B&S 18hp - RPMs vary on hills


Old 03-24-07, 06:34 AM
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Join Date: Mar 2007
Location: Hickory
Posts: 11
B&S 18hp - RPMs vary on hills

I recently picked up a used 10 year old MTD lawn tractor with a 18.5 hp Briggs & Stratton TwinII I/C engine (model 42A707). First thing I did when I got it was to replace the fuel & air filter, both spark plugs, and change the oil & gas. It seemed to run fine and I cut the yard with it.

However I noticed that the motor RPMs vary when I go up or down hill. The motor runs noticeably faster downhill, and slower uphill. The RPMs also seem to slow down slightly when I do a tight turn. Also, fuel leaked from the carburetor after I ran it, so I went ahead and did a complete carburetor rebuild, including the fuel pump membranes/gasket. Again it runs fine and no longer leaks fuel, but still has the RPM problem... what next?
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Old 03-24-07, 07:44 AM
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Unless the change is drastic and or effects performance, it sounds normal. The difference being a static/no load condition and load condition. A ten year old engine is likely to suffer some compression loss which may make these conditions more noticeable.
Downhill=No load, the engine can coast as gravity aids in power.
Uphill=Load, the engine has to work harder to pull, against gravity, the weight of mower and operator as well as cut the grass.
Tight Turn= Also a load condition as turning the wheels increases friction causing the engine to work harder.

If it is to a point where it seems it is faltering, you may be able to adjust the carb. Valve clearances can also be checked and corrected.

Make sure the fuel lines have no restrictions and are not soft enough to collapse easily. Also try loosening the fuel cap a bit and note any change under adverse conditions.
Old 03-24-07, 11:04 AM
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Join Date: Mar 2007
Location: Hickory
Posts: 11
Thanks for the reply. That makes perfect sense, it just seems more noticeable than I thought it should. I think the fuel lines are fine, but I'll try the gas cap trick, too.

I have one other question about this motor: *** EDIT: I will start a new thread with this question ***

Last edited by Monty NC; 03-24-07 at 05:16 PM.

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