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pollocma 04-21-2019 10:22 AM

Mower misses at mid and high RPM

I have an older Snapper 48" walk behind with a Kohler cv15t engine (single cylinder). It starts fine, idles but when I raise the RPMs to mid way or higher it misses and backfires. It did not do this when I owned for the first couple years I owned it.

I've replaced the carb (and just cleaned it again), ignition module / coil (set the gap to .010 - .012), plug (gapped to .035), fuel line, fuel filter. Yesterday, while it was doing it I sprayed starter fluid (while running) into the carb inlet and it kept on doing it or got worse. Made me think a spark issue but like I said the coil, wire and plug are all new and there really aren't any other components on the electrical side. I did order an inline spark tester where you can see the spark and I will compare with other engines I have. Is there a way to quantify the coil, spark ignition system (multimeter, otherwise)?

I will also go ahead and replace the vacuum line between the OHV and the carb inlet (likes like it has small cracks) but I don't think that's the problem. My other thoughts now are timing, maybe compression (but seems like has good compression when pulling starter cord). For timing, the lifters look like they come right off the cam and up into the head (no chain etc.), and the ignition side the ignition coil meets with the flywheel magnet so I'm not sure how these could be too far off but I suppose something could be worn etc.

Just tired of playing with it with mowing season coming. Would rather be sitting with the wife and a glass of wine than cussing out in the driveway.

Any thoughts, thanks Mike.

Tractor-Holic 04-21-2019 11:17 AM

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 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..

Tractor-Holic 04-21-2019 11:22 AM

Re: Mower misses at mid and high RPM
Just noticed the forum this is posted in--!...

This forum is where members ask questions about how to navigate the forums and any tech issues they may be having...a moderator should move this thread to the Snapper forum..

TobyU 04-21-2019 10:31 PM

Re: Mower misses at mid and high RPM
Looked for other new threads to see if it moved but didn't find so replying here.
Unlikely timing as mentioned above. I don't think it ws mentioned but flywheel key often comes should be steel on this kohler and will not be sheared but if all else fails can check it..I have seen flywheels installed with no key or it slipped out on assembly an laying on top of crankcase.

First thing: get a new plug an gap it at .028.
the spec is .030 and .035 is too wide and makes it hard on the coil to fire.
I have had more than one Command that had weak coils that would not run on .030.
.025 is fine to test it out.
Try this first. A plug can be damaged by a simple drop even in the box when new.

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