Hi guys. Thank for I need your help again.
I bought a model 7268 used 2 weeks ago to mow my acre of hills. It ram fine but vibrated a lot. I replaced the blade with a new one and that may have helped a little but not much. All the vanes are still in place on the flywheel and all the magnets. I just finished replacing the crankshaft because I thought the other one was bent but that didn't eliminate the problem. The vibration is not too bad when the motor is not running hard but it surges and vibrates a lot when it is surging. Any suggestions as to what the problem could be? Preciate it.
Mark, it is a model 7268, serial 9790726. Runs strong and the deck is in near perfect shape but for some reason it vibrates a lot when the engine is running fairly fast.
on the muffler look at the tringle plate that the three bolts go through it should have about 010 gap all the way around it. lb had a gauge tool to set this but it can be done with a shim. When not even they will vibrate like a blade way out of balance
#7
FuzzyDriver
To equalize the distance between the shaft and the muffler bracket I use the sheet metal from a Coke can. Roll it into a cylinder that fits inside the muffler bracket, let it expand, and then slide it onto the crankshaft. Remove it after tightening the bolts to spec. And, ABOVE ALL, wear gloves! Those cans can really cut you!
I did the pop can between the crankshaft and the triangle plate and that did not improve it. Any other ideas?
#10
tom3
I wonder if the blade adapter might be out of whack in some way. those things aren't made real strong I think. I'd pull the spark plug, rig up a guide pointer and rotate the blade around and see that it runs the same depth up and down and the same distance (runout) from side to side.
#11
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.
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.
#12
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.
#13
FuzzyDriver
Ted, our posts crossed. Good, clear post. Just BTW, the 7268 is a F100 engine.
#14
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
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.
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.
#19
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.
#20
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.
Again guys, thanks for all the help. The blade I have does not have the two notches in it on the trailing edges like the ones I've seen for sale on Amazon etc. It does have the built in stiffener (edges of center area curved up 1/4" or so, and embossed laterals on each side of center hole, for rigidity apparently) and is like the one shown in the Lawn Boy manual on page 9-15.
I don't have a wide enought work bench to set it on so I just lay down on the drive and spin the blade (ignition switch off) and I really don't so see any difference in the blade height as it spins. It definitely does not make any half moons in the grass. The cut is perfect. The power is great, especially for such a light weight mower. It is a sweet sweet mower but just vibrates too much. Has very good power even though I have it set to "Light" on that slide thing on the carb. I am thinking that maybe I need to mess with the carburetor. Is there a chance that the surging could be related to the vibration? I mean I know it vibrates more when it surges to high rpms, but does fixing the surge every cure the vibration at both low and high rpm's? As I was laying on the concrete a minute ago I also notice that gas was leaking like a seive out of the carb. Do the sell complete carbs for this? Are they had to rebuild. Can anyone tell me the correct part number for a carb and carb kit for this 7268 with what someone said is a F100 engine? Is there a parts manual out there somewhere?
I mowed my front hill with this thing this afternoon and it looks really nice.
#22
Teds
Blade track isn't something that can be eyeballed, you have to actually measure the blade tip height on a level surface at 4 points. If it were that obvious, it would shake itself apart.
Maybe I shouldn't have put so much emphasis on checking blade track, but it is an important and part of the troubleshooting for vibration. The collar could also be bent. I would say generally that a genuine LawnBoy blade is much superior to the counterfeit in both materials and construction. Make sure you have the real McCoy. Buy a new one. Lots of crappy parts sold today. A genuine blade is more likely to be balanced, but it still needs to be checked, especially since vibration is a problem in this case.
They don't cost that much more. I also noticed, as mentioned previously, a noticeable further reduction in vibration simply by flipping the blade around. I noticed this by accident, I like to sharpen the blade a few times a season. (A dull blade tends to "shred" the grass blade tips versus a sharp, clean cut. Dull blades can invite certain turf diseases, a sharp blade is important not just for appearance, the grass "heals" the wound much faster.)
Anyhoo, I'd swap the blade back on, and once in a while noticed immediately a bit higher level of vibration. Not bad, but noticeable. 50/50 chance of putting it back on the same way at random, right? So now I have the crankshaft end marked for proper alignment, and mark the blade so it gets put on the same way end for end, too.
The next question is whether you re-clocked the blade in relation to the crankshaft, or not?
How this is accomplished depends on engine series, but how to do it is outlined in the manual. Excess vibration can rob RPM and power too. What RPM is it running at on "Normal" and "Low"? Tachs are available for about 5 bucks or so - really handy for tuning.
I took an outside caliper and measured the height of the blade at the 4 different points around the circle. All checked good. Within a sixteenth inch at most. I then took the blade off and rotated it 180 degrees. No improvement. Unless someone has another suggestion I'm going to take this to my Lawn Boy mechanic. He's now back from Florida.
I took mower to my Lawn Boy mechanic this morning and ran it for him and he said that the degree of vibration I have is the nature of the F engine vs the 1973 D engine model I used to have that had little to no vibration. I took it to another Lawn Boy dealer across town and ran it for him and he said it should not have that much vibration. He also gave me a used but straignt crankshaft blade mounting flange. I have not tried it yet but will in a bit.
#25
FuzzyDriver
If your blade is not aligned right with the crankshaft, rotating it 180 degrees will have no effect. It would still be out of alignment with the crankshaft by the exact same amount. Is this blade rotation you did based on what the manual says? The manual shows it 90 degrees to the crank on C & D engines and 0 degrees (or 180 degrees, heh heh) for F engines (in other words, aligned with the crank for our 7268's).. Anthembassman's post shows the manual page.
Fuzzy,
The blade is parallel to the piston for this f engine per the manual.
I put the "new" blade mounting flange on and again no affect.
I think I have run out of ideas.
#27
FuzzyDriver
We're saying the same thing, just with different words. When the piston is at TDC it is aligned with the crank, so it's really the crank that you are aligning to - you just can't tell where the crank is for sure except at TDC (well, I guess BDC, too...::sigh:. The pictures in the diagram are confusing. They both show the blade 90 degrees to the crank, and even keep the "BLADE 90.." verbiage. I guess they are 'before' pictures because step 5 says to mount the blade in the direction of the file mark, and the drawing shows the blade 90 degrees to the file mark for our F engines.
Fuzzy- I am saying that when the piston is TDC then my blade is parallel to the direction of movement of the piston in it's cylinder. In the other engines, when piston is at TDC then the blade at that point is perpendicular to the direction of movement of the piston in it's cylinder. Actually I have tried it both ways and did not sense that much difference anyway. There must be something internal to the engine that is out of balance slightly. Would the bottom or top crankshaft bearings be the cause? If I turn the mower on its side with one end of the blade straight up, when I grab the end of the blade and pull up I can move it very slightly, but I would think that some free play would be necessary.
. The pictures in the diagram are confusing. They both show the blade 90 degrees to the crank, and even keep the "BLADE 90.." verbiage. I guess they are 'before' pictures because step 5 says to mount the blade in the direction of the file mark, and the drawing shows the blade 90 degrees to the file mark for our F engines.