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I have a John Deere LT150 with a 15HP Kohler Command engine (CV15S) that appears to be running rich, and the spark plug has black sooty deposits consistent with a rich mixture. I replaced the unserviceable air filter and fitted a new spark plug.

It has a Walbro carburettor that does not have a fuel solenoid, and the choke Is opening and closing correctly.

I removed and cleaned the carburettor, drained the fuel tank, and filled it with fresh fuel to no effect.

I performed a pressure test on the carburettor in an inverted position, and it successfully maintained 5psi, indicating that the needle and seat are sealed and there are no grooves on the inlet needle.

I removed the carburettor a second time and bent the plastic tab on the Plastic float slightly, as it appeared to be a little high. While I had the carburettor off, I filed off the limiter on the idle speed mixture screw to allow fine-tuning. Adjusting the idle speed mixture screw had some effect but not enough to correct the problem.
A carby kit here in Australia is $74.

It has hydraulic lifters, so there is no valve lash to set.

I had considered that the fuel pump might be supplying too much fuel, but the float should prevent an oversupply if this were the case. I tested the fuel pump pressure at the carburettor, which is putting out 2.9 psi. I was unable to find the specs for this plastic manual pump. A new pump is $181.


Any thoughts?

Two things come to mind. Air bleed that is stopped up or a porosity in the body of the carb allowing it to draw more un metered fuel in to the cylinder.

hlw49 said:

Two things come to mind. Air bleed that is stopped up or a porosity in the body of the carb allowing it to draw more un metered fuel in to the cylinder.

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Fuel pressure psi is in range. What are the symptoms the engine is having for you to think it is running rich besides a fouled spark plug? Any change in performance after installing a new air filter? Is the choke fully opening and closing? Is it blowing out black smoke when running?

How hard do you run the engine? Does it run poorly? Smoke? Many folks mistake a rich mixture for spark plug(s) being the incorrect heat range for how the engine is operated. If an engine is operated where the spark plug(s) tip is not reaching about 900 degrees F it will not be in the self cleaning range and will carbon foul.

Tiger Small Engine said:

Fuel pressure psi is in range. What are the symptoms the engine is having for you to think it is running rich besides a fouled spark plug? Any change in performance after installing a new air filter? Is the choke fully opening and closing? Is it blowing out black smoke when running?

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The initial problem was it would run for a while and then cut out. I replaced the coil with a genuine Kohler coil, which appears to have fixed the problem.

Then I noticed It was running rough, and I've been around engines for long enough to know an engine that is running rich.

I've listed everything I have done, and nothing has made a difference. There was a slight change when I filed the limiter off the idle speed mixture screw and adjusted it. But I could not adjust it enough to correct the condition. This was a further indication that it was a fuel problem.

It doesn't blow smoke when running, just a little when started on choke.

What is the maximum pump pressure?

Are you running the proper plug? A cold plug will do this every time.

It has a Champion RC12YC, the plug they tell you to use.

I don't think trying a hotter plug will make a difference. You can tell by sound that it's running rich.

Any other ideas.

You may have a bad plug. Have you checked the float level? If the level is too high it will cause a rich running condition. Here is the service manual for your engine. https://www.mymowerparts.com/pdf/Ko...13-CV14-CV15-CV16-CV460-CV465-CV490-CV495.pdf

Did the rich running start at the same time that you replaced the coil. Or some time later. Just wondering if a fixed timing coil got replaced with a spark advance coil, and now the timing is off.

50% chance it's the carb. our eyes are incapable of detecting every fault. Can't you find a replacement carb on ebay or amazon that will ship to Australia? Prices there are always way low.

So how is the exhaust, no small animals there to fill it up are there And is the vent from the bowl clear so that pressure cannot build up in there.

Etbrown44 said:

50% chance it's the carb. our eyes are incapable of detecting every fault. Can't you find a replacement carb on eBay or Amazon that will ship to Australia? Prices there are always way low.

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Thanks for your reply.

I agree. It's probably a carburettor issue. I'm leaning towards getting the carburettor kit due to not having had the best success with Amazon/eBay carburettors.

Ive bought a dozen ebay and Amazon carbs and never had one fail. Guess you can look at the amazon feedback first. Let us know if it works.