I have a John Deere 318 lawn mower my father in law gave me. It had been sitting in a barn ever since his father died collecting dust. I changed out the oil and transmission/hydraulic fluid (which seemed to be in good shape) replaced the fuel lines, fuel filter, oil filter, cleaned out the gas tank, replaced the hydraulic steering piston (it was leaking fluid and could not be repaired), cleaned the carburetor and fuel pump, cleaned it up and got it working again. After all of this was done it worked great for about a month. Now it’s idling rough. I adjusted the needle on the carburetor and the same thing happened.
Basically it runs smooth while I have it at half idle. When I increase the throttle it begins to act like it’s either getting too much gas or too much air. I place my hand over the air intake and it idles smoothly again, but by doing this I have put the idle speed back to about half way. I adjusted the needle on the carburetor and the same thing happened. When it begins to idle roughly I notice the carburetor coughing (I guess this is an accurate description) gas. Currently I have the needle screwed all the way in. I thought maybe it could be due to my transmission/hydraulic fluid may be a little low since it will really act up while the blades are turning, and is in gear. But, I’m not sure of this.
My father in law did say his father had to continually adjust the carburetor to get it to idle correctly. So… Any ideas.
Any ideas would be appreciated.
If the carb has always required fiddling with, it may be a good candidate for rebuilding or replacing. (Or someone may come along and say that model is always a pain in the butt, and you had the bad luck to inherit a 318.)
If you put your hand over the carb and it smooths out then it’s running lean (your hand cuts down airflow which increases fuel mixture). If the adjusting screw is bottomed out then I can’t imagine the carb is functioning properly. Kinda hard to guess without looking at it. You could have something partially blocking the fuel line but it appears you’ve covered all those bases.
You could check to see if there is a vacuum leak by spraying starting fluid around the carb with the air cleaner ON. Tough to do with a lawn mower because you’ll likely get the fluid sucked into the air filter. Also a dangerous thing to do.
Is it possible the adjustment screw is backing out due to vibration? When you adjust it start from full in and count the revolutions until it runs right and check it again when it fails.
I checked it yesterday after I let it run for a while and it was right where I left it. Should I take the spring off and see if it can go in any further? Would a new needle be able to solve my problem? The current one doesn’t look to be in bad shape, but I do not have anything to compare.
Could you just build a wood/metal cover for the carburettor to cut down the intake airflow (as your hand is doing?)
Bit ghetto, I know.
If we are talking about the mixture needle it should not be all the way in. Covering the carb making it smooth out tells me the mixture is lean. Screw the mixture screw out about 1.5 turns. and see if it runs better. If it does, then turn it out another 1/2 turn and see if it gets better. If it does, repeat. When it stops getting better, screw it in 1/4 turn and see if it gets better. Again lather rinse repeat.
Don’t take the spring off, the needle is bottoming in the hole, the spring isn’t preventing anything.
If you screw it out and notice no change something is plugged inside the carb, and it will need to be disassembled.
Thanks Rick,
I followed your advice and it’s running smoother. I haven’t got it just right yet since other distractions/priorities (my kids) kept me from the last fine tuning, but, for the most part, the problem is solved.
Thanks to everyone for the advice.
-Jacob