Small Engines (Lawn Mowers, etc.): Lawnmower air intake problem, little butterfly, butterfly valve

Question
From the original question text ... "bought a new air filter, soaked it in oil (30W) and wrung it out, and put it on the mower. "

First thing I thought of, too. Already tried that. :)



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Followup To
Question -
Thanks for the reply, Mark!

In answer to your question...

"a plastic plate with a tiny hole that is located inside the carb throat(where the air passes when the air filter is taken off)"

- YES that is it exactly!!! It hinges in the middle. When it opens, the mower runs faster.

This motor @ 3.75hp seems stronger than the mower I had before from wal-mart which claimed to be 5.5hp. Maybe the sprint motor is better than a run of the mill B&S motor?


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Followup To
Question -
hi Mark!

I just bought a used lawnmower with a 3.75 HP B&S engine. Not sure what year... It says "sprint" on the deck.

The first time I fired it up I cut the whole lawn no problem. The next week, I pulled it and it sputtered and smoked and sputtered and coughed. I removed the Air filter, and it started running much better instantly. I shut it down, bought a new air filter, soaked it in oil (30W) and wrung it out, and put it on the mower.

I started it up and it sputtered and choked etc. I pulled the air filter off again and it started running better. I found that if I turned the little butterfly valve that controls the amount of air the engine gets, it ran faster and stronger the further I opened it up.

I wedged a small toothpick sized twig in the mechanism so that it could not shut all the way, put the air filter back on and the mower fired up and ran (a little fast, but OK) I can still control the speed somewhat with the throttle.

My question: What SHOULD I do to make it run properly? What mechanism causes this valve to move on it's own? How far should I tear it down and clean / rebuild it to get everything doing what it should?

I only paid $40 for the lawnmower... in OK condition. Is my best bet to leave the stick wedged into the valve and be happy that it runs?

Am I going to burn it up running it like this? It seems like it might be running a little lean (more air than it needs) (Is that lean or rich?)

Any ideas would be welcome!! - Thanks Mark!
Answer -
Hey Chris,
The mower in this case is/was running lean but is now with this toothpick wedge is mounted it hold this valve open. What I do not know is where the valve is located. Is it on the carb. Is it or does it look like a golden plate(or it may be plastic)with a tiny and hole that is located inside the carb throat(where the air passes when the air filter is taken off. Please let me know where this valve is located and I will answer your question properly.
PS. Thats a pretty good price for a B&S Sprint engine
Answer -
Hey Chris,
I know your problem. Try replacing you air filter and take the wedge out of the valve. It should stay shut. Replace filter and let me know how it works out.

Answer
Hey Chris,
Try cleaning the carb. Soak the carb and gas line for about two hours. Then use pipe cleaners or compressed air. Re-soak the carb in fresh gas and re-install. Also try replacing the carb diaphragm(the black flexible thing in the carb). If this dosent work I dont know what will.        :(Please write back if this doesnt help.