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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?

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.

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.

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.

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.

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)

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.

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.

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.

Rivets said:

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.

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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.

Charlie_S said:

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?

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Just thought that it could be some carbon deposits on the top of the piston causing a ~~~glowing~~~ and thus a pre-ignition condition. Igniting the air/fuel mixture too early on the low end could cause that popping effect but as the engine speeds up, the timing would be closer and thus reduce or eliminate the popping/ pre-ignition. Wouldn't hurt to check the piston and head for carbon deposits. Hope this helps.

I suppose carbon buildup could be at least part of the issue, though compression reading isnt excessivly high like it normally would be with alot of carbon on the piston.
the backfire/popping issue is at all throttle positions though not just one particular engine speed so whatever the cause its consistant when under no engine load scenerio.
The real stumper to this whole issue is everythings backwards, when problems arise they usually manifest themselves under working conditions not no load conditions.

Your engine is somewhat of a mystery. Popping and backfiring from my experience has been timing issues and valves and these were addressed on here. On older engines bad points and condensers going out could cause popping and backfiring. Just wondering what were your measurements on both intake and exhaust valves cold and hot? Other mechanics suggested a stretched exhaust valve when it warmed up. That would be enough to make the valve stay off its seat slightly and still have enough compression to fire. I know you ran a crude compression test and obviously it has enough to run. As far as my previous suggestion of carbon deposits glowing, it's a remote possibility. Most of the time glowing carbon makes one diesel and sputter on after you turn it off. I'd measure the valve clearances and possibly pull the head just to see what things look like. Let us know what you find.

When the exhaust guide moved up in mine (over time) it apparently ran (well, by the way) until insufficient mixture was being admitted and or excess burned mixture was left upon intake charge (couldn't escape) to ignite. This because the exhaust valve was only lifting about .100" The torquey engine, huge flywheel and governer really covers a host of sins in these engines IMO. As noted compression was ok even when the engine went to one cylinder (Intek 24 V) . :smile:

In my opinion, you are going to have to do one of three things to get to the bottom of this problem.
1. Perform a compression test with a gauge to get an actual reading. Your thumb can be pushed off with as little as 30psi.
2. Perform a cylinder leak down test, to see if the cylinder is actually sealed.
3. Pull the head and inspect all the parts to visually verify they are good and working properly.

Without this we are only guessing, which is going to lead to a false diagnosis, big bucks spent, wasted time and frustrated individuals.

Rivets said:

In my opinion, you are going to have to do one of three things to get to the bottom of this problem.
1. Perform a compression test with a gauge to get an actual reading. Your thumb can be pushed off with as little as 30psi.
2. Perform a cylinder leak down test, to see if the cylinder is actually sealed.
3. Pull the head and inspect all the parts to visually verify they are good and working properly.

Without this we are only guessing, which is going to lead to a false diagnosis, big bucks spent, wasted time and frustrated individuals.

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If you've noticed, almost every test recommended has been poo-poo'ed and dismissed, won't even give it a second thought to try what's recommended. I'm finished with this one. Have fun guys.

Personally, without hearing it, I would suspect it is only unburned excess fuel contacting the hot muffler. If otherwise the performance is satisfactory, I wouldn't worry about it. If it is something that "could" become serious, you should "start" to notice changes in the engines sound and performance before any serious damage happens.

Lawnranger said:

If you've noticed, almost every test recommended has been poo-poo'ed and dismissed, won't even give it a second thought to try what's recommended. I'm finished with this one. Have fun guys.

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IF you'll accurately read what i wrote previously, I did mention compression was 110, and that i did a primitive leak down test holding my thumb over the plug hole BECAUSE at this point i dont have the capability to do an accurate leakdown.
Sadly, unlike some apparantly I have real farm work that needs done so i cant jump to all suggestions at everyone's beck and call.
Some suggestions i dont give a second thought to simply because several of you misread my origional post as to mean it ran bad when mowing which is not the case.
In Fact, a couple did catch on and realize its just the opposite, Thus some suggestions are not relevant, and others while they have some merit, but still dont make sense simply because it runs well when working but not well static with no load.
Still, when time allows i will check a few things suggested, but again, I dont always have the luxury of jumping to things at everyone elses schedule so for those of you who made helpful suggestions thanks. to the rest that dont get it other things do take priority sometimes go **** yourselves, i'm outa here. And herein lies the main reason i seldom do forums is the assholes that dont realize things cant always be done immediately to they're liking.

Charlie, I taught small engine repair for 30+ years to individuals for 14-65 years old. I have also worked in the field for over 25 years. During that time I have dealt with true idiots and experienced professionals. Some of these people shouldn't be allowed to change an air filter, but they think that are mechanically minded. That could be both the idiot and the "pros". You came to this forum looking for help. That means to us that you wanted our input. Most of the proffesionals here realize that people who come here don't have the money or a vast amount of time to do the work immediately. We understand that you may not have the tools we recommend using. With that being said look at it from my side of the postings. I cannot see the engine or equipment you are working on. I must use your descriptions and my past experience to visualize what you are doing and then try to give you suggestions as to how to solve your problem. I know that most people don't had all the tools I recommend using, but still hope that they might be able to borrow one. Some threads are not completed or solved for weeks and even longer, due to both sides not having the time test, research, or post back. I also make mistakes and ask people to do things more than once, because I don't reread every thread completely, to review what has been asked for or been done. Being on this forum is like a classroom, many individuals wanting help at the same time. Do you realize that many of the pros on this forum spend time looking up parts or researching manuals to help individuals solve their problems. We don't get paid, but just enjoy passing on our knowledge to those who appreciate our help. You called all of us a bunch of names and told us what we should do. How would you feel if those terms were used toward you, when someone disagrees with the knowledge you have in your chosen field. We disagree with each other all the time, but that doesn't mean that we can't learn from each other. I know that because of your comments no one is going to help you, but I do hope that you solve your problem and learn from this. Rivets

Charlie_S said:

IF you'll accurately read what i wrote previously, I did mention compression was 110, and that i did a primitive leak down test holding my thumb over the plug hole BECAUSE at this point i dont have the capability to do an accurate leakdown.
Sadly, unlike some apparantly I have real farm work that needs done so i cant jump to all suggestions at everyone's beck and call.
Some suggestions i dont give a second thought to simply because several of you misread my origional post as to mean it ran bad when mowing which is not the case.
In Fact, a couple did catch on and realize its just the opposite, Thus some suggestions are not relevant, and others while they have some merit, but still dont make sense simply because it runs well when working but not well static with no load.
Still, when time allows i will check a few things suggested, but again, I dont always have the luxury of jumping to things at everyone elses schedule so for those of you who made helpful suggestions thanks. to the rest that dont get it other things do take priority sometimes go **** yourselves, i'm outa here. And herein lies the main reason i seldom do forums is the assholes that dont realize things cant always be done immediately to they're liking.

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WOW!!! Now there's a real attitude of gratitude! Sheesh!