Kohler Command Twin Running Rich

[Mr Mower Man]

I have a Craftsman lawn tractor in the shop that really has me flummoxed. It's powered by a Kohler CV730-0043. The customer's original complaint about the machine was that the exhaust smell was so strong when he mowed grass with it that he had to change clothes afterward. Furthermore, it seemed to lack power. He said he changed the oil and spark plugs a while back and it seemed to do better for a little while.

I could tell right away when I started the engine that it was running a little rich. The first thing I noticed was that his air filter was filthy, so I took it off to see if the rich condition stopped. It didn't. I also noticed that the orange seal under the air filter was gone and the clean air side of the air filter was dirty. His oil was black and a little low as well. I checked the spark plugs and saw that they were badly fouled with a lot of carbon. I also noticed that there was a lot of black soot around the exhaust pipes and muffler. There's no hour meter on the tractor, so I don't know many hours the engine has on it. The serial number on the engine is 3506801731, so I guess that means it was built in '05, right? It could have a lot of hours on it.

The first thing I did was replace the fouled plugs with some new NGK's. The engine sounded nearly OK at first. Then I activated the PTO and let the blades run for a little while. I could smell the rich exhaust. When I disengaged the blades, the engine was clearly blubbering in a rich running condition. So, with a little help from my Kohler tech support, here are all the things I checked:

Compression. Cylinder #1 was at 165 PSI and cylinder #2 was at 160 PSI. Service manual recommends a minimum of 160.

Leakdown test. Both cylinders both had low leakage (around 15%-20%), and I couldn't hear any air escaping the intake or exhaust.

Valve movement. Kohler tech support told me it sounded like I might be losing an exhaust cam lobe, so I should check to make sure all my valves were moving about 5/16". I checked them, and they were moving that distance.

Timing. Kohler tech support also advised me to verify that the timing wasn't off. I didn't pull the flywheel, but the key didn't appear to be sheared to me.

Carburetion. I actually put the carb in my ultrasonic cleaner and put a rebuild kit in it. I thought perhaps I had a partially blocked air bleed. This didn't work.

Accelerator pump. Kohler tech support advised me to disable the accelerator pump by taking the hoses loose. If the rich condition stopped, then there was a problem in the accelerator pump. I removed the hoses and nothing changed. That wasn't it either.

Balance test. The engine was running at about 3050 with both cylinders. It ran at 2970 on cylinder #1 by itself, and at 2870 on cylinder #2 by itself.

Ignition. Weak ignition can be a cause of fouled plugs. Since cylinder #2 was about 100 RPM's slower than cylinder #1 in the balance test(and the exhaust fumes seemed to be stronger when running just on #2 -- but that could've been because that's the side the muffler was blowing the exhaust and that's where I was standing), I decided to swap ignition coils and spark plugs to see if that would make cylinder #1 run 100 RPM's slower than cylinder #2. No change whatsoever, so that isn't it.

Restricted exhaust. I wondered if the muffler might be plugged, so I removed it and ran the engine. Aside from being a lot louder, there was no difference in the rich running condition. I even removed the exhaust pipes to inspect them and the exhaust ports in the cylinder heads. While everything was lined with carbon, nothing appeared to be plugged.

Crankcase breather. I noticed that the breather hose was spraying some stuff into the air intake, so I removed the hose from the breather. This actually did change the way the engine ran just a little. It wasn't quite as rich, but it was still rich. So that wasn't my problem either.

Crankcase vacuum. Just in the interests of being thorough, I made sure there was vacuum in the crankcase when the engine ran with load and with no load. There was.

Oil condition. As I mentioned, the oil looked ready for a change. And I wasn't sure but that it might have been a little bit diluted because of the rich running condition. I thought this might be a cause of the spray coming from the breather hose into the air intake. So I changed the oil and filter. In the process, I discovered that the oil filter nipple was loose, so I'm glad I decided to change the oil so I could tighten it. And I think the oil had been slightly diluted, since what's coming out of the crankcase breather now seems to be less wet. But it's still running rich, so that wasn't really my problem either. Interestingly, the engine still does run less rich when I disconnect the breather hose from the breather. I'm assuming this is normal, and that the breather isn't causing my problem. After all, it still is rich when I stop the breather from feeding air into the carburetor.

The only thing I can think of that I didn't do was to actually remove the cylinder heads to visually inspect them, but I don't think I need to. But then again, maybe I do, since I haven't discovered the problem yet. So I can think of two things the problem could be. Either both ignition coils are weak, or the jets on my carburetor are worn and the carburetor needs to be replaced. Neither one seems very likely to me, but I don't see what else could be wrong, given all that I've checked. I'm leaning toward trying a new carburetor to fix it, but I'm not confident about that and that's a $265 carburetor.

What do you all think? What have I missed? Surely I've missed something.

[38racing]

Did you shut off the fuel supply to see if the rich condition went away as the fuel level in the bowl decreased? And in your carb work you don't mention checking the float.

[bluemower]

someone may have previously worked on the carburetor and changed the jets or the fuel level in the float bowl.

Another possibility is a failed diaphragm in the pulse fuel pump. Fuel is located on one side of the diaphragm and a spring and vacuum pulses are located on the opposite side of the diaphragm. If a hole develops in the diaphragm, fuel can be drawn into the crankcase through the pulse hose. In addition to fuel diluting the oil, fuel fumes in the crankcase can be expelled through the breaker hose - directly into the air stream entering the carburetor.

[Mr Mower Man]

Did you shut off the fuel supply to see if the rich condition went away as the fuel level in the bowl decreased? And in your carb work you don't mention checking the float.

A brand new plastic float was included in the rebuild kit, and is not adjustable anyway. But I looked at the float level during the rebuild just out of habit, and it looked good and level to me. And yes, I did clamp the fuel line shut to see if the engine ran better just before it died. No such luck.

[Mr Mower Man]

someone may have previously worked on the carburetor and changed the jets or the fuel level in the float bowl.

Another possibility is a failed diaphragm in the pulse fuel pump. Fuel is located on one side of the diaphragm and a spring and vacuum pulses are located on the opposite side of the diaphragm. If a hole develops in the diaphragm, fuel can be drawn into the crankcase through the pulse hose. In addition to fuel diluting the oil, fuel fumes in the crankcase can be expelled through the breaker hose - directly into the air stream entering the carburetor.

I'm not sure how someone could've changed the jets, at least very easily. I looked up the carburetor and I don't see any of those individual parts available for sale. Doesn't mean it couldn't have happened, though. But even if it did, the only fix for that is to replace the carb since I can't get any of those parts individually. I don't believe the fuel fumes from the breather are causing my problem, since it still runs rich when I disconnect the breather hose, just not quite as bad.

[KE4AVB]

Some the carburetors in this series had an air bleed main jet. Don't know this particular carburetor has one but if it does it could be partially plugged causing an over-rich mixture. I recently having problems getting some carburetor passages cleaned. A couple there some very hard like a grain sand or hard corrosion plugging some jets.

[Mr Mower Man]

Some the carburetors in this series had an air bleed main jet. Don't know this particular carburetor has one but if it does it could be partially plugged causing an over-rich mixture. I recently having problems getting some carburetor passages cleaned. A couple there some very hard like a grain sand or hard corrosion plugging some jets.

This carburetor does have a small removable air bleed jet. The passage feeds directly into the area where the top plate is. It looked clear, but I didn't do anything drastic to open it up, either. You suppose I should attack that jet?

[KE4AVB]

If it is clean and clear then I would not do anything drastic; unless, I had a replacement jet. Either way it be advise to size it beforehand as opening it size would permanently lean the mixture just like enlarging regular jets permanently enriches the fuel mixture; always have a backup jet on hand in case you go too far.

[TandEtractors]

Maybe the needle and seat aren't sealing. Don't think the seats are replaceable .
Did you check if the needle and seat could hold a vacuum ? with the carb held upside down and applying vacume to the fuel inlet of the carb.


Still sounds more like a blocked air bleed or wrong main jet though.

[Mr Mower Man]

Maybe the needle and seat aren't sealing. Don't think the seats are replaceable .
Did you check if the needle and seat could hold a vacuum ? with the carb held upside down and applying vacume to the fuel inlet of the carb.

No, I didn't check that. But the needle was new with the rebuild kit, and as you say, the seats aren't replaceable. The fix for that would be a new carburetor. I'm planning to run the idea of replacing the carburetor past Kohler tech support tomorrow. And if they do anything but tell me for certain I'm barking up the wrong tree, I think I'm gonna try it and let you guys know what happens.