Small Engines (Lawn Mowers, etc.): B&S 16.5hp motor running rough at high throttle, murray riding mower, welbro

Question
QUESTION: I have an aging Murray riding mower with a B&S 16.5HP engine (model 31C707 with a Welbro carb). It's been running pretty good but all of a sudden it does not run well as the RPMs increase(and more gas is applied). It will run smoothly at low speed or idle, but when throttled up it will start running roughly and frequently 'cough' and pop - practically stop running - then restart for another few seconds. I have cleaned the gas and air filter. Checked the sparkplug (it's sooty black). And removed and blew out the carb. I am going to remove and strip down, clean and rebuild the carb, but not confident that is the problem. If it was a car I would say that the timing is off. Don't know enough about small engines to go beyond what I am doing. Any suggestions (or more questions?) Thanks

ANSWER: Timing on your engine is preset with no adjustment. A sooty black plug is telling me it's running to rich at high speed. I would maybe suspect, either the float is out of adjustment if it is a adjustable type with the metal tab on it, or it could maybe be allowing fuel into it. Shake the float close to your ear, and listen for fuel sloshing around in it. Second, I would suspect a worn needle, and seat. This of course would be after I made sure the spark plugs were good, and the choke wasn't hanging up. Where you say the engine is aging, I'd maybe pull a quick compression test too. .......Tom

---------- FOLLOW-UP ----------

QUESTION: Tom, thanks for the prompt and clear recommendation. I definately seems then to be running rich. The plug is new and I'm going to put a kit in the carb. Two quick questions: There's a small, apparently electrically initated, solenoid like device that screws in and holds the bowl to the bottom of the carb and has a needle valve that is spring loaded on a stem that inserts into the main jet on the carb and has two wires leading into it (one to ground). I can't find what this is in any of the manuals - is it some kind of fuel shut off or something else? Second question - what should I expect for compression on the test. Thanks very much in advance.... John

Answer
Your compression should be around 130-170PSI.....The solenoid you have mentioned is a electrical activated fuel shut off to the carb. to prevent after run when the key is turned off. When it is defective you will have no fuel supply to the carb, a different problem then your situation.......Please remember to rate the answers it always so appreciated........Tom