Hello,
 
I have a mower that runs pretty poorly in that is runs and stutters. I can see puffs of black smoke from the exhaust every time it stutters. The spark plug goes black with soot, indicating rich running.
 
I have serviced that carburetor with new diaphragm, gasket and cleaned it all with carb cleaner and compressed air.
Twice actually and I also removed the main jet for cleaning.
 
None of made any difference and it runs exactly like before.
 
When looking into the carb, onto the main jet, I can see petrol drops coming out there and flying baout and ever time I see a big drop coming, the splutter follows.
  
Here is a video of this:
(Sorry, it was hard to hold the torch and the camera at the same time with precision)
 
EDIT: This forum does not seem to allow links (perhaps because I am still new). 
Please paste the text below into your browser to get to the video clip.
 
youtube.com/shorts/8uKIvhzcnhk
 
 
I am not not sure what next to do. The carb is clean and apart from some wear in the throttle shaft, I can't find anything wrong with it.
 
Do I need a new carburetor or is this problem caused by something else in the engine perhaps?
 
Best regards
Bernt
 
 

Well, the air filter hold down screw HAS to be installed or it'll suck fuel out 
of that orifice..
 
Your diagnosis sounds correct, running too rich.
 
Do you have a large air adjustment screw on the pull starter side (on the carb)?
Turning it in clockwise LEANS out the mix.
 
If you can post your engine model #, it would help..
 
 
.

Also, check the tightness of the THREE HEAD BOLTS closest to the exhaust.
They tend to loosen over time.  
 
It almost sounds like you have a head gasket leaking (loose head bolts)?
Spray some soapy water around the head to cylinder and look for any  bubbles
(before the engine gets too hot).  
 
.

Hello and thanks for helping.

I can confirm:
The head bolts are tight.
I have repeated the test with the airfilter bolt installed, it is the same. I see the same pattern of fuel flying out of the main jet like shown in my video.
There are no adjustment screws on that carburetor.

I am not able to locate a model number for the engine. I think it is a 3.5 perhaps? (Despite watching a video from B&S showing the 3 locations of the model number.)
The mower is Rover.
I have pictures of the mower and engine sides but the forum does not allow me to upload them.

Tks for the update.  
 
Try a new (known as good) spark plug and check for good spark
with the plug grounded to the head.
 
As I noted earlier, it sounds like more than a rich running condition.
 
A bad coil, spark plug, may not work 100% of the time.  Excess fuel 
can build up / load up W/O spark 100% of the time....
 
You may also want to check the flywheel key, it's not sheared and 
that the coil is installed correctly (if machine is new to you). 
 
.

Hello,
I have since borrowed a complete tank with carb from a friend's mower.
I bolted it on, added fuel and it runs like a sewing machine. Despite his primer bulb not working. I do not see fuel flying and percolating from the jet like on mine.

I still cannot upload pictures or share links. Can a mid please fix that?
Here is another video:

youtu.be/0tEaa5rRF54?si=QVKOlBOB8SQSwfg9

So now what is the problem on mine?
I stripped it again.
It really all looks fine.
Any ideas greatly appreciated.

Well, with your successful swap, the issue IS in your carb. 
 
Something worn, something missed, etc...  You may want to bring up 
the carb parts list and compare to yours...
 
 
.

I am not so sure now.
 
That carb is plastic and only has bout 4 parts to it.
I did a lot more more troubleshooting by swapping parts from good to bad etc by elimination.
 
Interesting find: I eventually swapped the good carb onto my fuel tank and the problem remained!!
So now I had the good carb also spitting petrol and percolating simply because it was on my tank.
 
I could not find ANY difference on my (now bad) tank to the other good tank. Same holes, same surface, not warped, nothing different.
 
So Lastly, I swapped the membrane and gasket from the good tank to my "bad" tank with the good carburetor.
 
The problem went away.
The cause of this problem has something to do with the membrane /gasket combination.
I do not know what. However, the thickness of the good membrane combination is 0.8mm thick and the bad combination is 0.43mm thick. 
So my mine is thinner to almost half of the good one. Mine is new though and I then checked my old one and it was the same. 
I am now wondering if the membrane material on mine  is letting some fuel through, causing this problem.
 
Surely someone out there has encountered this?
 
To summarize:
1) The engine recently started running badly, running just like I video. I have had the mower for 16 years.
2) I ordered a kit and replaced the membrane/gasket and cleaned the arb and tank.
3) After this, the engine ran as badly as before and I could not find anything wrong with the carb etc. I started this thread.
3a)I observed and videoed fuel spluttering about from the main jet or somewhere near it.
4) Swapping the tank/carb with a borrowed really old one fixed the issue.
5) Discovered that the issue seems caused by the membrane/gasket combination as swapping that for the really old borrowed one fixed the issue - or so it seems.
6) I still do not know what the actual cause of  the problem is.
 
Best Regards
Bernt
 

Was the carb kit / gaskets OEM or aftermarket?
 
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Aftermarket.
Why?