Kubota Z421 - Hard to get blades spinning.

[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?

 

[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.

 

[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.

 

[BIGBREW]

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.

 

[BIGBREW]

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.

 

[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.

 

[slomo]

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.

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?

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.

 

[BIGBREW]

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.

 

[BIGBREW]

No surprise.

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

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.

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

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

Probably running on one dead cylinder.

Does not have one dead cylinder.

 

[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.

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.