Ok to Increase RPMs on Briggs and Stratton Mower?

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  #1  
Old 10-04-11, 03:12 PM
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Ok to Increase RPMs on Briggs and Stratton Mower?

I have a 21" push mower with 6.5 HP Briggs and Stratton engine, and was wondering if it's ok to increase the RPMs. I mowed some newly planted rye grass today and the mower could barely cut it because the grass blades are so thin. It mostly bend the tops off rather than cutting them. I was able to adjust a lever with spring attached and increase the RPMs enough to cut the grass, but am not sure if it is safe to leave it like that. I don't think it's the blade because I sharpened it recently, but I did it myself with an angle grinder, so it may be dull--doesn't feel very sharp.
 
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  #2  
Old 10-04-11, 04:20 PM
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If the blades don't feel sharp then they are probably not sharp. I would get a new blade or have a professional sharpen it before increasing the engine's rpm.
 
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Old 10-04-11, 04:34 PM
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Not to be rude but are you sure the blade is not on upside down. Have a good one. Geo
 
  #4  
Old 10-04-11, 11:38 PM
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Not a good idea to increase the RPM of your mower's motor. Best case is it will shorten the life of the engine, worst case is the engine will self destruct the next time you use it depending on how much you sped it up. If the grass you are cutting is so immature, I would have waited to cut the new grass until it had developed thicker blades and/or made sure the mower blade was sharp.
 
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Old 10-05-11, 05:28 AM
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I've cut new grass that was very tender and it still cuts and I've run blades so dull they just had a smooth rounded cutting edge and it still cut (ok, tore) the grass. I'm wondering if Geogrubb isn't onto something. Maybe the blade is upside down.
 
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Old 10-05-11, 10:03 AM
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The engine is designed at the factory to run 3600 rpm but the government says that the blade tip speed is not to exceed 19,000 feet per minute. So, your mowers established engine rpm is based on the length of the blade (and some other less important factors). The longer the blade the lower the engine rpm must be to meet the standard. Let me say this; I don't care if your engine were only running at 2000 rpm, the blade is still going to provide a good cut...if the blade is sharp and ground at a good angle and is not installed upside-down.
 
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Old 10-06-11, 09:41 AM
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Come on fellas, the blade is not upside down. The grass is new and very thin. Once I sped up the motor, it cut fine, and it has always cut the more mature grass with no problem. I planned on getting the blade sharpened anyways, but simply wanted to know if I could increase the RPMs. Sounds like a bad idea.
 
  #8  
Old 10-06-11, 09:59 AM
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Originally Posted by mossman View Post
Come on fellas, the blade is not upside down. The grass is new and very thin. Once I sped up the motor, it cut fine, and it has always cut the more mature grass with no problem. I planned on getting the blade sharpened anyways, but simply wanted to know if I could increase the RPMs. Sounds like a bad idea.
I have to admit I was thinking the same thing about the blade. It might sound crazy to you, but for us in the trade, upside down blades are a regular occurrence, so it's not too far out there to suggest. I've seen blades that were worn out upside down. I can't imaging how frustrating it must have been for that operator trying to cut their grass. A good sharp blade with the air lifts not worn off the back will cut new winter rye and the finest/thinnest of grasses.
 
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Old 10-06-11, 02:59 PM
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"The engine is designed at the factory to run 3600 rpm but the government says that the blade tip speed is not to exceed 19,000 feet per minute"

Is that why today's mowers no longer have a throttle control? I miss the old mowers where you could idle down thru the short/sparse grass and throttle up when you hit the thick grass. Seemed to take less fuel too
 
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Old 10-06-11, 03:19 PM
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No offense intended by anyone, I'm sure moss.....but I've done it myself. And I've been sharpening my own blades and mowing my lawns for 30 yrs. (now I just let the wind blow the trash off the gravel...lol.)

Sometimes the way that seems right...just isn't.

Wanna hear how many times people on here have said..."Yeah, checked that twice..", and then came back all hang dog?
 
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Old 10-07-11, 05:17 AM
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My dad recently put all three blades back on upside down. Luckily he caught the mistake before putting the deck back on the tractor.

I don't think the rpm's or blade tip speed has anything to do with no more throttles on mowers. I think it's a cost saving trick. Throttle levers and cables cost money so leaving them off saves a dollar in the cost to build a mower.
 
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Old 10-07-11, 05:53 AM
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Ok, fine. So I'll ask....what side of the blade faces down? As is, the side that cups downward (near the shaft) is facing down, which I'm pretty sure is how the original blade was installed when I bought the mower. Again, it cuts my other grass just fine. Would an upside down blade cut grass at all?
 
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Old 10-07-11, 06:35 AM
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A round piece of steel (or plastic for that matter) turning at 3000 rpm will certainly shear grass; 3000 r's is moving right along! Your terminology may be different than mine and I don't know what you mean by "cups downward" and especially since each manufacturer has different blade designs and since you didn't mention what brand mower you have, or even if you have an OEM or aftermarket blade installed. Here's how best to know if the blade is on correctly (on 99% of modern mowers), your business end will have two angles: one parallel to the ground and one at roughly a 45-degree angle (these two angle meet at the actual cutting edge). The angled line (and the one you would sharpen) is going to face the engine, not the ground.
 
  #14  
Old 10-07-11, 07:27 AM
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Easiest way I guess would be to pull the rope a little and see which direction the motor turns. The sharp edge of the blade should be leading in that direction.

I have a push mower for trimming. One day while cleaning the underside I noticed the wrong side of the blade was shiny. Oops, backwards. So yes, it will cut being on backwards.
 
  #15  
Old 10-07-11, 10:06 AM
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Originally Posted by mossman View Post
Ok, fine. So I'll ask....what side of the blade faces down? As is, the side that cups downward (near the shaft) is facing down, which I'm pretty sure is how the original blade was installed when I bought the mower. Again, it cuts my other grass just fine. Would an upside down blade cut grass at all?
Remove the plug wire, tilt the mower on its side, carb up, when the engine is running the blade will be turning in a counter clockwise direction as you look at the blade with the mower tilted, the cutting edge should be the leading edge as it turns, the lift wings at the end of the blade should be pointed upward if the mower were setting flat. Have a good one. Geo
 
  #16  
Old 10-07-11, 04:13 PM
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Blade is installed correctly! It is dull. I tried sharpening it myself but evidently did a poor job. Will be purchasing a new one tomorrow for $15. Thanks for the input. Thread closed.
 
  #17  
Old 10-08-11, 04:19 AM
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Blades are easy to sharpen, all you need is a grinding wheel or even a grinder. You just need to make sure you take an even amount off of both sides to keep the blade balanced. What went wrong on your sharpening attempt?
 
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Old 10-11-11, 06:16 AM
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In my opinion, a bench grinder with a 7", minimum, diameter wheel is the only way to sharpen a blade. Angle grinders, die grinders and bench grinders with smaller than 7" wheel just don't do a good job. We use a 3/4-horsepower, 10" bench grinder at our shop but most homeowners who only occasionally use one can't justify the expense. I know this is DIY but in some instances it's best to leave some work to the experts and if you have a local shop you can rely on, it's best to have two sets of blades and leave the sharpening to an expert. Same thing can be said about chainsaw chains...you wouldn't believe some the the homespun sharpening jobs we see here at my shop!
 
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Old 10-11-11, 06:33 AM
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Unless a blade was really dinged up (back when I had grass), I just clamped it in a vise and used a file. No overheating and it really didn't take that long. Since I would normally tip the mower up to clean out the deck every month or so, the extra 5-10 min to touch up the edge was no big deal. Oh...but I did have 2 blades. After the swap and cleanup, I'd just sharpen the used one and hang it on a nail in the garage.
 
  #20  
Old 10-11-11, 10:04 AM
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Crap, should have kept the old one. Too late. I put a new blade on and will be using the mower later today. The old blade actually felt sharper than the new one does, which is odd. Maybe it is the RPMs.
 
  #21  
Old 10-11-11, 10:09 AM
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Mower blades should not be sharp like a knife...they just rapidly dull. I always sharpened mine more like a shovel blade or splitting maul. Not sharp enough to cut you..but sharp enough to penetrate and split. Since the blade is moving so much faster than a shovel or maul, it does the same job.

I guess butter knife vs pocket knife is kind of close to what I mean. Compare the edge of a new blade to what you sharpen.
 
  #22  
Old 01-13-12, 10:39 AM
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I just realized I've been using two-year-old gas in my mower. That could be what my problem is, and that would explain the white smoke and rough idle.
 
  #23  
Old 01-13-12, 11:06 AM
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As to the blade upside down, we get tenants calling frequently with electrical issues and our first question is always whether they've checked the breaker. Often, they take offense just like mossman did here. We then tell them this story:
A few years ago we did not ask this question and instead drove over to investigate a similar question from a tenant. Once on site, we reset the breaker and all was fine. The tenant then asked, "Gee, do you think I should give back my PhD in electrical engineering?"
 
  #24  
Old 07-30-12, 03:02 PM
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I have a 6.0 hP that wouldn't start-it has been probably 2 months since I used it-and I took the plug out and noticed it was bad. I put a new plug in and now it starts but it doesn't run at the same speed as it did before. Is that indicative of bad gas? I just put new gas in it when I tried to start it before I changed the plug. Would it have anything to do with the plug gap? Don't understand why it lost power. It was running fine before I let it sit-we're in extreme drought and I haven't had to mow in a while, like since before Father's Day!!
 
  #25  
Old 07-30-12, 06:47 PM
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Does it do it without the air filter? Have you checked the throttle shaft to be sure it isn't stuck? Is there tension on the spring at the right front of the engine above the carburetor?
 
  #26  
Old 07-31-12, 11:01 AM
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New blade doesn't do a very good job either. It cuts the fescue and zoysia fine, but it misses blades of bluegrass here and there, so I have to go over those areas twice. Very frustrating.
 
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