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Whats wrong with my new (to me) mower?


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My father-in-law pays to have his lawn care done and he had a nice Craftsman riding mower he wasn't using any more so it followed me home. :D

All I have in it so far is $35 for 2 new tubes for the back tires. I'll have to get a new battery for it. The one thats in it has a date of 2002 on it. It's a 12 horse Briggs and Stratton w/ electronic ignition.

Anyway...I put fresh gas in it and used the jumper cables and my pick up and she fired right up...but it's ain't running right.

I took some video of it with my birthday present (Flip Video) and up-loaded it to youtube.

YouTube - 12 horse engine running bad

Whats wrong with a small engine that runs like the one in the video? :confused:

Edited by KevinD
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It's starving for fuel so the problem could simply be the tuning screw on the carb but modern gas varnishes up pretty quickly so it may have all sorts of trash in the carb or the inline gas filter. Since it runs well enough, change the filter then buy a can of Chemtool and dump it in the gas tank. Open the throttle and let it sit and run until it runs out of fuel. Put in fresh gas and adjust the hi-speed screw on the carb. If none of these things fix it, you'll have to clean and kit the carb. That's is no big deal if you have a little mechanical aptitude.

There may also be an adjustment on the governor - but it's best to get the right Craftsman manual and see if they address it there.

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Dose it run like that at high speed? If so the main metering jet, in the carburetor is partially plugged. If it only dose it a idle the idle mixture is not right. Either way the carb. will need the be rebuilt. The surge up and down is the engine governor working when RPM drop the governor opens the throttle. When RPM increase the governor closes the throttle. A new spark plug would help as well. It is running too smooth, and fact that it is running tells me it can't be a electrical problem.

There are no idle or high speed adjustment on this carb. They are adjusted at the factory and sealed. The only adjustment you have is the float level inside the carb bowl.

Hope this helps

Edited by LarryM
Carb adjustment addition.
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I agree with the above, however it may also do this if the air filter is real dirty. Chances are though it is a fuel issue that is causing it to pulse. If it isn't the air filter (you will know as soon as you take off the air filter if it is that) then I would pull the carb off (make sure the engine is off and cold!) and gently remove the bowl. It will probably have some junk in the bottom that should be cleaned out. Then remove the float and inlet valve clean well with some carb cleaner and reinstall. This short and easy procedure has cured about 70% of the times I have had this problem brought to me... If not this then it is time for a kit and a real cleanup.

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Thanks guys. I'll be pulling the carb and going with Ironhorse's "short and easy procedure" first. I'll also get a new plug, air filter and in-line fuel filter while I'm at it. And depending on how it well comes apart I may need fuel lines too.

Failing that what do I need to ask for in a rebuild kit when I go to the shop?

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  • 1 month later...

I work in automotive repair. It's sometimes nightmarish to see what the lack of a tune up can do to an engine. Go with your instincts about replacing the plug, air filter etc. If you're thinking of rebuilding the carb yourself make sure you know what you're getting into. Sometimes its worth the money to have someone familier with small engine carbs rebuild it. I attempted my own rebuild on a snowmobile carb once, and I just couldn't get the darn thing set up right when I was done.

Edited by Hammered
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