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Greetings. Re: Craftsman 17.5 HP 42" Mower with Briggs and Stratton 31C700 Engine. The engine runs smooth at lower speed but when I accelerate, it sounds like a machine gun. I found that throttle spring was disconnected and I put that back. I also put a new plug in. No difference. Thanks for any help.

First lets make sure there is nothing hitting the pulleys on the bottom of the engine, as well as anything hitting the flywheel. From there lets valve clearance, and determine if the noise is coming from the crankcase area or maybe something on top of the piston.

ILENGINE said:

First lets make sure there is nothing hitting the pulleys on the bottom of the engine, as well as anything hitting the flywheel. From there lets valve clearance, and determine if the noise is coming from the crankcase area or maybe something on top of the piston.

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I have the engine cover off and that exposes the end of the air intake pipe and I can see that opening as I sit in the seat, I start the engine and have the throttle at about half way up and the engine runs smooth. Now, when I push the throttle lever up to increase the speed, the engine starts popping, not like a back fire pop, but like a steady popping. It's almost like it's going to choke out, but it doesn't. It seems like it came on over the length of time it took me to cut the lawn which is about an hour. I guess you could say that I was racing to finish the job because I thought it was going to quit. After I got finished, I thought that it might be the valve clearance since it was getting hard to start. While I finished readjusting the valves, I also put a new plug in. Too, I noticed that the throttle spring (that very thin one) had disconnected so I connected that back on the throttle lever. It started up much better since I adjusted the valves but the problem still exists.

The really thin one that wraps around the throttle lever is a damping spring and has little to do with the overall running,\.
It is there to smooth out acceleration & deceleration so you dont get over run when the throttle moves.

If your machine gun sounds like it is in the exhaust, pull of the top cover and check the flywheel key.

bertsmobile1 said:

The really thin one that wraps around the throttle lever is a damping spring and has little to do with the overall running,\.
It is there to smooth out acceleration & deceleration so you dont get over run when the throttle moves.

If your machine gun sounds like it is in the exhaust, pull of the top cover and check the flywheel key.

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OK, the flywheel is OK. I did say in my earlier post that I had the top cover off and it exposed the air intake pipe. Of course, the machine gun like noise is coming from the exhaust but appears to come from that air intake pipe. Having said that when I accelerate, it sounds like that machine gun like noise is coming from that pipe because I can see some smoke being expelled from it and there is accumulation of gas which leads me to believe that there is too much gas going into the engine when I accelerate. Too, I have a brand new plug installed and when I took it out, it was black. I just have find out whey that is happening.