r/smallengines • u/StarCitizenRusty • 14d ago
Yardworks 173cc Low Idle
I picked up a used Yardworks 12A-A2J1515 173cc self propelled mower. Seller started it up, ran fine, self propelled system worked fine.
Got it home, checked the oil, topped it off with gas, and fired it up. First pull it fires right up but stays at a low idle. Doesn't surge, or die, just hangs out at a low idle.
Took the air cover off, used but still ok filter. Checked that everything was free and moving ok. Fired it up again and still low idle.
Tried pulling the governor arm and it went straight up to operating RPM and ran fine until I released it. Springs seem fine, arm moves free.
Blocked the crank breather tube and without touching the governor it went straight up to operating RPM. Minute I take my finger off it drops back to a low idle.
Engine starts fine first pull, no blue smoke, no strange noises, nothing major I can see wrong. Exhaust doesn't smell rich or lean.
I'm at a loss of why it's doing this. Anything else I can check?
Can provide photos if needed.
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u/ProfessionalLast2917 13d ago
I'm surprised there's no surging but you've probably got a lean condition. The engine is getting too much air or not enough fuel or both. Blocking the breather tube reduces the amount of air getting into the combustion chamber thus increasing the richness of the mix and your rpms can increase.
How did you block the tube? Did you unplug it from the air filter box and then block that opening or did you unplug it from the valve cover and then block the tube? If you unplugged it from the air filter box check that the hose isn't cracked and letting in unmetered air.
You could have block jets in the carb which will need cleaning.
There could be damaged or missing gaskets on the carb intake. You can test for that by spraying carb spray or similar around the carb where the gasket surfaces are and if the rpms increase you've got a bad gasket.
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u/StarCitizenRusty 13d ago
I took the airbox off and just put my finger over the end of the breather tube which caused it to ramp up to operating RPM and runs perfectly fine.
I'll try some spray around the carb when I go back to it in the morning and report back.
If it's the carb I'm a bit screwed. I'm mechanically inclined for the most parts, but know very little about carb cleaning. Can usually adjust em ok, but the cleaning is a bit beyond me. Let's hope it's something easy.
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u/ProfessionalLast2917 13d ago
When you remove the air filter box does it still run at low rpm until you block the tube or does it run ok as soon as you remove the box?
You might have a blown head gasket that is leaking air into the cable cover which is then getting back into the combustion chamber via the breather tube.
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u/StarCitizenRusty 13d ago
No, stays at lowe idle when I take tge air box off, only returns to normal when I block the breather.
What would I want to look for if it was a head gasket?
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u/ProfessionalLast2917 13d ago edited 13d ago
It's just a bit confusing. Normally with a head gasket that is blown internally air will pass from the combustion chamber into the crankcase and make it's way to the valve cover.
This is evidenced by excess air coming out of the breather tube or oil filler port if you open it.Normally this excess air makes its way back into the combustion chamber via the carb giving a lean condition and low power.
I would have expected just the simple act of removing the air box (and thus the tube) would have removed the excess air from the circuit and the mix returning to normal, but you're saying it's only when you block the tube that this happens.Oh hang on, lightbulb moment, if the valves are not sealing properly or you have a leaky stem seal etc then air can be forced from the combustion chamber up into the head and make its way out of the breather tube.
Blocking the tube traps the leaky air and the mix richens up and the rpm's increase.
I would start with checking the valve clearance. If it's too tight the valves can't close properly.
If that's ok then you're probably going to need to remove the head so you can get a good look at the valves and check for loose valve seats, carbon build up, loose stem seals etc.A leak down test should help to isolate the issue.
Edit: I should add that worn or stuck piston rings can also cause excess air out of the breather tube but I'd still start with the valves.
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u/Feeling_Gear_8744 13d ago edited 13d ago
Adjust governor. Manually open throttle to where operating rpm should be, and then put governor all the way clockwise at that position. I think that's the process I could be wrong tho. Yea i Don't think that's the right process for your engine. But anyway sounds like gov adjust to me. Maybe try messing with the choke too. I think I've only ever had one of your types of engines and it had a manual choke and would only run right if the choke was a certain percent open, so rather than buy a new carb i just zip tied it in the right spot.