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Another Briggs12 series puzzle

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Another Briggs12 series puzzle

Postby 38racing » Mon Sep 16, 2019 5:38 pm

2010 126t02-1525 . auto choke version. Friend has had since new on ariens self propelled walk behind. First time it would not start. I found really dirty air filter. He pulled most of crap off and it started. Now he has new filter. Issue now is that it starts and runs fine but then slows down. Maybe even stops. No black smoke when it slows. He said it started after tipping for cleaning. On slow it's just like you idled down. If left alone it may go back to speed. If you stop it will restart but at slow speed or maybe fast. I've verified it is not losing spark and changed plug anyway. Have tried loosened fuel cap and replaced with known good one. Fresh fuel and a heavy seafoam treatment. Verified that choke is open. Verified that as rpm drops the governor allows spring to open throttle plate. Verified that intake manifold is tight. Pulled carb. Verified all passages pass carb spray. Noticed float elevated. Changed needle and seat and brought it back level. Removed tank and verified flow out good but still blew into outlet with compressed air. Removed vent hose from air filter back cover. After each thing I did I would test run. It would run for varying periods but eventually slow down. I've cut most of my property doing tests. Just when I think it's fixed it slows. I left it running and left home on errand. It was running when I got home. Don't know if it had slow periods. Wondering if when he tipped it could he have done something to the breather. I tried loosening dipstick as it faltered. Not 100% sure if it was reversing slow down. A first manometer test showed vacuum existed. Interesting was that while doing that test it never slowed. I was looking for a change in vacuum when it slowed. Not having 3 hands I also had it blow oil all over the place. Supper arrived at that point. I'm planning to have wife help with another manometer test and I'll check torque on head bolts. Beyond that I'm pretty much out of ideas other than swapping breather with a running engine or maybe a moving valve seat?
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Re: Another Briggs12 series puzzle

Postby KE4AVB » Mon Sep 16, 2019 6:48 pm

Try a different known good ignition coil. Might be failing electronics.
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Re: Another Briggs12 series puzzle

Postby hanz63 » Mon Sep 16, 2019 7:35 pm

Though you said that every passage is clean, recheck the little vent hole in the brass orifice as you look into the venturi. These plug when the air filters are dirty.
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Re: Another Briggs12 series puzzle

Postby 38racing » Mon Sep 16, 2019 8:51 pm

KE4AVB wrote:Try a different known good ignition coil. Might be failing electronics.

Spark on tester is is steady when it's running slow but I may have to try that.
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Re: Another Briggs12 series puzzle

Postby 38racing » Mon Sep 16, 2019 8:53 pm

hanz63 wrote:Though you said that every passage is clean, recheck the little vent hole in the brass orifice as you look into the venturi. These plug when the air filters are dirty.

Tested that one . got great spray. So good it had me wondering if it was that good on that other unit I have to prime so long.
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Re: Another Briggs12 series puzzle

Postby 38racing » Mon Sep 16, 2019 9:17 pm

After supper I set up manometer again working it while the wife started mower. Pulled 8" of water. I then started it and changed the rear belt on an Mtd varidrive. It missed once or twice but never slowed or stopped. I cut some lawn and it stayed running. Parked it running while I went back to work on the Mtd since belt was just a 5/8 x 44 so adjustment is required. It ran for long time with no problem. I stopped it and filled tank. On restart it ran slow for 5-10 seconds and kicked into high. Ran for another hour I'd guess with no slowing. I shut it off for tonight. I'll refill and try it again tomorrow sitting and cutting. The only thing now is that I have breather hose off air cleaner backing plate with a cap on the plate hose nipple.
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Re: Another Briggs12 series puzzle

Postby 38racing » Mon Sep 23, 2019 10:13 pm

I ran it for a couple extended runs and then took it to him. Yesterday he told me that he cut the front lawn and half the back and then it stopped. He waited an hour and then finished the backyard. I know it sounds like a bad coil but I'm having problem understanding how a bad coil would have it slow to idle but not stop completely during my tests and fact inline tester showed a spark. And it woyld restart and either run slow or maybe even fast.
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Re: Another Briggs12 series puzzle

Postby KE4AVB » Tue Sep 24, 2019 7:25 am

38racing wrote:I ran it for a couple extended runs and then took it to him. Yesterday he told me that he cut the front lawn and half the back and then it stopped. He waited an hour and then finished the backyard. I know it sounds like a bad coil but I'm having problem understanding how a bad coil would have it slow to idle but not stop completely during my tests and fact inline tester showed a spark. And it would restart and either run slow or maybe even fast.

Ever thought of that you might have a failing spark plug that might shorting partially? A shorting plug will not show up using an inline tester nor would the inline tester show a timing issue.

Also with electronics involved just anything possible from reduced spark to timing issues. I had bad coil electronics that threw the ignition timing so far off that it literately sat the air filter on fire. Another other one wouldn't even fire the plug on compression but ignite the exhaust with a nice 12" flame. That one nearly sat my pants leg on fire. I even had one act like the governor was out of order oscillating the engine.

Of course if wasn't for me working with diesels I would have never thought of a 2 cycle dieseling because of a bad spark plug. I was kind lucky at the time to have an inductive tach connected when the rpm limiter kick in. The tach showed zero rpm yet the unit was running wildly at full throttle. It turned out to be a bad spark plug. Even the Stihl techs couldn't find that one, just said nothing was wrong with the hedger.

About the way to know for sure is to change the plug to a known good plug and test. If still does it then trying the coil with a known good coil and test. Then if the problem goes away then you know one of the two was the problem; otherwise, the hunt continues.
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Re: Another Briggs12 series puzzle

Postby 38racing » Tue Sep 24, 2019 8:40 am

Had already changed in a plug from a running engine. And running another engine on his old plug. Looking up coils I see thzt the auto choke may use different coil so I'll have to be selective in finding a test one from another engine.
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Re: Another Briggs12 series puzzle

Postby 38racing » Tue Oct 08, 2019 6:44 pm

So he kept trying to cut his grass but if it started it would always stop at some point. Now it won't start. Just puffs a bit. Inline tester shows spark on 2 different plugs. Puff appears to be spit out the carb. So I'm going check flywheel key and pull valve cover and check valve movement. Then change coil if those things look ok.
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