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New Walbro Carb... Engine Still Running Rich

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Luke Dalton

New member
Oct 16, 2023
Tulsa, OK
Hey everyone,

Reaching out to this forum for advice because I just don't have the knowledge to go any further. I purchased a 2020 Cub Cadet XT1 LT50 with around 200 hours on the clock on Facebook marketplace earlier this year. He wasn't asking much for it, and I may have gotten what I paid for. Before I ran the mower, I changed the oil, filter, air filter, and plugs. Originally, the mower would run but ran rough on full throttle - it idled perfectly fine below 50% throttle and I mowed with it several times before getting tired of not being able to run it at full power. I ordered a knockoff chinese carb from Amazon and it ran even worse than previously. I also discovered that there was gas in my oil - I'm assuming this happened while I was turning over the engine many times as I was trying to get the thing to run with the knockoff carb, but I'm not sure.

I ended up ordering a Walbro carb directly from Kohler (or Cub Cadet, either way it's legit) and installed it. I also drained the oil/gas, flushed it with 2 new quarts, and then installed another 2 quarts of oil and a new filter. With the new carb on it runs better than with the Chinese carb, but still not as well as it originally did - originally, I could get it to idle smoothly at low power - now, it runs rich at any power. When running, the engine bucks and smoke comes out the exhaust. The only time it's run normally is when the mower is running out of gas, which lasts for 10 seconds or so. I have also reset the governor, which didn't need much of a reset and I haven't noticed any difference. I've also noticed that my new plugs were black and slightly wet when I pulled them out the other day - I let them dry overnight and attempted to clean them before reinstalling. No difference in operation.

In summary, here are the things I have done to the mower:
-New Walbro carb
-New oil pump
-New plugs (I have removed each while the engine is running to test the other plug, and they are both working, even though dirty)
-New oil and filter
-Governor adjustment

If the carb isn't the issue and the mower is still running rich, what else could my problem be? Thank you SO much for your advice.

I'll start out buy saying I'm not familiar with that newer engine. But a good basic diagnostic test would be a compression test. Even better would be to do a Leak Down Test. if you don't have the equipment for this many Auto Supplies loan tools. Most will have the compression test gauge, some may have a leak down test.

Report your results back here.

Show us some pictures of the new carb. Most new ones were set on a standard setting and have the adjusting needles covered over with wax/putty. some you can dig the putty out and remove the needle, clean it and reinstall to actually adjust it. When you remove the needles gently count the turns it takes to close them all the way and record that. When you reinstall them, turn them all the way in to closed, then back them out the number of turns you recorded. use this as the starting point and try to closes them some to lean it out.
Disassemble and clean your new carb. It too was probable made in China and may not have been put together clean. Spray carb clean down all the passages, use a magnifine glass to see the tiny holes and spray carb cleaner through them with the straw.
It’s still fuel. Carb is not delivering good fuel and air mix. If the model doesn’t have adjustments it’s either not jetted correctly or and most likely the float is adjusted wrong. As was said their sll Chinese carbs do it is what it is. I rebuild or swap a couple hundred carbs s year and brand new OEM or aftermarket ones still usually need something cause quality control is not good. I open new ones and go thru them before install. Measure keys make sure needle seats and hold pressure and make sure float is at proper level. You’re just getting too much fuel and it’s overflowing into the engine. That’s it. Take it to someone who knows what their doing and get it right
The jets on that are almost certainly not adjustable. Sounds like it might be jetted a bit too rich, or the float level needs to be set differently such that the float closes the needle at a lower gas level in the float bowl.

A plugged air filter will also do this, as Ron is alluding to.