Mowing Problem with 72" Finishing Mower

Fatch · May 31, 2007

I have a B7800 with a mid-mount 72" finishing mower. I believe the mower is from Kubota, but I'm nost sure as it came with the tractor when I bought it used. My problem is that it doesn't cut well, leaving streaks of grass only partially cut, especially when turning. I wonder if my problem is one of the following:

1) I removed and sharpened the blades a few weeks ago. I failed to notice the direction each blade was mounted and so, I remounted in a random way. Do the blades (three of them) need to be mounted in some particular orientation?

2) I am going too fast. I typically am in 2nd gear moving "slowly". I have tried 1st gear, running at the max of this gear.

3) I am running the tractor at too an RPM. I typically run it at 1700 RPM but I have tried 2000 RPM.

Any suggestions??

bialecki · May 31, 2007

I have a B7800 with the 60" MMM and I found the same thing. The things I did to correct this was:
Make sure all the settings were correct; on the 60" MMM there are 5 wheel adjustments, one was off. On the 72" I think there are only two mower deck adjustments. Make sure the "setup" is correct, you will need to reference a Kubota owners manual for that, or maybe someone has one can pdf it to you. I also cut mine at a PTO RPM of 2000 to 2200. Since my B7800 is a hydrostatic drive I cut at a walk pace and I am careful at the corners, I usually do a criss-cross cutting orientation to assure all matted grass is indeed cut.

JPinnell · May 31, 2007

The 72" MMM on my new 20hr old B7800 did cut less than perfect when new. I mow running at least 2000rpm in the middle range. Anything less and the deck won't "suck" enough to lift the grass. 2200rpm does a better job, but I'm still in the breakin period and shouldn't run full PTO rpm yet. I also found the factory edge on the blades to be beveled, but flat on the very edge. Sharpening them up and mowing at 2200rpm has greatly improved things. IF the grass is very thick and over 8"tall I do have to cross cut. The front R4s do seem to matt the grass on 90º turns. I do a 270º turn in a clover leaf pattern every 3rd lap or so and that seems to clean up the matted stragglers.

Also depending on how used your blades are you may need new blades or signigicant grinding to true them up. The very ends of the blade get worn and rounded off. You need a sharp blade edge and the end needs to be squared off and not rounded. If you're getting tufts in 2 stripes in the center of the mowing path on straight stretches your blade ends are too rounded and you're not gettign an even cut.

I had an exMark LazerZ previously and the Kubota deck is pretty close with good sharp blades and adequate rpm.

ronjhall · May 31, 2007

Fatch said:
I have a B7800 with a mid-mount 72" finishing mower. I believe the mower is from Kubota, but I'm nost sure as it came with the tractor when I bought it used. My problem is that it doesn't cut well, leaving streaks of grass only partially cut, especially when turning. I wonder if my problem is one of the following:

1) I removed and sharpened the blades a few weeks ago. I failed to notice the direction each blade was mounted and so, I remounted in a random way. Do the blades (three of them) need to be mounted in some particular orientation?

2) I am going too fast. I typically am in 2nd gear moving "slowly". I have tried 1st gear, running at the max of this gear.

3) I am running the tractor at too an RPM. I typically run it at 1700 RPM but I have tried 2000 RPM.

Any suggestions??

1. There is no set position for the blades. Just make sure they are installed with the lift angle up.
2. I run my B2910 (same tractor as B7800) in Mid range.
3. Cut with engine at PTO speed (2900). This will give maximum lift from the blades. You may find it a little unnerving in the beginning. Once you get used to it. It will seem wrong to run at any other speed.

twodogs · Jun 1, 2007

I have a b7800 with a 60" mmm. I cut in mid range and I set my rpm at 2500 (I think). If you look on the mower pto lever it tells you where to set the engine, & I am almost positive it is 2500 rpms. It cuts great, so long as the deck is cleaned out & the blades are set as the others have mentioned. The blades are great - rarely need sharpened due to their heavy construction. I think this mower is well built & would put it up against almost anything out there. I think it cuts very well.

Lockport · Jun 1, 2007

IMHO, I think 60" is the max width you'll get a half decent cut on a relatively flat ground and setup correctly. I think if demand called for anything larger, I would be looking at a gang momer. Northern tools used to have them for cheap. Just my $0.02!

crbr · Jun 1, 2007

Fatch said:
I have a B7800 with a mid-mount 72" finishing mower. I believe the mower is from Kubota, but I'm nost sure as it came with the tractor when I bought it used. My problem is that it doesn't cut well, leaving streaks of grass only partially cut, especially when turning. I wonder if my problem is one of the following:

1) I removed and sharpened the blades a few weeks ago. I failed to notice the direction each blade was mounted and so, I remounted in a random way. Do the blades (three of them) need to be mounted in some particular orientation?

2) I am going too fast. I typically am in 2nd gear moving "slowly". I have tried 1st gear, running at the max of this gear.

3) I am running the tractor at too an RPM. I typically run it at 1700 RPM but I have tried 2000 RPM.

Any suggestions??

A few more thoughts:
1) The height setting(s) may be off somewhere (as pointed out elsewhere here).
2) Air pressure in one of your tires is low or unequal to the others
3) A spindle shaft has been impacted out of plum / level
4) The mower may be mounted in a fashion that is not allowing the leverage to operate equally. Look at the mounting holes of the lever arms and confirm both are the same. They are just under your foot platform.

I mow with the 7800 / 60" MMM in med range, at 2200 RPM, wide open pedal. Mows like a champ.

ducky · Jun 1, 2007

Hey ronjhall, I see you have a b2910 also. How wide cut mmm do you have on that tractor and how tall grass will it cut? I have a b2910 too and was thinking of getting a mmm for it. I need to cut some tall grass down to around 6" high and need a little better finish that a bush hog gives.