Engine runs rough for a while, then smooth for a while, repeat

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My Craftsman DYS4500 (Kohler "26hp") lawn tractor-mower started running really rough the other day. I was mowing, and suddenly it just didn't want to drive the blades very well, so I disengaged the blades, took it up to my shop, and just when it got near my shop, it started running clean again. I mowed for about another hour, and then it ran rough again for a couple minutes (long enough for some $#@!%#@ to fly) and then cleaned up and ran fine. The next day, it was running rough and wasn't showing any sign of running better, and so I pulled the carb.

Carb is clean. Suspiciously clean, I'd say, because I've *never* had it apart, and the tractor's probably about 10 years old. The float was clean, everything seems peachy. I haven't put the carb back on to see how it runs now, but I'd be surprised if anything's different.
Fuel filter is clean and fuel flowed out of the outlet just fine (onto the ground, before I had the catch jar in place ).
Air filter is "clean enough". Spark plugs were replaced last fall, and look fine now - no obvious fouling.

Any ideas?

Only interesting thing I can think is that a couple days before this happened, I had the front end jacked up off the ground, so the whole tractor was tilted quite a bit (my main pulley had fallen off and I had to clean out the threads to get a new bolt in to hold it on - a bit more clearance was helpful). Maybe something got stirred up in the tank? - but, the fuel filter is pretty clean...

 

[Gary Fowler]

Replace the spark plugs. I had a boat engine do the same thing and drove me nuts chasing fuel problems. Even removed the plugs, visually inspected, cleaned and replaced. Boat shop mechanic immediately knew what the problem was and just told me to buy a set of plugs to replace the shorting out ones. He said the Japanese plugs were bad about insulation breaking down and would do just as you describe your lawnmower doing.

 

[kthompson]

Gary that is interesting. Have a mower with a Kawasaki doing same thing. Back in shop second time and and the person said it sounded like fuel issue but it could be electrical. Think will call shop and pass on what you said. Funny thing is on mine have replaced fuel filter even though fuel was flowing fine through it and it would run fine for say 15 minutes and then issue begin again. Could be the time to change the filter (walking to shop to get filter and pliers and then to walk back to mower and replace the filter was probably 10 to 15 minutes) allowed engine to cool enough to allow connection issue to close.

 

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Gary that is interesting. Have a mower with a Kawasaki doing same thing. Back in shop second time and and the person said it sounded like fuel issue but it could be electrical. Think will call shop and pass on what you said. Funny thing is on mine have replaced fuel filter even though fuel was flowing fine through it and it would run fine for say 15 minutes and then issue begin again. Could be the time to change the filter (walking to shop to get filter and pliers and then to walk back to mower and replace the filter was probably 10 to 15 minutes) allowed engine to cool enough to allow connection issue to close.

I agree and will definitely try swapping the plugs - cheap and easy enough to do! Will report back with results.

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[PILOON]

Replace the spark plugs. I had a boat engine do the same thing and drove me nuts chasing fuel problems. Even removed the plugs, visually inspected, cleaned and replaced. Boat shop mechanic immediately knew what the problem was and just told me to buy a set of plugs to replace the shorting out ones. He said the Japanese plugs were bad about insulation breaking down and would do just as you describe your lawnmower doing.

Hopefully your mechanic does not suffer from ego problems.

I have learned the hard way that U should never even suggest what a problem (much less the cure) might be.

Recent example:
My Kia demonstrated a vibration problem coming from the rear end.
My car was on the lift for annual tire change and I asked (was even on the work order) them to start her up in order to determine if it was a wheel bearing, drive shaft or whatever.
Was told to mind my business and leave it to the pros.
Well he showed me as to how much a pro he was as he refused stating that it would de program my computer (???)
Lots of dealers simply 'shotgun' (mainly to sell parts) as well.

Give me a small independent any time!

 

[randyjs]

That's how mine was acting up before one coil finally went dead.

 

[WVBill]

I had a Zero Turn with a 19HP Kawasaki that exhibited the same issues. Changed all the filters - didn't help. Then Google came to the rescue and I found that the air filter on that engine/carb was supposed to have an "integrated bypass" or something (this was over 5 years ago so memory is thin). Anyway, even though the filter appeared clean, somehow the bypass was clogged and the first replacement, while listed for a 19HP Kawasaki, was NOT the right filter for the 19HP Kawasaki with that particular carb.

Not saying you have the same situation but might be worth it to replace the air filter with one that matches the EXACT part number of the OEM filter.

 

[tomplum]

It does sound like it is dropping a cylinder. Plugs are a great start. Should it reoccur, you will want to post the engine model and spec off the tag. Some Kohler twins it is a simple coil swap, others are more involved.

 

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So, I did "something" wrong putting it back together it seems. I put the carb & air intake back on, noticed that the connector to one of the points was loose (probably my root cause in the first place that I didn't check previously - I pulled the connectors and cleaned them up to get all the lawn fuzz out and dielectric greased and put it back together).

When I crank the engine, it'll catch briefly, and then cough black smoke and die, and seems to be way too rich (my pathetically uneducated guess).

From what I can tell, this carb only has a single adjustment, a screw that goes in the top just behind the air filter. Not sure how that should properly be.

This is the kohler twin cyl ohv 26hp engine - don't know the model as there's no apparent tag on the engine; the only tag I can find anywhere is on the mower body itself.

 

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When I pull the carb off after failing to start, a bit of gas dripped out from the front of the carb where the air box goes.

Here are some carb pics.