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AR15.COM
8/2/2011 1:36:04 PM EDT
I have a Bolens riding mower that I purchased from Lowes in 2005, mower has a 15.5hp Briggs & Stratton engine, it has had good preventative maintenence since I purchased it. The problem is this, I got the mower out to cut the yard and when I moved the throttle to full speed the engine runs rough and a little smoke comes out of the exhaust.

The engine seems to run ok when the throttle is backed off to a slower speed.

I have noticed that the last few times I have cut grass this mower is using more gas to cut the same amount of yard, I cannot figure that out either.

This morning I changed the spark plug and fuel filter and put in fresh gas, but it is still running rough at full throttle. I am thinking the problem is the carburetor?
Thanks in advance.

**Update** I purchased a can of Seafoam and ran some through the carburetor and added some to the gas. It took a couple of days of treatments before the engine started to run ok, I cut my backyard this evening and the mower runs good. Thanks again for everyones advice.
8/2/2011 2:11:10 PM EDT
[#1]
You could try to run a heavy mix of Seafoam through it.
8/2/2011 2:30:02 PM EDT
[#2]
Make sure the air fiilter is clean and nothing is blockihg the air intake port
8/2/2011 3:11:09 PM EDT
[#3]
Sounds like carb problems.  Is the smoke black?  If so, you have a situation where it's too rich.  Check the air filter first.  If it's ok, then clean the carb.
8/2/2011 3:14:30 PM EDT
[#4]
Check your air filter.

Does it rev up fine when the PTO is not engaged(ie when your blades are not turning)?
Have you cleaned your deck lately?
Are your blades sharp?

Do you use it year round or do you store it in the off season? If you store it how do you prep it for storage?

8/2/2011 4:41:18 PM EDT
[#5]
What about that crap that's in gas now...I'm having a brain fart I can't remember.
8/2/2011 4:53:52 PM EDT
[#6]
Main jet is partially plugged. Very few of the new engines come from the factory set correctly, they are set lean to begin with [thanks epa] and it takes very little to make them run rough.
8/2/2011 5:00:09 PM EDT
[#7]
Check the vent hole in your gas cap.
8/2/2011 5:00:23 PM EDT
[#8]
Thanks for all the replies, the air filter is clean, blades are good, and the mower has been running fine with no problems, until today. Tomorrow I will get something like Seafoam and clean the carburetor.
Thanks again.
8/3/2011 8:58:20 AM EDT
[#9]
Had the same problem with the same exact mower.  The engine head gasket is starting to leak.  
8/3/2011 3:17:19 PM EDT
[#10]
Sounds crazy but make sure the engine mounting bolts are tight I had this happen on a similiar mower and couldnt believe it,
8/4/2011 8:39:00 AM EDT
[#11]
Quoted:
What about that crap that's in gas now...I'm having a brain fart I can't remember.


Ethanol.

And yes, it can make small engines run oddly.