Too Much Vibration. Model 7268

[Teds]

It is very important that the blade is straight, sharp, and balanced.

The first thing you want to check is the blade track itself. Small differences will affect the quality of the cut in a big way. Shims (washers) of .025" and .035" are used to prevent "half moon" swirls and scalping. Placed as required between the muffler housing and deck. This doesn't affect balance per se. But it does - the blade first needs to be tracking correctly before fine tuning the balance. So check track first.

The Toro people have kindly posted the LawnBoy maintenance manuals that show this, and other techniques. If vibration levels are high on the "D" series engines then they suggest aligning the piston to TDC and then aligning the blade 90° to this. File a line into the end of the crank to maintain this alignment. This makes a big difference. Then - further try it both ways - that is, once the blade is installed perpendicular to the piston at TDC run it up, note the vibration level, shut it down and flip the blade and try it again. For whatever reason a blade may fly or track different.

Here: https://lookup3.toro.com/ttcgateway/acrobat/manuals/lball40.pdf


Proper track and balance will make for a smooth running machine. Check RPM with a tach, it is important that blade tip speed isn't crippled. They were designed for about 3600 on "normal". If you get the track and balance right and the RPM and the rest of it setup well it will reduce fatigue quite a bit and give the high quality cut that LawnBoy was famous for.

 

[FuzzyDriver]

No, no! It HAS to be a Coke can! 8^D

You know, I just re-read your initial post and see that you say it is surging. Are you seeing the engine speed going up and down a lot when you are cutting? I'm trying to understand whether you are saying the vibration is causing surging, or surging is causing you to feel vibration. And only when at run speed, or only at run speed under load? Do you have good throttle control?

Along a different line, do you have the big blade washer the 21" calls for? I don't know if absence causes vibration, but the manual says the 21" is supposed to have one.

By coincidence, I have a messed up 7268 on my workbench right now.

 

[FuzzyDriver]

Ted, our posts crossed. Good, clear post. Just BTW, the 7268 is a F100 engine.

 

[AnthemBassMan]

Did you align the blade how Lawn Boy describes? Some mowers it makes a difference, some it doesn’t. On your F Series engine, set the piston at TDC then align the blade so it runs from carb to spark plug. Also make sure your blade adapter isn’t bent. New ones are easily found.

L8R,
Matt

 

[Bill52]

Thanks to all you guys for helping me out on this. I truly appreciate it!
To answer your questions to me: Yes the engine goes up and down when cutting. The vibration is OK when it is down mode but is excessive when it surges. The surging to high rpm causes the excessive vibration. Vibrates most when surging and not under load. From what I read on page 9-15 at the link Ted sent, this 21" does not require the large washer. I had been using one but took it off and that seemingly had no affect on the vibration. Also not using stiffener. I also aligned the blade parallel with piston TDC as per note for F engines on page. Still vibrating.
As soon as I figure out a good way to track the blades I will post that. Not sure how I can see underneath mower at the 4 different points. When I have the mower tipped up on it's side I can see that the blade tips are equidistant from the side of the deck and at the same height at one particular place on the deck but if I understand it right I need to check the blade tip at 4 points 90 degrees apart. I am thinking I could cut a Root Beer can in two with one end just long enough to touch the driveway when it is taped to the bottom side of the blade. Then if I can hear it contact the ground as I spin the blade through its full 360 degrees it should be good and need no shims. I am unclear as to where the shims go. Do they go under the 3 mounting bolts that hold the engine to the deck. Are they essentially washers?
Where are the Lawn Boy manuals posted? I looked everywhere (but there) and could not find.

 

[AnthemBassMan]

Here you go.

Lawn Boy Manuals

L8R,
Matt

 

[Bill52]

Many thanks for that manual Matt.

 

[FuzzyDriver]

Bill, does your mower have the TRI-CUT Lawn-Boy blade on it? Page 9-14 and 9-15 show that the original blade type for our 21" 7268's was flat in the center area and called for the washer and stiffener. There is a guy (not me) selling NOS TRI-CUT 20" blades on eBay with some nice pictures where you can see how they have a "built in stiffener". My 7268 does indeed have a TRI-CUT, so mine also should not have the washer and stiffener. I turned it over to look just now and the stiffener is gone, but whoever did it still has the washer, but on the wrong (nut-side) of the blade. So I just checked and all my OEM L-B blades are TRI-CUT. I never really thought about it before, I was just going from the manual.

 

[Teds]

Measuring Blade track or height at 4 locations is kind of a pain in the you know what. If you have a level workbench to place it on helps. Some variance in blade height is allowed but should be able to tell right away if it's off badly. The spec is 1/8" high or low, closer to spec or exceeding it the better.

Yes, they are essentially washers. I didn't opt to purchase the special LawnBoy branded shims, but cannibalized a washer from another piece of equipment. Have to actually remove the engine, and then the upper muffler plate, the shims go between the plate and deck. On mine the measurement was off from side to side, so I added one shim. I wanted to avoid trying to get it perfect - sort of like sawing off chair legs, it never ends well. Maybe this winter I will take a closer look at getting everything dialed in closer as it will get torn down anyway, but for now I'm happy. An out of track condition will be more noticeable at lower cutting heights.

Not saying this will fix the vibration problem but I would want to check blade track first for sure.

 

[Smithsonite]

I haven't seen this on any push mower yet, but I've seen it plenty on lawn tractors - check the engine mount bolts. I've seen some on (cheap) lawn tractors where I was amazed the engine hadn't fallen right out, lol.

Worth a check, anyway.