Is there a way to fix the problem of grass left uncut between the blades? It is particularly bad between the left and center blade. I removed the relatively new blades (installed about 2 months ago), and rotated them, L to C, C to R, R to L, and I get the same result...2 stripes of grass the entire length of my mowing, which means I have to mow over the same path a second time, but slightly moved to the side to make sure my blades cut the 2 stripes that are left between the 3 blades. The grass between the left and center blade is up to 12" wide, while the grass left between the center and right blades is about 2-4" wide. I have the ProMaster 260.
I spent a considerable amount of time researching this after my zero-turn (Toro Timecutter) had the same issues with leaving grass uncut in the center of the mowed strip. I actually wrote a computer simulation of grass being cut with various blade configurations on mowers to better understand it. Here's what I learned; maybe it will better help you understand the problem.....
1-Multiblade System Issues: If you have a multi-blade system (2 blades or more), and the blades don't overlap (they aren't staggered from front to back) then it is most likely impossible for the bladed to cut all the grass in the center. The only way they could is if the bladed are 'synchronized' so that they can overlap the middle of the mower, but can never collide. You can easily check this by looking under the mower and turning the blades by hand. If they never over lap the middle line of the mower, then they will always leave some grass uncut in the center. Some design 'blow the grass over', increasing the odds of it being cut by the blades on either side, but it never works 100% on these designs.
2-Circular Blade Issues: Grass that enters the deck in the center of each blade is twice as likely to be cut as it can be cut by the front of the blade, then there's a chance it will be cut by the rear of that same blade just before it leaves the rear of the deck. Grass near the sides of each blade is statistically have as likely to be cut, although it's odd's are a bit better because the blade sides cover more area due to horizontal movement. In addition, because a blade is circular, once side of the blade (relative to the mover's left or ride) is moving slightly faster forward and the opposite side is moving slightly slower--both because their velocity is added/subtracted from the forward speed of the mower (assuming it is moving). So the (from above) counter-clockwise side of each blade is actually cutting slower, and is more likely to miss or fail to cut a blade of grass. Finally, the radial area that a blade covers is less the further the grass is from the center of rotation for the blade--therefore grass is far less likely to be cut the further it is from the center of each blade, which is why uncut strips usually seem to be on the left and right of each blade (and the center, which is actually the right side of the left blade and the left side of the right blade combined). Always run the blades at maximum speed. Slower rotation increase the odds of missed blades of grass. The only blade configuration that is 100% consistent across the cutting deck would be a 'horizontal blade', i.e. a "real mower" like the old-fashioned push mower with a 'rolling blade'.
3-Cutting Blade Factors: The faster a blade rotates, the more likely it will hit a blade of grass, not to mention the less likely the grass blade will be pushed over instead of cut: important with 'wire-like grass'. This is also why blade sharpness is important. If a cutting blade has large areas with 'dings' or dull spots in it, those areas will push over grass instead of cutting it.
4-Mower Speed Factors: This relates to several of the above items. The faster you mow (forward velocity of the mower) the more likely grass will be missed because of grass being close to the edges of the blades, blades not having time to 'stand up' after being pushed over by wheels, and various other things tied to items above. Everyone hates that you can't mow faster, but the truth of the matter is that the slower you travel on the mower, the less likely grass will be missed.
5-Check under your deck for 'clogs' of grass. I noticed that mine was missing more than normal, and it turned out there was a bunch or soggy grass caked on the sides and center of my deck, preventing the blade from cutting on the edges and center of the deck properly. Giving it a good scrubbing totally cleared out the problem. I now wash the underside of my deck with a custom tool ever cut.
Hope that helps--most important is to cut as slow as you can, with blades rotating as fast as they will go, with a sharp blade and a clean deck and know that every circular blade system has variations of the exact same issues (it's all just math!). It was a learning experience studying this and writing my simulator, but now when I mow I find myself visualizing exactly what's going on when I see a problem.
