Briggs model 098902 0110-01 running really rich. puts out black smoke when idling. Could this be a diaphram problem?

Yes, black smoke is too rich, blue is oil and white is steam/condensation.

Surging is normally caused by air entering the inlet manifold or governor.

All Carburetors have adjustment
Needle height
Main Jet size
Idle Jet air control (often hidden under a silver disc).

surging

https://www.google.com/search?q=18hp+Briggs+and+Stratton+surging

Carburetor adjustment

https://www.google.com/search?q=18+hp+briggs+and+stratton+carburetor+adjustment

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Hello John,
Can you provide additional vehicle information (Year, Make, Model, etc.)?

Best regards,

Sounds like the carb needs cleaning, it's running too rich. First check the choke cable is not broken and travels full range.

try cleaning the air filter as it is probably blocked

Fouled plugs may be caused by oil leaking into the cylinder. Does it smoke when running (cold), are you using up the crank oil. Black smoke is excess gas comsumption, blue smoke is oil burning. If your plug is fouled with black carbon, you are most likely running to rich.

Bad carb or fuel injection issues- the system is fuel rich (black smoke) and also fuel rich system can make the idle low. Many things can cause this but thses are just a few.

First check the air filter. If the filter is stopped up then it acts like a choke which makes it run rich, which in turn will cause some smoking to occur. It will also cause it to run unevenly. You can remove the filter and start it to see if that was it, or it is possible to get a really good idea by looking at it. If there is a screw that holds the air filter assembly down on top of the carburetor, then put the screw a little bit into the hole it goes through before starting. If you have one of the flat filters then you do not need to do this.

If you still have this problem after the filter, then let me know and send me the model and type number of the engine.

THE PROBLEM SOUNDS LIKE THE CARB IS RUNNING TO RICH OR TO MUCH GAS IS BEING PUMPED INTO THE CYLINDER. IS THERE ANY SMOKE AND WHAT COLOR SMOKE

look for a vacume leak. black smoke means it is running very rich