEARINGS which the lawn crew owner, could not see, unless he gutted the spindles from the deck CSI style. With the belt off, do all the spindles rotate smoothly and silently?

We checked the play in each spindle, they only have 207 hours on them, unlikely that one of them went bad, but we checked them anyway. You don't need to be a detective to understand when a bearing is going bad.

slomo said:
Should be more than enough if Kubota made it that way.

Maybe it is the electric clutch and voltage, something I never thought of.

slomo said:
Probably running on one dead cylinder.

Does not have one dead cylinder.

Wednesday at 10:27 AM#10

StarTech

It should be either a Z421-KWT-3 OR Z421-KWTNC with a Kawasaki FS730V-AS17 engine 24hp.
The deck would be a RCK60P-400Z.

BIGBREW said:
Ya know, just because we are on here looking for information does not mean that we are stupid.

We checked the play in each spindle, they only have 207 hours on them, unlikely that one of them went bad, but we checked them anyway. You don't need to be a detective to understand when a bearing is going bad.

Maybe it is the electric clutch and voltage, something I never thought of.

Does not have one dead cylinder.

Just because there is only a few hours on a bearing doesn't mean they can't have problems. I one mower here that was constantly snapping belt. I had the customer pay a close attention as when it was happening. With that info I finally found one bearing that had what appeared to be minor catch during testing. It was enough to snap the belts on start up it happen to have been resting in that spot. Can even be rust in the bearings.

And some bearings have poor lube installed in them from the bearing factory which causes early failures. This deck uses 6205-RST bearings [6205-ZZ]. There should be grease zerks on the spindles but I see none in the images of the spindle housings so it is highly you ever greased the bearings.

From your further descriptions of the problem it is leaning to a poor clutch engagement problem and might be that the FT-LB torque of the clutch is too low for the deck load. Can even be that the grease in the bearings is hardening.

Or you can have grass and other crap like baling twine or plastic bags wedged between the blades and the spindle housings or between the spindle shafts and the bearings in the lower spindle housings.

So it can be a multitude of problems combined.

Wednesday at 10:53 AM#11

S

slomo

BIGBREW said:
Ya know, just because we are on here looking for information does not mean that we are stupid.

It was no surprise the FRIEND couldn't find a problem. Nothing about anyone being silly.

BIGBREW said:
We checked the play in each spindle, they only have 207 hours on them, unlikely that one of them went bad, but we checked them anyway. You don't need to be a detective to understand when a bearing is going bad.

Hours mean zero in mowers. People can tear up an anvil in record time. Play is only one factor in bearings being good. See that CSI reference stuck. Good for ya'.

BIGBREW said:
My only complaint is that I have to be 1/2 throttle and up to get the blades to start spinning

Totally normal. Three blades and bearings at a dead stop takes some Gundy to get going.

You know you are supposed to be mowing, running, idling at MAX revs all the time right? As in 3600rpm soon as you let go of the key?

Is this an air cooled engine? If so better keep the revs up.

Wednesday at 2:31 PM#12

B

BIGBREW

slomo said:
No surprise.

How about the spindle BEARINGS which the lawn crew owner, could not see, unless he gutted the spindles from the deck CSI style. With the belt off, do all the spindles rotate smoothly and silently?

Should be more than enough if Kubota made it that way.

Probably running on one dead cylinder.

No, but when I pull it out to mow this week, I will definitely check that. Thank you.

Wednesday at 7:27 PM#13

A

Auto Doc's

Seems that some folks went off on a tangent about spindles but missed the fact that you are trying to engage the deck at idle or near idle RPM.

Don't get me wrong, checking spindles is very important and they do have issues, but noise would be obvious when the deck was run for any length of time.

Wednesday at 7:58 PM#14

StarTech

Not all deck spindle problems make noise. Ever had plastic baler twine jam in the spindles?

What was mention was there were a startup stalling sound which can be either the PTO clutch slipping under the initial start up load or the belt slipping under the start up load.

Thursday at 8:20 AM#15

B

BIGBREW

Auto Doc's said:
Seems that some folks went off on a tangent about spindles but missed the fact that you are trying to engage the deck at idle or near idle RPM.

Don't get me wrong, checking spindles is very important and they do have issues, but noise would be obvious when the deck was run for any length of time.

I ran on old Simplicity 61-inch zero turn before this one. That one you could pull the pto at a pretty low rpm. Guess I am just getting used to the new one yet. I went from hay wagon to suspension seat, so this is quite an upgrade for me. Just want to make sure I am running the new mower properly. Thank you for the input.

Thursday at 8:45 AM#16

A

Auto Doc's

BIGBREW said:
I ran on old Simplicity 61-inch zero turn before this one. That one you could pull the pto at a pretty low rpm. Guess I am just getting used to the new one yet. I went from hay wagon to suspension seat, so this is quite an upgrade for me. Just want to make sure I am running the new mower properly. Thank you for the input.

Was that Simplicity a hand operated lever PTO?

Thursday at 3:26 PM#17

B

BIGBREW

Auto Doc's said:
Was that Simplicity a hand operated lever PTO?

No, it was a pull switch. They would just engage pretty easy is all. Guess I am just not used to my new one.

Friday at 4:43 AM#18

Tiger Small Engine

BIGBREW said:
I ran on old Simplicity 61-inch zero turn before this one. That one you could pull the pto at a pretty low rpm. Guess I am just getting used to the new one yet. I went from hay wagon to suspension seat, so this is quite an upgrade for me. Just want to make sure I am running the new mower properly. Thank you for the input.

How fast the engine is running to engage the PTO is another topic that many people disagree with. I have been hitting the PTO on my Scag at wide open throttle now for almost 2000 hours (original PTO clutch). Your mileage may vary.

Friday at 12:50 PM#19

A

Auto Doc's

Hi Tiger SE,

I have likely heard most of the traditional debates. (LOL)

The ones who disagree have likely never measured the charging system output to compare idle versus full throttle. You have an electric PTO.

How quickly and firmly an electric PTO engages depends heavily on having high charging system output, the highest system output is at full throttle.

Some people like to claim high RPM creates excess "shock load" on the deck belt and spindles but I just cannot find any proof to support that (armchair) opinion.

Being a 40+ year (certified) technician has taught me that it is better to put the (imagined) shock load on a cheaper to replace deck belt than the expensive engine.

Friday at 1:06 PM#20

S

slomo

As Auto Doc was hitting at, at low revs, could be insufficient voltage to fully engage the PTO.

Makes total sense.

Saturday at 8:36 AM#21

O

oldntired55

StarTech said:
It should be either a Z421-KWT-3 OR Z421-KWTNC with a Kawasaki FS730V-AS17 engine 24hp.
The deck would be a RCK60P-400Z.

Just because there is only a few hours on a bearing doesn't mean they can't have problems. I one mower here that was constantly snapping belt. I had the customer pay a close attention as when it was happening. With that info I finally found one bearing that had what appeared to be minor catch during testing. It was enough to snap the belts on start up it happen to have been resting in that spot. Can even be rust in the bearings.

And some bearings have poor lube installed in them from the bearing factory which causes early failures. This deck uses 6205-RST bearings [6205-ZZ]. There should be grease zerks on the spindles but I see none in the images of the spindle housings so it is highly you ever greased the bearings.

From your further descriptions of the problem it is leaning to a poor clutch engagement problem and might be that the FT-LB torque of the clutch is too low for the deck load. Can even be that the grease in the bearings is hardening.

Or you can have grass and other crap like baling twine or plastic bags wedged between the blades and the spindle housings or between the spindle shafts and the bearings in the lower spindle housings.

So it can be a multitude of problems combined.

just a quick note here, i have seen several craftsman and jd mower decks fitted with grease fittings at top of spindles. yet on disassembly, found sealed spindle bearings, both sides not 1. my thought always was... how can the grease get into the bearings?? i dont think it was intended to force its way past the innermost seals///??// i had an old craftsman 42 inch , i put new bearings in and popped out the inside seals... i ran that deck for 12 yrs, on the same set of bearings... just sayin

Saturday at 8:47 AM#22

StarTech

oldntired55 said:
just a quick note here, i have seen several craftsman and jd mower decks fitted with grease fittings at top of spindles. yet on disassembly, found sealed spindle bearings, both sides not 1. my thought always was... how can the grease get into the bearings?? i dont think it was intended to force its way past the innermost seals///??// i had an old craftsman 42 inch , i put new bearings in and popped out the inside seals... i ran that deck for 12 yrs, on the same set of bearings... just sayin

Yes the OEMs are putting many cases -2RS sealed bearing which will not accept grease via the zerks. Now the -ZZ and RST can have grease force into them as they have trash seals which are not fully sealed. Even those need the inner seal removed.If you try forcing grease into the -2RS the inner seals tends to collapse into the bearing race jamming the bearing assembly.

Saturday at 9:38 AM#23

P

pwardxe126

BIGBREW said:
Bought a used Kubota Z421 60" with 207 hours on it. Great mower, super comfortable. My only complaint is that I have to be 1/2 throttle and up to get the blades to start spinning. Friend of mine has a lawn care business, he came and took a look at it and said that he could not find anything wrong with it. All the spindles are good. It is the smaller motor though, maybe 23.5hp. Anyone else have this issue with their Kubota?

I've had the same problem before. It was the PTO clutch. When engaged, it is supposed to slip momentarily. If it doesn't, it's such a hard jolt engaging all at once and stalls the engine.

Saturday at 10:30 AM#24

A

Auto Doc's

Deck spindles come with lifetime grease already packed in the bearings... Don't they?

Most that I open up only have a minimal single shot of grease that resembles thinned Vaseline.

Grease fittings on most modern deck spindles that I see are just an illusion that you are buying a serviceable part. I open them up and find that most bearings have plastic inner seals.

For ones without grease fittings, I drill and tap them for a fitting that I can position for ease of access in the side near the base.

I disassemble new replacement spindles and remove the inner most seals, then pack them by well hand before reassembly. After that, they only need a couple of shots of grease once a season to maintain them and improve their life.

Saturday at 10:49 AM#25

R

RevB

Full throttle......

They do make electronic soft start units that will ramp up the engagement voltage over a second or two. Also senses engine RPM to adjust the ramp rate. Ogura makes them and there's a company in NY state that makes them as well.

Why Does Your Zero-Turn Need a SoftStart Clutch Controller? | GDI

Having smooth clutch engagement every time you turn on your PTO is nice.Less of a teeth-rattling jolt when you engage your blades would certainly make any operator happy, but that isn’t why your commercial mower needs a SoftStart controller. Learn

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