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MTD with Briggs engine (Can't read the model # information, old mower) idles great but when I engage the mower it chokes and dies. I need help diagnosing the problem. I've removed and cleaned the carburetor, replaced oil, spark plugs, fuel filter. Any ideas?

Dies as in power turned off or dies like running out of fuel?

As soon as I engage the pto it spits and sputters and dies

Idles great and speeds up great until the blades are engaged

And I did get the engine info,
31c707 0603 e1

Ok i just reread your first post. Is the engine at idle when you engage the PTO?

I've tried it all kinds of ways, idle and throttled up, same result

Ok so Im going to go the safety switch/PTO switch way. Check the safety switches to see if the are working and in place. The seat switch is most likely not working. But we did just help someone that the PTO switch had ants inside of it and causing the problem.

If all of this checks out then carb will need to be cleaned.

Seat switch seems to be working ok. Is it possible I missed something when I cleaned the carb or didn't put it back together correctly? I found a Welch plug in the cleaning solution but not sure if it came from this carb??? If it is missing could this be the cause?

Would it idle ok without the Welch plug?

A welch plug plugs a whole in the carb. Could air be getting in through that whole? Yes it could. Chances are it was just covering an adjustment screw you are not supposed to adjust. (well at least in the EPA's eyes)

Clean the carb again.
Dirty carb
Down and dirty: youtube.com/watch?v=VBL-BUj0c6E
More technical: youtube.com/watch?v=nAk_WDt3AD8