Why do some Honda engines oscillate at idle?

I thought it was just my HRX217 lawnmower that oscillates wildly during idle. But then I rented a Wacker trash pump with a honda engine, and it would also oscillate when in the full throttle position whenever it lost prime. Then I rented a pressure washer with an 11HP honda engine and this too would oscillate heavily at idle when the water was not being sprayed.
The fluctuation is pretty severe, as if the engine is going to stop, it doesn't sound normal at all. Since it happened on 3 engines in a row - I'm starting to think it could be the gas I'm using. My gas cans have a mix of 89 and 91 octane from Mobile. Could it be that the octane is too high? Am I supposed to use 87 octane instead?
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It would be the govenor making them run bad, its not normal.
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ransley wrote:

Or air leakage. Worn throttle shaft vibrating or loose at intake can cause similar symptoms.
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Could it be that all three engines have worn out parts? I suppose that could be possible since two of them were rentals that are subject to heavy abuse. But the other one, my HRX217 mower started doing this after only a few months. So it was practically brand new. Is it just my luck with Honda engines?
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I have had Briggs & Stratton engines do this.
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The interesting thing is that all 3 of these engines would run great under normal load (while cutting grass, while pumping water, while spraying). The fluctuation only happens at idle or under non-operating load.
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wrote:

Makes me think of fuel / air adjustment, maybe a fine tune of the screws.
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iwdplz wrote:

I believe that's what the engineers refer to as the governor hunting. There must be some way to increase the hysteresis of the speed control system for those engines. I've seen a lot of small engines fluctuate under non- load conditions and sometimes something as simple as a new muffler will straighten it out. When my friend's chainsaw was running poorly, surging and such, we got the manual out and adjusted every needle valve on the thing by the book and it solved the problem.
TDD
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I have a 5 kw generator with a Tucumse engine and it does the same thing. It will surge under a very light load or no load. Put about a 1000 watt 120 volt load on it and it runs smooth.
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When I took my small engine repair course, the teacher called that "hunting". As in "hunting for the proper speed". The problem is typically that the spring for the governor gets rusty, stiff, painted, etc. The governor spring is far more essential than people understand.
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wrote:

I also have a hrx217. To stop this I just put a new air filter on. The old filter looked fine but a new one solved the problem.
I have also seen oscillations from running the engine to lean.
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I tried a new air filter, changing the spark plug, changing the oil. I looked for any mixture adjustment screw but I was told that the HRX217 doesn't have such adjustment. Anything else I can check? It started happening when it was relatively new after just a few months so it would be less likely to be an air leak or rusty spring etc.
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Here's a picture of your carb. note the air mixture screw near where it bolts on to the manifold.
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wrote:

I should have said. " I've seen oscillations from running small engines lean." Not specifically the Honda. I never looked into a carb adjustment, don't even know if one exists because the new air cleaner solved the problem.
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