Too Much Vibration. Model 7268

[Bill52]

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.

 

[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.

 

[Bill52]

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.

 

[Bill52]

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.

 

[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.

 

[Bill52]

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.

 

[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.

 

[Bill52]

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.