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Effects of increasing engine RPM? #5333600 01/27/20 11:09 AM
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Avery4 Online Content OP
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Hello everyone, I am wondering about what affects increasing the governed RPM will have on a small engine. To be more specific, I have a Harbor Freight water pump that is powered by a Predator 79CC 4 cycle engine that I use to pump water out of my crawl space and backyard when it rains as my house is on very low land and I live in a swamp. I often have 6-8+ inches of standing water in my backyard after it rains that I am pumping out as my house and garage floods if I don’t.

So as you may imagine, pumping hundreds of gallons of water out of a flooded backyard takes quite a bit of time and I am not a patient person, so I removed the screw on the engine’s throttle lever that prevents it from being moved to the wide open position, and the pump now moves significantly more water, allowing me to finish pumping and get on with my life a lot sooner.

I have been running the pump wide open the last few times I used it and so far everything is still going good, and I have put several hours on it this way. No odd noises or no oil consumption or anything else disturbing other then the muffler glows red after running it wide open for a few minutes, which isn’t too surprising.

So my question is what risks am I taking by running the engine wide open instead of at the stock maximum throttle setting? Will the engine simply require maintenance sooner and experience a little more wear, or is it likely to catastrophically fail in the near future from running at a higher RPM than stock? And if so, which parts of the engine will suffer?

Thank you very much in advance!

Re: Effects of increasing engine RPM? [Re: Avery4] #5333607 01/27/20 11:14 AM
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Increased RPM will increase the temp of the engine oil, especially with an air cooled engine. Going up one grade may help. Do you have one of those temperature guns?


If you want the job done right......do it yourself.
Re: Effects of increasing engine RPM? [Re: Avery4] #5333608 01/27/20 11:15 AM
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supton Offline
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I believe the Predator engines are popular with the go-kart crowd. I'd look to them to see what they are doing--maybe they will report that a stock engine will live at 5k for x hours. Or that after 3 hours & 4k the con rod typically lets loose...

Turning it up of course will wear the oil and engine faster. How much I have no idea. The fact that it's spinning that fast, to me, says it's not overloaded--otherwise it couldn't rev that high. Doesn't sound like it's doing this day in and day out (in which case I suspect this is the wrong setup anyhow).

Worst case, how much is a replacement engine (with coupon, of course)?


2011 Toyota Camry, base, 2.5L/6MT, 201k, hers
2010 Toyota Tundra DC, 4.6L/6AT, 157k, ours
1999 Toyota Camry LE, 2.2L/4AT, 223k, his
Re: Effects of increasing engine RPM? [Re: Avery4] #5333611 01/27/20 11:15 AM
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One or both the engine and the pump will wear faster.

You don't say how fast the engine is running.

It is quite possible that the engine is governed not for the engine, but for the pump.


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Re: Effects of increasing engine RPM? [Re: Snagglefoot] #5333619 01/27/20 11:21 AM
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Avery4 Online Content OP
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Originally Posted by Snagglefoot
Increased RPM will increase the temp of the engine oil, especially with an air cooled engine. Going up one grade may help. Do you have one of those temperature guns?

I found that the oil is running at around 190 degrees max, at least in this cooler weather, which seems good to me. However, it is always filled with fresh synthetic oil, so that hopefully helps. Currently I am running Pennzoil Ultra Platinum 5W30.

Last edited by Avery4; 01/27/20 11:21 AM.
Re: Effects of increasing engine RPM? [Re: Avery4] #5333632 01/27/20 11:31 AM
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All small engines use an aluminum connecting rod. Aluminum has a finite fatigue life and the far higher loads due to higher RPM and more frequent cycles rapidly decrease that lifespan. As such, it's good to know that most will hold up to intermittent 5000 RPM use and constant 4000RPM use without early demise.

Keep in mind that you are better off modifying the governor spring to achieve the desired RPM than you are to operate it wide open. As an unloaded condition (sucking air) can result in an 8000 RPM rod-throwing event.

NOTE: I have a Kawasaki 4HP water pump and I run it at 4100 RPM. Have done so for years. I use 5W-40 M1 TDT oil in it. It pumps incredibly better at 4100 than it does at 3400 (stock RPM)

Last edited by Cujet; 01/27/20 11:33 AM.

People who count on their fingers should maintain a discreet silence.
Re: Effects of increasing engine RPM? [Re: Avery4] #5333634 01/27/20 11:32 AM
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governance is for load, so generally small engines can take a bit more during heavy load but should not be run as such always or one should expect a direct correlation between expected engine life and any increase(or decrease) of extreme temperatures or pressures.


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Re: Effects of increasing engine RPM? [Re: Avery4] #5333673 01/27/20 12:03 PM
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Thanks everyone, I really appreciate your help. I never thought about the possibility that the RPM may be limited for the pump’s sake rather than the engine’s sake, but that’s a great point. Also a great point about the engine possibly over revving if I suck air, but the way the governor is set up, it will not actually over rev if load is decreased, it will decrease throttle and still maintain a reasonably steady RPM.

Another question- Would rejetting the carb be inadvisable? This engine runs way lean from the factory to the point that it will sometimes die out if throttled up too quickly without the choke part way on, and it always runs smoother on part choke. I am thinking of very slightly widening the hole in the carb jet to allow slightly more fuel flow if that wouldn’t be a bad idea. This may have the benefit of keeping the engine a little cooler too since lean mixtures burn hot. Thanks

Re: Effects of increasing engine RPM? [Re: Avery4] #5333794 01/27/20 01:51 PM
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Every Briggs & Stratton engine-powered water pump I have ever owned,
be it Pacer, Monarch, or Red Lion - runs at WOT when under load....

but not at a "much higher RPM" like you experienced.
These engines are just lugging away while pumping 60-90 gallons a minute.
I tried to do what you did, only to find they were already at WOT.

By the way - - you can always sell that little 79cc water pump and get the 212cc version.... just sayin'......


"The evidence demands a verdict".
(Re:VOA)"it's nearly impossible to actually know the particular additives that are in there at what concentrations."
Re: Effects of increasing engine RPM? [Re: Avery4] #5333800 01/27/20 01:55 PM
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Originally Posted by Avery4

Another question- Would rejetting the carb be inadvisable? I am thinking of very slightly widening the hole in the carb jet to allow slightly more fuel flow if that wouldn’t be a bad idea. This may have the benefit of keeping the engine a little cooler too since lean mixtures burn hot. Thanks

I do on every small engine that needs it.

Since the EPA killed adjustable-needle-valve carburetors, "rejetting" is required. I use a torch tip cleaning tool for this job.

BE CAREFUL - - it's easy to remove too much metal ............. VERY EASILY!!!!


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"The evidence demands a verdict".
(Re:VOA)"it's nearly impossible to actually know the particular additives that are in there at what concentrations."
Re: Effects of increasing engine RPM? [Re: Avery4] #5333809 01/27/20 02:05 PM
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I agree, if you are capable re-jet for sure. And yes the crankcase temp will come way down with a good power mixture. heed the warning ...too much fuel and the cylinder will wash down, oil will dilute and KaBang will follow. Revving it a bit higher with the governor spring wont hurt and pump will push more water ...it is not a problem for the pump unless you're pushing head pressure to get it up over a hill into a drain. Listen to the engine if you rev it higher ...a good rumble is working, ...screaming in agony is bad ...listen to talk to you.

Re: Effects of increasing engine RPM? [Re: Avery4] #5333844 01/27/20 02:51 PM
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I'd suggest a bigger pump/engine combo.
It looks like your battle with water is long term.
Mother Nature> Harbor Freight.


"If there must be trouble, let it be in my day, that my child may have peace." --- Thomas Paine
Re: Effects of increasing engine RPM? [Re: Avery4] #5334135 01/27/20 07:44 PM
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As mentioned aluminum rod engines are governed to 3400 RPM mostly for longevity. The Gravely one cyl lawnmower I have blew a rod on the way back to the barn. This 14 HP engine has a very heavy flywheel and the governor was set to 3600 RPM and when it blew it free wheeled for about 40 feet in complete silence. Set the gov to 3400 after replacing the rod and piston. Aluminum rods have a shorter service life than steel. So yes a bigger pump and motor is a better solution but where is the fun in that.


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Re: Effects of increasing engine RPM? [Re: Avery4] #5337091 01/31/20 07:46 AM
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But a tach, I like sirometers. B&S, Tecumseh, Kohler, max 3,600; Honda & Chinese max 4,200, 3,800-4,000 is fine.

Re: Effects of increasing engine RPM? [Re: Avery4] #5337836 02/01/20 12:37 AM
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Not sure why you choose that small HF engine.The 212 cc 6.5 hp is always on sale for $99. I put one on my lawn edger to replace an under powered Honda 110cc and it works great:


https://www.harborfreight.com/engines-generators/gasoline-engines.html

Last edited by user52165; 02/01/20 12:38 AM.

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