Re: Mower misses at mid and high RPM
Sounds like you've replaced everything ignition wise and the fuel system,so maybe its a mechanical problem inside the engine..other than maybe the "kill" wire grounding out somewhere intermittently or a bad ignition switch,it sounds like you've checked a lot of things and ruled them out as suspects..
I guess with hydraulic lifters there is no "valve adjustment" on these Kohlers,I have not worked on any like yours--never had too..so that pretty much rules out the valves being out of adjustment..
It may have a valve spring that is weak or broken--that will let the valve open and close OK at lower speeds,but let it not react rapidly enough at higher rpms and cause a misfire or backfire out of the intake or exhaust..worn valve guides can let the valves not seat properly at high rpms also..
Which side the backfire comes from is your clue whether its the intake valve or exhaust valve at fault---but sometimes when a camshaft lobe wears down,which reduces the amount the valve opens and also limits how long it stays open,that can lead to a high rpm misfire too--and the valve that cant open enough,if its the intake valve,you lose power,as less fuel & air mixture gets in the cylinder..
if its the exhaust valve not opening far enough and long enough,not enough exhaust can get out into the muffler,and the engine cant inhale enough fuel air mixture to run right at higher rpms..--it also can let the built up exhaust, puff back out thru the intake & carb,fooling you into thinking the intake valve is burnt or not seating perfectly..
Unfortunately its not uncommon now to see worn camshaft lobes on many small engines--some engines now even have PLASTIC camshafts !..(which is ridiculous in my opinion!)..
The fact todays motor oils have reduced the ZDDP and zinc content makes old flat tappet camshafts lose their lobes pretty rapidly--practically everything automotive has roller camshafts now and dont need those oil additives as much,but air cooled engines DO..this is why some people like using diesel rated oils or special ones formulated for older engines..lets the cam lobes live longer..