View Full Version : 25 Kohler miss on full throttle

02-09-2008, 02:19 PM
I have a 25 hp Kohler on an Exmark and it has a miss or slight backfire at full throttle. It isnt burning all the fuel. Im hear there is some kind of advance module. Can I disable it?

02-10-2008, 02:33 PM
model is CH25S spec # 68695 serial # 3124809131 if that helps. It is not always a consistent miss/backfire but sometimes it is really worse. Sounds like a burnt valve sometimes but sometimes it is non existant but almost always is happening now. I looked up and there is a module speed advance 24-584 09s that I am thinking of buying to try but dont want to just plug and try if there is any help id appreciate it. Restobob?

02-10-2008, 04:11 PM
Does that engine have one coil, or two?
I'm thinking one. But I had a Briggs with two that had that problem, and it was a bad coil. If you have 2 coils, you can pull one plug wire, then the other and see if the popping stops, and switch the coils to see if it swaps cylinders.

02-10-2008, 04:19 PM
It has 2 coils but it does make some sense that 1 of the coils could be weak or going bad so I will give that a shot. Thanks!!!

02-10-2008, 04:41 PM
is it blowing black smoke?

02-10-2008, 05:35 PM
Seems like some black smoke with unburnt fuel smell like if the choke was stuck.

02-10-2008, 05:39 PM
My 27 Kohler consistently does that if it gets wet. Can be from washing it, rain, or just a really damp day. One of the coils is going bad, I need to replace it before I start in the spring.

02-10-2008, 07:34 PM
It seems to be worse the longer the machine runs but rain hasnt made it a whole lott worse.

02-10-2008, 09:03 PM
if it is blowing black smoke the your problem can be with the air, fuel, or spark.
To rule them out start the engine when the black smoke comes out remove the air filter and pre cleaner if it gets better clean the pre cleaner and get a new air filter. ( better check to make sure there isn't a ton of junk in there before you start it, that way it doesn't get sucked down in when the engine is running. Even if your air filter looks clean it can still be the problem. Next check the plugs by pulling one side's wire off then replace it then do the other side if the engine stops the opposite side coil is bad. then taking them out and watch the arc between the electrode one at a time.It should be blue. If it's a fuel problem you should probably take it to a dealer because there are a lot of thing to check on that side. You can also check the valves and compression with a tool called a leak down tester. Briggs and Stratton makes a nice one. At my shop 9 times out of ten it's the air filter or pre cleaner is dirty or some times clogged with a fine mist of oil that you can't really see. One more thing, if the coil or plug is bad you would notice more smoke and a loss of power when you are cutting or trying to go up ahill

02-10-2008, 09:27 PM
hyave you been up with the oil changes?

i had it happen once where we never did oil changes on a brand new exmark 25hp all year.....the oil sump clogged with black burnt oil sludge and it would give me no power on one side...it wass like the valves were staying open and i wasnt getting any compression...cause the top end wasnt getting oil

it would only do it once in awhile after being warmed up, tried replacing valves, rockers and springs and it took disassembling the whole motor before i figured it out

hopefully its nothin that crazy

02-10-2008, 10:56 PM
Well it does really work worse on a hill so it could be a weak coil or plugs but I think I tried the plugs with no difference. It does seem to make more of a backfire/miss when it goes on a hill angle in a particular direction. . The machine was purchased used so I do not know the oil change history and I do believe it was a long while between changes but I pulled valve covers off and it was clean as a whistle so the carbon sludge might be there but not on the top end.

02-10-2008, 11:34 PM
Ok I think specifically on Exmarks (and toros) the Kohler is one engine where choke is just past WOT, so whatever you do make sure you're not accidentally choking it if only a little, that might be all it is, unless you have a separate control for the choke.

On another note if you haven't changed the oil since you got it, ehm, yeah it be a good time for that otherwise just keep it on schedule.

02-11-2008, 02:43 PM
When I unplug a spark plug on one cylinder it slows some but smooths out if i unplug the other it slows a lott but still spits and backfies does that mean the one that is still hooked and spits and backfires is weak and or bad?

02-12-2008, 07:43 PM
THANKS for all the help! It turned out to be a weak coil wire connection on one of the coils. Now runs really strong with no backfire or miss. It does diesel after shutting off and I cant figure that out?? I unplug the plug wires and it keeps going?? any ideas?

Two Seasons
02-13-2008, 01:57 AM
dieseling usually means you have carbon build up in the cylinder head. Carbon gets red hot and acts as the "spark" for continued combustion until it fuel starves.

Seafoam is a product that helps to break carbon down and blow it out the exhaust.