# Briggs and Stratton 12HP compression?



## dkruitz (Feb 20, 2010)

Tried starting my estate sale find mower I bought last year. No luck - I have spark, fuel, but found compression to be down at the 60psi mark - pretty low in my opinion. Pulled the head, cleaned the carbon on everything and found some scoring in the cylinder - nothing all that deep. Also noticed the motor says 12HP but the riding mower says 11HP - appears that this motor may have been swapped in to the mower at some point and would explain the electrical harness issues I've run into previously. I ran this mower late last summer with no problems. Could I have sticking rings from sitting for 4 months?

What kind of compression should I expect on a 12HP single cylinder side valve B&S?


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## SABL (Jul 4, 2008)

Any scoring of the cylinder would indicate broken rings IMO. That may explain the low compression. Other than coil/pick-up failure the first thing to check is the "shear-key" in a no start situation. Rodent damage to wiring is also possible as well as safety switches being activated. The presence of spark and fuel (even with low compression) points to a timing issue *if* the spark is strong and the fuel is fresh with no water contamination.


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## dkruitz (Feb 20, 2010)

Is 60psi OK for a side-valved 12HP B&S?


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## dkruitz (Feb 20, 2010)

Found it! The battery terminal had gotten corroded over the winter and wasn't kicking out enough juice to really turn it over with gusto and hot spark. I cleaned it all up, charged it to peak, got hot spark and it fired right up.
I'll keep an eye on the cylinder scoring and compression at the end of the season - at least I now have a baseline of 60psi cold.

Thanks for the help!


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## Basementgeek (Feb 7, 2005)

Compression is hard to check because of the way the cam is ground.
For a truer reading is to spin the engine backwards.

BG


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## bozodog (Nov 27, 2009)

> Found it! The battery terminal had gotten corroded over the winter and wasn't kicking out enough juice to really turn it over with gusto and hot spark. I cleaned it all up, charged it to peak, got hot spark and it fired right up.


That, and what you have gone through, is a perfect reason for "KISS". Keep it simple, stupid. (not pointed at you, everyone that reads a little problem into a great big one) 

Every trouble shooting effort should always start with the fastest, easiest, cheapest, most obvious fixes.


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