# Tecumseh engine idles up and down



## bushkanaka86 (May 19, 2015)

I am working on my mower which has a Tecumseh 640174 engine (model 143.995502). It didn't start at all, so I took the carburetor completely apart and replaced the float and was able to get the mower to start. 

However, while I was able to cut the grass, the whole time it ran the engine continued to rev up to cutting speed and drop down to idle. I could see the throttle butterfly constantly moving back and forth. There is no choke or manual throttle control on this model of mower. 

I read that this probably means the engine is running lean and I need to clean some holes in the carburetor. I ended up completely taking the carburetor apart 3 times and soaking everything down with carburetor cleaner. I made sure the main jet was clear as well as all 3 holes in the bowl bolt. After cleaning it 3 times, I can't find anything else that could be plugged.

The engine still runs but it still does the constant revving. The only solution I found so far was to completely tighten the idle screw so the engine idles at cutting speed. I don't know if this is good for the engine though.

Thanks for any help you can give!


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## 30yearTech (Sep 14, 2006)

Base on your description I would think the problem lies in the low speed or idle circuit of the carburetor. A plugged pilot jet, or low speed pickup could be the culprit. You can use brake or carburetor parts cleaner in the spray can to spray through the circuit to see if it's open and flowing.

Read though this info, your carburetor should be similar

http://outdoorpowerinfo.com/repairs/tecumseh_carb_640017.asp


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## bushkanaka86 (May 19, 2015)

I appreciate your response. As I mentioned, I did tear down the carburetor 3 times and thoroughly clean the pilot jet and bowl bolt as that website mentioned. However, reading through that site gave me 2 ideas of what it could possibly be.

1) I didn't notice any brass low speed jet like in the diagram. There were no other screws I could take out in the carburetor. I also didn't notice a low speed jet hole in the side of the pedestal but I wasn't looking for that one. I would have seen the brass jet though, I think.

2) My float was bad and I bought a new one. They didn't have a new metal one for me so I got a plastic one. The new one came with a spring that my metal one didn't have. I installed the spring like it was diagrammed and it provides a constant, low, upward pressure on the float. Could that possibly be causing it?


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## sgtrudz (May 15, 2015)

Bushkanaka,I am coming in late on this thread, so excuse me please. I just have a simple que. How old is this unit ? Before Tecumseh sealed the idle mixture screw, they were adjustable. It was a common mistake for a mechanic to be holding the carb upside down when removing the idle mixture screw while cleaning with carb cleaner, welding tip cleaners , etc. There was a small balance rod internally that floated up & down according to engine vacuum. If the idle mixture screw was reinserted & even just snugged up, It would damage the rod, making it[ the idle function] irrepairable. Again, that's only on older Tecumseh models.You mentioned looking for a brass screw.The whole carb was painted white, so it [ the idle mixture screw on the side] wouldn't appear brass. Forgive my intrusion, as 30year tech Knows alot more than I. Good luck, sgtrudz


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## bushkanaka86 (May 19, 2015)

sgtrudz said:


> Bushkanaka,I am coming in late on this thread, so excuse me please. I just have a simple que. How old is this unit ? Before Tecumseh sealed the idle mixture screw, they were adjustable. It was a common mistake for a mechanic to be holding the carb upside down when removing the idle mixture screw while cleaning with carb cleaner, welding tip cleaners , etc. There was a small balance rod internally that floated up & down according to engine vacuum. If the idle mixture screw was reinserted & even just snugged up, It would damage the rod, making it[ the idle function] irrepairable. Again, that's only on older Tecumseh models.You mentioned looking for a brass screw.The whole carb was painted white, so it [ the idle mixture screw on the side] wouldn't appear brass. Forgive my intrusion, as 30year tech Knows alot more than I. Good luck, sgtrudz


Thank you for your response. My carburetor is not painted white so I assume it isn't the older model you are talking about. On the carburetor itself, there were only 2 things that could be unscrewed, the bowl nut and an idle screw on top that simply is there to stop the throttle butterfly. As far as I can see, there aren't any other screws. This mower was given to me so I am not sure on the exact year it was made.


I did have another idea. If I hold the solid link between the governor and the throttle, the engine reaches the correct RPMs. The governor appears to be adjusted as much as possible to the full throttle position so I can't adjust it but I am wondering if the governor spring has lost some tension and needs to be replaced.


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## geogrubb (Jul 28, 2006)

I think the bowl nut, main jet, is not clean, refer to the pic below for the small hole near the top of the nut, I use the wire from a twist tie to clean it. Have a good one. Geo


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## 30yearTech (Sep 14, 2006)

bushkanaka86 said:


> On the carburetor itself, there were only 2 things that could be unscrewed, the bowl nut and an idle screw on top that simply is there to stop the throttle butterfly. As far as I can see, there aren't any other screws.



The pilot jet screws in on the side and is covered by a little plastic plug that needs to be pried out, to gain access to the screw head of the pilot jet.
The illustrated parts list of the carburetor used on the model you listed, shows the pilot jet.


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