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Hey folks, I hope you can help me.

I have a 2004, 300 hrs old L120 that belches black smoke on throttle up. Too much fuel at idle, right? It runs fairly well at midrange and full throttle and at idle After watching umpteen you tube videos I have done the following: new air filter, two new ignition armatures, two new sparkplugs that light up the testers perfectly, a new carburetor (insuring correct throttle and choke alignment), valve lash adjusted properly.

To my knowledge, there is no mixture adjustment in these carburetors so I suppose everything is set correctly when bought new. It doesn't backfire as I would expect with an unburned mixture. So what do you think?

Why does my machine stutter then cough a cloud of black smoke when throttling from idle?

Thank you for your thoughts.

Jules

Check the choke is not flipping on slightly when the engine throttle up .

We’ll need a little more info before we can help you. Once we have that, we can give you some ideas what to look for.
1. Please provide us with the engine brand and all engine numbers. We need to know what you are looking at.
2. After the initial puff of black smoke, does the engine even out and run smoothly with no more smoke?
3. How long have you been experiencing this condition?
4. Was the new carb you installed an OEM or aftermarket carb? What was the part number?

Thank you, bertsmobile1.
Checked choke; no movement at throttle up.

Thank you, Rivets
1. Engine : Briggs & Stratton 20HP 655cc V-Twin 406777013931
2. After initial black smoke engine evens out and runs smoothly throughout throttle range with no more smoke. Throttles down smoothly and idles without a miss. Throttle up and, again, black cloud.
3. The machine sat idle for five or six years then was sent to shop in a no start condition. The shop found bits of rubber filler cap gasket in tank and filthy OEM carburetor. Tank and carb were cleaned, fuel lines blown clear, oil changed and air and fuel filters replaced, then filled tank w/ ethanol free fuel. When returned last week, the tractor cranked right up and idled fairly well though the aforementioned black belching at throttle up was evident and when running at full throttle it popped and coughed lightly throughout usage. When throttling down the engine would intermittently backfire through the exhaust. I should note no mower load applied as I use the machine to tow a small dung cart as I pick droppings from the pastures on the wife's horse ranch (yeah! I'm a lapdog).
On seeing the black smoke and hearing the popping and coughing, I thought I could improve on the shop's work so I researched you tube and completed the work detailed on my previous post. The only improvements I effected however, was eliminating the popping and slight coughing of the machine as it ran under power.
4. The carburetor is aftermarket bought from Amazon. https://www.amazon.com/dp/B076J59G4T?psc=1&ref=ppx_yo2_dt_b_product_details
I've, of course, saved the OEM carb and intend to fully analyze it.

Thanks so much for everyone's help.

Jules

Don't know about being careing & loving husband but you are definately Bezos's lap dog.
He thanks you greatly for giving him even more money as his $100,000,000 yatch is now 3 years old so needs replacing.

So you bought the cheapest unbranded carb from the den of thieves & expected it to work perfectly ?
Do you have conversations with the fairies at the bottom of the garden as well ?
Blowing black smoke on acceleration is usually because the fuel level in the float bowl is too high thus then the carb gets the stronger signal excess fuel is drawn into the cylinder so you get black smoke.
The other possibility is the timing key is cracked so the spark is slightly too far retarded or one of the magnetos is breaking down .
As you are not putting a big load on the engine there is the possibility that the main jets are too big (or even missing) on the new carb which could have come from a batch of 10,000 defective ones rejected by John Deere & then sold to cheapskates Via Amazon and these carbs are so bad they have to "give away" trinkets in order to get people to buy them.

Mower engines are mostly fixed timing engines so in order to make the engine go faster you supply excess fuel for a brief moment.
If you watches the flame you would see it go from blue to white then yellow then back to blue again when it reverts to normal speed.
Governed engines are a bit confusing to talk about as the engine speed is fixed so high idle & full throttle are the same speed , just with a different opening on the throttle butterfly.
Now if for some reason all of the richer mixture is not burned then you get black smoke
So it might need new magneto unit, or slightly hotter plugs or projected electrode plugs or different jets in the carb or a float adjustment .

A factory that makes quality parts casts their name in the side of the carb and puts a batch / serial number on them so they can be traced.
A company that makes defective from new rubbish does not put any identifing marks on the product so no one can prove they made them .

Thanks bertmobile1.
I agree entirely with your cheapo carb assessment, but I had to have something immediately before I rebuilt the OEM unit which I intend to return to service. Interestingly, after I replaced both armatures and sparkplugs and tested ignition system with satisfactory results (spark testers lighted consistently at all throttle settings.) the mower was belching black smoke with the OEM carburetor installed so I figured the only thing left for me to do was rebuild the OEM unit. But I didn't want the tractor down for any appreciable length of time, thus the offending purchase.

Loved the information on mower engines. I had thought about installing hotter plugs and I will look into adjusting float level. I don 't know if I can change the jets but I will research the topic. If ever I find the solution I will post anew.

Thanks again for your imput.

Jules

Get a leakdown test done and double check the ignition timing.
I rather feel you have a bad head gasket or leaking float valve
A float valve unable to fully close off the fuel supply is very common with engines that have been sitting wet for a long time .
EPA regulations require the overflow to be internal so you can not see or smell it happening but blowing black smoke is the give away .

bertsmobile1, excellent info. Purchased leak down tester yesterday. Will check ignition timing.

Jules

you have bits of rubber o-ring in your breather, clogging it up a bit.

Thanks Joed756,
I cleaned out the intakes when I changed the carb.

126jp said:

a new carburetor (insuring correct throttle and choke alignment

Click to expand...

Was this an Amazon/Ebay Chinese carb or OEM?

Look at the carb float height setting. Or a carb needle partially or fully not working. Might pressure test the carb to rule out the needle and seat.

Thanks slomo, never thought to press. test the Amazon-probably Chineze or Korean carb.

Sounds like carburetor flooding, in according with Slomo.