Effects of increasing engine RPM?

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537
Location
USA
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!
 
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17,501
Location
NH
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)?
 
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11,563
Location
Illinois
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.
 

Avery4

Thread starter
Messages
537
Location
USA
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.
 
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10,495
Location
Jupiter, Florida
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)
 
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1,408
Location
Gone Fishing
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.
 

Avery4

Thread starter
Messages
537
Location
USA
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
 
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10,001
Location
Waco, TX
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'......
 
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10,001
Location
Waco, TX
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!!!! [Linked Image from images-na.ssl-images-amazon.com]
 
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1,065
Location
Arizona
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.
 
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866
Location
New Hampshire USA
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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2,090
Location
missouri
some water pumps will lose efficiency from cavitation if too much RPM Usually small engines are safe to 4000 rpm. Above that they may overheat. You always shorten the life if increased RPM, but the difference from 3000 RPM to 3600 is slight, From 3600 to 4000 is much higher. I would not exceed 4000 rpm. I bet somewhere there is a app for your smart phone to measure RPM.
 

Avery4

Thread starter
Messages
537
Location
USA
Thanks everyone for your responses. After several more hours running wide open, the only problem I have had is that the plug wire was smoking from the heat of the muffler since it was about 1 inch below the glowing muffler. I turned the cap so the wire is now pointing down and the wire is more like 1.5 inches away from the muffler and I put some fiberglass insulation between the bottom of the muffler and the plug wire and it hasn't started smoking again yet. I actually took the heat shield off the muffler, wrapped it in fiberglass, and put it back on to keep the heat from the muffler off of other things like the fuel tank and carb, which were getting pretty warm. Insulating the muffler helped this significantly. Good point about the engine overheating from running under more load. The highest oil temp I have been able to measure is around 230, which seems quite reasonable to me considering that my Briggs powered mower runs an oil temp of 280+ degrees under load on a hot day. Surely the oil temp will be higher in the summer though, I will keep an eye on it. Getting some kind of tach would be a good idea so I know how fast I am running the engine, it's on my list of things to do.
 
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4,086
Location
Central Virginia
FWIW the aftermarket is big for the H-F Predator 212. You can get rods, pistons, big carbs, intakes, heads, etc. to make the power you need. The go-cart and minibike crowd loves those things. For your stock engine I would turn it up a little (as others said under 4K) and maybe drill out the jet a size or two bigger with a mini drill bit you can get on Amazon pretty cheap.
 

Avery4

Thread starter
Messages
537
Location
USA
I have an update. I bought a cheap eBay header pipe for this engine as well as an adjustable carburetor. I bought the header pipe so the heat from the muffler isn't heating up the carb, gas tank, valve cover, head, plug wire, etc and I bought the adjustable carb so I could adjust the air/fuel ratio to keep the engine from bogging and dying out when trying to throttle up. What an improvement, I can now throttle up without it dying out and things aren't being fried by the heat of the muffler. Unfortunately I did experience my first catastrophic failure after probably 20+ hours of running it this way. I got distracted because my dog got out, which resulted in me not paying attention to the suction hose and air getting sucked in, which resulted in the engine overrevving. I initially thought the engine threw a rod because the engine made that type of clunking noise and suddenly came to a stop, but when I cranked the engine over I could feel that it still had compression so I knew it was something else. Basically what happened is the magnet on the flywheel came apart and destroyed the plastic cooling fan as well as the ignition coil. Luckily I have a parts engine since the first pump I bought had a cracked block and the store could not accept the return since it once had fuel in it, so I showed it to them, they gave me a new one, and I ended up with a parts unit. I pulled the flywheel and coil off that one to replace the failed parts and I'm back in business. I now think the original flywheel that failed may have been defective/poorly balanced from the factory because the engine vibrates a LOT less with this flywheel on it even at lower RPMs, this is definitely the smoothest this engine has ever ran. I made an adjustment to the governer so it will still run wide open under load but but will not rev up any more if the same thing happens again, which is what I should have done in the first place. Oh well, live and learn.
 
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9,071
Location
Marshfield , MA
Thank you for the follow up. The 99$ Predator I re-powered a snow blower with 5 yrs ago made me a fan. I recently bought a 5.5 Kw generator powered by its big brother
 
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