Lawn Mower Idle Oscillates

My Murray lawn mower idle oscillates up and down, I can see the spring on the carb moving about 1.5".
I there a spring kit or some kind of throttle control that may fix this this?
I rarely use this lawn mower and am about to put it up for sale, just thought I'd fix this problem before advertsiing.
Its a Murray riding lawnmower, model 30552x12a w/a Tecumseh 10 HP
Thanks for any input..
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I learned about this in my small engine repair course.
If the spring on the throttle gets a bit hardened (painted, rusty, etc) it doesn't respond right. So the engine speed "hunts" for the right speed.
Replace the spring between the throttle and the carb.
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Christopher A. Young
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Yup, could definitely be a spring. I read a little book on repairing small engines from the library recently, and it also suggested it could be any spring or linkage which connects the governor system with the throttle.
You may have to remove the engine cover to fix this one, but the good news is that if it's packed with grass or other debris, cleaning it may resolve the issue for you (anything that binds or blocks the spring(s) or linkages from proper operation).
Good luck,
Stephen Kurzban
Buck Wheat wrote:

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a spring won't fix it. you have junk in the carb. the idle feed passage is clogged causing it to try and run off the main fuel circuit. pull the bowl and clean thoroughly. up in the center of the carb tube is a asmall passage running at a 45 degree angle towards the top. this is where the restriction is. good luck Chip
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hunt or oscillates can be the spring but in all my years of working and teaching on small engines it is not a common failure. there is a list of possible here is but a few, air filter plugged, worn throttle shaft carb, intake manifold leaking, governor problems (worn or misadjust) linkage worn bent or missing, fuel problems.
as I said the list is long but when one runs into a problem with a spring it is more common it will either be stretched and the rpms are too low or modified some how and has too much tension making rpms too high.
good luck
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Actually, it would not start about a month ago, so I took the bowl off and cleaned out the carb and hoses. I had to turn the bowl little by little to get it started again, I think maybe the bowl is still not correctly positioned.
I'll take it off and try to find the right position, I have not seen any markings to line-up..if there are any(?) I'll check all other parts for clogging, etc.
Thanks for taking the time to reply, it's a riding mower and I dread trying to get it to the shop.

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If it's a Tecumseh engine, and there's a flat area in the fuel bowl, this goes directly under the hinge area of the carb, so the float can drop as far as it needs to.
Bob

carb,
worn
spring it

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That's exactly what I needed to know, I tried several positions yesterday w/the float bowl, all yielded the same result...no change, I'll correct it today. I was able to cut the lawn as once it is in full throttle it doesn't seem to oscillate.
What does the screw/adjustment at the bottom of the float bowl do?
Thanks.

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The screw at the bottom is the main jet needle valve.
Tecumseh carbs have three adjustments: idle speed which will be a screw possibly with a spring wrapped around it, that sets how far the throttle plate can close off the fuel/air supply; a low speed needle valve which is often located on top or on the side near the throttle shaft, and a high speed or main needle valve located under the fuel bowl.
The low speed (often called idle mixture) needle valve is adjusted for maximum RPM at idle. This is usually about 1-1/2 turns from fully tightened. Turn it slowly each way, noting where the engine runs rougher or slows down, then set it in the middle. Do this adjustment first.
Second would be to set the idle speed. This is often around 1800 for rotary mowers but could be slower for tractors, i.e. 1200 - 1400 RPM. If you make a significant adjustment here, go back and repeat the idle mixture adjustment.
Lastly is the high speed mixture. Set the engine at idle speed, then set it to full throttle using the main speed control. Do NOT use a finger to manually open the throttle at the carburetor as the engine is governed and should limit its speed to around 3000 RPM. Adjust the mixture so the engine accelerates without hesitation or backfiring. This will take several tries, and is also usually around 1-1/2 turns from fully tightened. Usually it will only take a change of 1/4 to 1/2 turn to get it to smoothly accelerate. You can do this under load as well. Clockwise will result in a leaner mixture, i.e. less fuel compared to the air going through the carburetor.
None of these adjustments should be way out unless someone pulled the valves out to clean the carburetor and didn't pay attention to where they were set beforehand. If you do need to pull them out, carefully turn each one in (clockwise) and count how many turns it takes until it seats fully, but do not exceed that position, then remove it and the spring. Some needle valves also have a rubber O-ring and even a flat washer at the end of the spring in the casting, so be careful.
These procedures are most likely outlined in the several good books on small engine repair. A trip to a local library might give you more detailed information. I'm going from memory as I usually make the adjustments without even thinking about them.
Good luck. Bob

and
list of

linkage
or
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Increases or decreases the gas jet. Richens or leans the fuel mix.
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Christopher A. Young
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Unless its a water drain, and NOT an adjustment... Hard to adjust JETS when they are FIXED.

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I have seen two styles of water drains: spring-loaded types that you push up with your finger and let stuff out, and an actual 12mm bolt with a gasket that unscrews. Usually these are mounted off to the side of the bottom of the bowl, not directly in the center, and it's hard to call them adjustments. I'm pretty sure the original poster was referring to the knurled screw and spring that screws up into the center bolt that also holds the fuel bowl to the carburetor. This does go into the tube that feeds the main jet, so if you stretch the previous poster's reply, he's got all the words there to convey the message, but it could use just a few more. I understood what he was saying, and it does indeed richen and lean the fuel mixture once the throttle is off the idle position. I think the original poster will find more than enough info here if he reads all of our replies.
Bob

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Mine is the 12mm bolt w/gasket that unscrews. I've moved it around and it does affect a change in the idle, but it does nothing about the oscillation. I tend to think it is "hunting" and could believe the carb is clogged.
My thought is that it was working, then I didn't use it for several months and it was covered w/tarp, then I put a new battery in and it would not start (it would start perfectly w/starter fluid, but when fluid was gone it cut-out.) So
I took the bowl off and checked the gas lines and yes there was a lot of "junk" in the bowl (and some each time I've cleaned it.) I have now taken the bowl off and sprayed the entire system with carb-cleaner and have not turned any screws. So I don't see how it can be anything other than the bowl, and I have followed the advice of a poster about the correct position, but it makes no difference, so it must be clogged and/or a spring has failed.
I really did not want to remove this carb, it looks like a real bitch because the screws are against the muffler and on and on...and I don't want to screw up the linkage/springs. As I say, it does run when at full throttle, but I'd rather have it running correctly at all speeds.
Thanks for the dialogue, I and others appreciate your responses.

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On a completely different path, the reminder that a tarp covering it forces moisture to be trapped, and that can be even more of a problem as heat buildup accelerates the deterioration process. Ever consider just turning a plate upside down over the rope-start portion?

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Well, the only way you can completely clean it is to remove it. Those bolts behind the muffler can be hard to get out, but an open end wrench and large screwdriver should get it started, then use finger power. Chances are good you'll need to replace the carb-to-engine gasket as well, as they tend to stick to metal if left in place for more than a few years. You can pick up the correct gasket at a small engine repair shop, or get some gasket material that's the same thickness at a car parts store - they might even give you a large scrap for free. Carefully cut the appropriate holes and you're in business.
There shouldn't be too many things connected to the carb. The throttle cable, maybe a spring, the choke linkage, but that's about it. The important thing is to remember what holes everything went into. Taking or drawing a few pictures is often a good idea. Get the shop manual for the engine - one that tells you how to overhaul the carb. Buy a can of good spray carb cleaner. An old toothbrush is also helpful for getting off massive quantities of varnish and crud. I won't go into the details here, but a good mechanical inclination is most helpful. Try to work in a clean area and be careful of loose springs and other parts that may fall out of the carb as you take it apart.
If it won't idle, it's probably the idle jet that's clogged, or the float is way too low, or the throttle plate is closed too tightly. Of course, it could also be something else that I didn't mention.
Bob M.

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Just get a carburetor rebuilding kit and use it. Usually the problem is the diaphragm in the center of the (B&S) carb is warn out. Part of this runs the acceleration pump. That's what causes oscillating RPM.
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