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Idles smooth, doesn't at higher rpms...??

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Re: Idles smooth, doesn't at higher rpms...??

Postby dart451 » Sat May 19, 2018 5:08 pm

Update:

Ok, I tried another carburetor, did not help the rough running of the engine. I also by-passed the fuel tank with a known good tank directly into the carburetor, still no difference. So, tomorrow I will pull the valve cover and check the valve clearance. I'm just about out of ideas... I hope perhaps an out of spec valve clearance might be the culprit!
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Re: Idles smooth, doesn't at higher rpms...??

Postby dart451 » Sun May 20, 2018 11:54 am

Ok friends, I believe I found the culprit to my dilemma! I read my very first post and I failed to include the information that the engine was completely void of OIL when I got it! The customer told me that it had some serious oil leaks...thought it was the transmission. So when I checked the oil, there wasn't any! So, I should have mentioned this because I believe the engine was run on low oil. I checked the compression and after pulling on the cord as fast as I can about 3 times, I barely get 15 psi of compression. Now the engine will start but I think under load of the tiller mechanism, it won't have enough power to get out of it's own way! I checked the valves this morning, the valve lash is very good...right within specs. I understand that one should do a leak down on these engines due to compression release mechanisms but I would STILL think I would get more than 15 psi! What say you wise small engine mechanics??
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Re: Idles smooth, doesn't at higher rpms...??

Postby dart451 » Sun May 20, 2018 12:19 pm

I just completed a leak-down test on the engine...passed with flying colors! I couldn't hear any leakage whatsoever and on my gauge in was in the SET location above the lowest leakage reading. So, I just don't know what this stupid engine will not run smooth!
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Re: Idles smooth, doesn't at higher rpms...??

Postby dart451 » Sun May 20, 2018 4:10 pm

Ok, I have a sound file in video format so I can let you listen to the motor...any suggestions other than what I've checked...I'm all ears!
https://www.youtube.com/watch?v=Ll0KeBg ... e=youtu.be
Last edited by dart451 on Sun May 20, 2018 5:50 pm, edited 2 times in total.
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Re: Idles smooth, doesn't at higher rpms...??

Postby Arkie » Sun May 20, 2018 4:51 pm

That you tube link did not work for me.

Before checking the compression again jump the push rod off the exhaust valve rocker arm so as to disable the compression release. (you can do this with your finger pressure pushing on the exhaust valve spring on a OH valve Briggs)
On some engines you need to hang onto the push rod so it don't fall inside, not sure about yours.

I had a single cylinder Briggs 12 hp that ran fair at idle but would sputter and spit at medium and high throttle (kinda sounded like exhaust valve leaking and spitting out muffler) and the magneto coil finally failed.
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Re: Idles smooth, doesn't at higher rpms...??

Postby dart451 » Sun May 20, 2018 5:51 pm

Sorry about that! I accidentally made the video private but now changed it to public so it should work!
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Re: Idles smooth, doesn't at higher rpms...??

Postby dart451 » Sun May 20, 2018 7:59 pm

So, anyone have a clue with the way this engine runs?
https://www.youtube.com/watch?v=Ll0KeBg ... e=youtu.be
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Re: Idles smooth, doesn't at higher rpms...??

Postby Arkie » Sun May 20, 2018 8:23 pm

dart451 wrote:So, anyone have a clue with the way this engine runs?
https://www.youtube.com/watch?v=Ll0KeBg ... e=youtu.be


You say the leak down test was oK, but the compression was only 15# probably due to compression release???
15# is suspect as too low even with compression release operating and the leak down could have occured with the valve seated good and gave a good test.
2 or 3 carbs later still same.
The engine was ran without oil is most likely a hint.

I would do another compression test with the exhaust rocker off the push rod. Do several compression test keeping a heads up for intermittent valve leakage, and watch the strength of the compression pulse as the engine turns over, like a seat loose and it don't take very much upset on those type OH valve stems and guides for a valve to slightly stick or gall in a guide, because the valve springs are not real strong. Push the valves down with your fingers and feel and watch for consistency in the two valves comparing one too the other. Also check for valve guide wear. We know that most likely it's either erratic compression or ignition. I assume you have installed a NEW correct type gapped spark plug. I've seen CHAMPION spark plugs act just like that.

But again that is the way the old L head Briggs sounded when the magneto finally completely failed that I mentioned about but had more of a spit out the muffler.

You can take a Briggs type inline spark tester or Oregon 42-087 like this one and use in in a shade or dark place and carefully watch the spark while the engine is running. Do not relay on a bulb type spark tester. A engine will run normally with the Oregon or Briggs spark tester inline.
.https://www.ebay.com/sch/i.html?_from=R ... +&_sacat=0

I don't like to just throw parts at something (unless process of elimination in this case and just a little bit bad)) but I suspect you can get a magneto for that engine for around $10 from flea bay. (and do try a new spark plug)

If it don't go then you eliminated most all except for valve/ erratic compression issue.
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Re: Idles smooth, doesn't at higher rpms...??

Postby dart451 » Sun May 20, 2018 9:47 pm

Really appreciate the info Arkie! Really do! I'll do more tests tomorrow...thank you!
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Re: Idles smooth, doesn't at higher rpms...??

Postby KE4AVB » Mon May 21, 2018 4:41 am

First what Arkie suggests first but may have to bite the bullet and pull the head. With 15# compression and it passing a leak down test suggest to me that you may have lower or mid cylinder damage. That is one draw back that the leak down test has as it tests only the upper cylinder, valve components, and the head/gasket where a compression test give general idea of the overall cylinder condition so both tests should be done and compared. As you gain experience using both you will find what a normal compression reading with the ACR active and with a leak down test you usually find the problem but not always without doing an invasive inspection.

Of the two that I overhauled of these slant OHVs the lower portion of the cylinder was seriously ovaled while the top was nearly perfect rounded though with some wear. As for the compression you should be seeing nearly 65-70 psi even the ACR active. Now of course both these were smokers and both passed by homemade leak down test but failed the compression test. Most times customers won't spend the money on an used engine to have it rebuilt due to replacement cost vs repair cost factors so I don't get many opportunities to do a rebuild except for myself.

I am also sitting on a Stihl 026 chainsaw that has 150 psi compression according to my compression gauge that is losing all that compression right at the top of the stroke due cylinder damage so it can't even hit.
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