Small Engines (Lawn Mowers, etc.): scoring, lean mixture, air leaks

Question
Can u explain the difference in piston scoring I.E. what causes intake side scoring and what causes exhaust side scoring

Answer
hi jess


piston scoring can be caused by a number of things  

scoring on intake side of piston  and is usually not as bad as ex side  

stale fuel can cause scoring on intake side ,  so can dirt entering in front of air filter
and also lean  mixture from carb setting -c/case air leaks ( from leaking gaskets / manifolds ) & air bubbles in the fuel system ( these lean out mix and make it run hot as well as not enough  oil in petrol machine is running on   hi loading of the saw and overrevving of the engine could also cause it

scoring on ex side of piston  is usually caused by lean mixture , air leaks - not enough  oil in mixture engine is running on or entry of dirt in front of air cleaner and overrevving of the engine

shows up as deep scores on piston / rings / bore  that will definately be caught by a fingernail  and would be very hard not to miss when muffler was removed
compression and power are directly affected  and units typically have low power , hard to start, wont idle , stall out  if they do sort of idle  it goes up then down  - inconsistant

most times if unit has bad / deep scoring all the way around piston incorrect  oil mixing has caused fault and unit would most times not be worth repairing

if so  better to buy new  as repair can be more than a new saw


cheers paul