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Need correct RPM for B&S Quatro 4HP on Poulon lawnmower


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07-15-09, 03:51 PM   #1  
Need correct RPM for B&S Quatro 4HP on Poulon lawnmower

I finally acquired a tachometer. But I can't find what the RPM should be on my lawnmower with a B&S Quatro 4HP. Can anyone tell me?

It's a two-speed. I'd like to know high and low.

I measured it at around 2200 and 2500, which I'm thinking is maybe a little high.

Thanks.

 
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07-15-09, 04:16 PM   #2  
Most small engines are rated @ 3600 rpm.. The only difference to this rule is the safety watchdog that says the blade tip can only travel so many feet per second.. ( I don't know the #) So the longer the blade,, the slower the engine for that rule,,, But the engine is rated 4 HP @ 3600 RPM... Confused now?? I am....Roger

 
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07-15-09, 06:10 PM   #3  
Roger is on the money, It's these "watchdogs" that tell the OEM's what to set RPM's at.

This is from the Briggs manual just to give you a idea of where it should be set according to the "watchdogs".



 
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07-15-09, 07:07 PM   #4  
Excellent. Thank you both. I've got (I think) a 20 in. blade, so I'm way below the rated limit. And since it seems to mow the grass just fine at 2200 (low), I might as well leave it alone. If I need to set it faster, I know I can.

 
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07-15-09, 08:52 PM   #5  
You might actually be better off to run closer to the recommended speed. The engine is designed to run there and momentum, lubrication, efficiency etc. all come into play.

 
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07-15-09, 09:25 PM   #6  
Posted By: BFHFixit You might actually be better off to run closer to the recommended speed. The engine is designed to run there and momentum, lubrication, efficiency etc. all come into play.
Ah. Okay, then, maybe that's a better idea. (But that chart doesn't hint at what the recommended "low speed" RPM would be.)

 
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07-16-09, 07:52 AM   #7  
Here is the formula, I borrowed it from somewhere. Have a good one. Geo

Formula for mower blade speed
RPM's on lawnmowers are actually set for maximum blade tip speed of 19,000 feet per second, and of course should not exceed the engine mfg specs. for the engine.


To calculate blade tip speed you would have to take the length of the blade say 21" and multiply that by the engine or spindle RPM's say 3200 then take the result (for this example 67200) then multiply this by 3.14 (pie) (211008) and then divide by 12 (inches in a foot) = 17,584 Feet Per Minute.

The maximum blade tip speed as set by government standards is listed as 19,000 feet per minute.

Blade length X RPM's X 3.14 / 12 = Blade Tip Speed in Feet Per Minute.

If you want to figure out maximum rpm's based on blade size then you could do this

19000 (divided by) the result of (Blade length X 3.14 (divided by) 12) = Max RPM's

Example:
For a 21" blade the formula would look like this:
19000 / ((21 * 3.14) / 12) = 3457
It carries out several decimal points, but you can just round down for the maximum blade tip speed.

The maximum RPM's you should set a 22" mower for is 3300.
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07-16-09, 09:13 AM   #8  
A good reference to have. Thanks.

But I wonder if we're talking about two different things: maximum allowable RPM, and ideal RPM. For my needs, I'd just as soon run my mower at the lowest speed that works well, and is good for the engine.

Does anyone know where I can find info on the actual recommendations for my particular machine? Or does everyone just use these general tables?

 
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07-16-09, 09:35 AM   #9  
For the reason Roger posted in post #2 above, There is no one concrete rpm setting.

Set your machine to 3300 and it will be happy......

 
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07-16-09, 10:44 AM   #10  
Posted By: 31YTech For the reason Roger posted in post #2 above, There is no one concrete rpm setting.

Set your machine to 3300 and it will be happy......
.

 
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07-17-09, 09:57 PM   #11  
Posted By: 31YTech Set your machine to 3300 and it will be happy......
I'll keep that in mind.

I also have a Toro Snowblower 540 that someone gave me recently, and I did a carb rebuild on it. I can't find an RPM guide on it in either the user manual or the Tecumseh L-Head engine manual. (It's a 5 HP from 1979.) Any thoughts on that? The manual says to shut it off it revs too high, but doesn't tell you how high that is.

 
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07-18-09, 06:03 AM   #12  
Tecumseh recommended RPM settings can be found on micro fiche card #30, They are listed by model/spec number.

Unfortunately you didn't post engine numbers and my fiche are at the shop for the weekend.

Without looking and just from experience I'd say 1200/1400 idle speed and 3200 high speed on this older engine....

 
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07-18-09, 11:24 AM   #13  
Posted By: 31YTech Tecumseh recommended RPM settings can be found on micro fiche card #30, They are listed by model/spec number.

Unfortunately you didn't post engine numbers and my fiche are at the shop for the weekend.

Without looking and just from experience I'd say 1200/1400 idle speed and 3200 high speed on this older engine....
That's what I get for posting by memory. It's actually a Toro 524: Tecumseh Model No. Hs50-67074b (5 H.p. Snowthrower Model No. 38040).

Folks were helping me over on this thread--http://forum.doityourself.com/outdoo...ml#post1593994 -- but no one had an answer on the RPM.

If you have a way to get the actual numbers for me, I'd appreciate it. Currently it's at about 3500 RPM at full throttle, and it dies if I bring it down below ~2200.

(That's in summer-time. I have no idea how cold weather will affect it.)

 
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07-18-09, 08:34 PM   #14  
If it's dieing when you throttle it down first make sure the throttle cable is adjusted so it's not hitting the kill switch too soon.. If not, than you're into carb adjustments &/or an overhaul with soaking it in a bath cleaner & installing a new kit... Nothing to it really & it's kinda nice to know it's done & you're not monkeying with it when it's snowing & -20 outside.... Roger

 
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07-18-09, 09:46 PM   #15  
Posted By: hopkinsr2 If it's dieing when you throttle it down first make sure the throttle cable is adjusted so it's not hitting the kill switch too soon.. If not, than you're into carb adjustments &/or an overhaul with soaking it in a bath cleaner & installing a new kit...
I already did the overhaul. I damaged the carb body slightly getting one of the stupid welch plugs out (it was in the idle circuit). I may be able to adjust it some, but probably I'll just live with fast idle, at least until I test it in snow and find out how much of a nuisance it is. (Already checked the kill switch.) Never had a snowblower before, so it's new territory for me.

 
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