Strange Kohler 15.5 hp issue

[Charlie_S]

Kohler 15.5 hp, in an older craftsman mower. Engine starts good hot or cold, but it backfires through the muffler when under no load at all throttle settings. Not a full blown out of time or fuel issue backfire, but sort of like the exaust popping one would get when throttling back with no muffler installed.
Engage the clutch it mows fine and no popping back through the exaust, (same with a cold engine also) power is good and mows well.
So to reiterate, the only time it pops back through the exaust is under no load with engine at normal operating temps. Cold engine under no load, or with the deck engaged mowing it runs perfectly.

If it were the opposite, with the engine acting up under load but running fine with no load, right away i'd start looking at plug, timing and possibly a jetting issue, or of course a carb rebuild due to plugged jets and so forth causing a lean condition under load.
Oil's changed regularly, as is air and fuel filter maintence. carb hasnt been opened up nor has it been tuned in several years. Spark plug burn is good, a bit darker than tan but not sooty black indicating overly rich or crusty from oil consumption.

Any thoughts?

 

[Lawnranger]

You mentioned that the machine is "older" so if it were mine I would start by running several tanks of fuel treated with Seafoam fuel system cleaner (available at walmart & auto parts stores) just to see if it helps. Without closer inspection it can be difficult to diagnose but you could check to see if the flywheel key is partially sheared. I have read about this problem on forums and even seen it first hand but the owner did not want to pursue a fix so we let it go as is and I have not read where someone had fixed the problem.

Check compression and perform a cylinder leak down test - possible valve/valve seat? If you think about it, theoretically the only way to get a backfire out the exhaust is for the exhaust valve to be open or leaking when combustion takes place or else how would expanding gas pop out the exhaust? I suppose if unburnt fuel hit a hot muffler it could make a backfire sound but then the engine would have to be running rich and that would show up in the color of the spark plug and you already verified the plug.

Those are the ideas that come to mind and make sure to tell us when you ultimately fix the problem.

 

[Charlie_S]

All good thoughts and as i have time i'll do further checking. As far as fuel treatments, i routinely use the mercury marine fuel additive as well as fuel stabilizer in my fuel cans when i fill them.
While a partially sheared key on the flywheel is possible, i discount that because that would bring timing issues as well as starting and running well under load issues as well. Worth checking but not priority I dont think as it runs well under load.
Compression test makes sense and i suppose an exaust valve thats burned a bit might be the culprit.
Again though if a valve is burned enough to cause any problems it should show up with hard starting and problems under load, not just after the engine gets hot and running with no load on it. It's one of those things one needs to hear and run the mower to really discribe the issue.
I guess the next step though is compression and double check the timing (again) and go from there.

 

[Lawnranger]

Don't discount performing a cylinder leak down test in addition to testing compression. Cylinder leak down will allow you to listen for air leaks in the intake, exhaust and crankcase and it's a valuable tool to have in your arsenel.

 

[Rivets]

I would definitely look at a problem with the exhaust valve. LawnRanger is spot on about fuel hitting the muffler created by a rich mixture, but this would have occured overtime a longer period of time. Both the spark plug and a good compression test would lead to this conclusion. You could try leaning out the fuel a little. My guess is a stretched valve or worn valve guide. Both these conditions would be consistent with backfiring. A stretched valve would be held open too long and a worn valve guide would allow the valve to cock a low speeds but run properly as engine speeds up.

 

[motoman]

Like a broken record I repeat... look at the exhaust valve guide positions in the heads. Only because it happened to me. There was lots of coughing and backfiring before losing the cylinder. (Intek 24)

 

[chance123]

I agree with all of the above. When you say "It is an older machine" How old is it? The "old" Kohler engines relied on the old leaded fuel to lube the valve stems. I often times find valves actually sticking or binding it their guides.

 

[Charlie_S]

by "older mower" i'm thinking early to mid 90s, as i bought it used in about 99 and it was several years old then, but well into the era of unleaded fuels.
Compression is good, a nominal 110 psi, so i'm not seriously considering valve problems at this point, as i'd think a bent/sticking/burned valve would result in lower compression. unfortunately my guage doesnt support a leak down test but turning the engine over by hand to compression stroke, holding my finger over the plug hole for 10 or 15 seconds does result in a little pressure escape when i move my finger, not the most accurate but does indicate compression is holding.
i'm starting to think about possible slight timing issue but that should manifest itself in hard starting/dieseling at shut down if it was off much.
All the above suggestions while good ones i'd think would result in running rough when under load, at least all the engines both automotive as well as small engines i've seen with issues would run worse with a load on them rather than running ok working them and having problems at a static (non loaded) condition.
I also suscribe to the theory of check/change/repair the inexpensive things first so next as time allows will be check timing, a new plug as part of the oncoming winter maintence, perhaps a little octane booster. I'm also wondering if some of the issue isnt thanks to the illistrious epa regulations that often result in less than optimal performance.

 

[Rivets]

Just reread the entire thread and I think we missed something. You have popping through muffler during no-load operation. We assumed exhaust valve problems, but missed another possible cause. If the engine is running rich, unburned fuel could be igniting in the hot muffler, causing the same symptoms. Under load the engine slows down slightly and allows the extra time to burn this rich mixture. What color are the plugs, do they show a lean (white) or rich (black carbon) mixture? Is the choke fully opening? Just some thoughts.

 

[Lawnranger]

Just reread the entire thread and I think we missed something. You have popping through muffler during no-load operation. We assumed exhaust valve problems, but missed another possible cause. If the engine is running rich, unburned fuel could be igniting in the hot muffler, causing the same symptoms. Under load the engine slows down slightly and allows the extra time to burn this rich mixture. What color are the plugs, do they show a lean (white) or rich (black carbon) mixture? Is the choke fully opening? Just some thoughts.

I addressed that in post #2 but he mentioned that the plug burn is good, a bit darker than tan but not sooty. I'm still thinking on this one and would definitely check the flywheel key for partial shear.