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I have MTD Gold 5.0hp self propelled lawnmower with Briggs&Stratton carburetor. Recently it started running very slow, with difficult start, I have to use the engine starting fluid. Self propelling is also going extremely slow. First I took apart the old carburetor and cleaned it up, but it did not fix the problem. Then I bought a new carburetor
replaced the old one but the problem still persists. I wonder if this could be caused by anything besides the fuel feed, like a sparkplug, or if there is some place where fuel line can get clogged and how should I check this malfunction. I'll try to replace the sparkplug tomorrow and see if it fixes the problem. If not, I would really appreciate any help on the possible cause. Thank you

$7 carb won't always fix the problem.
over time fuel line can break down and clog up the line it's self or the carb. Either replace or blow it out with compressed air.
And by running slow, you mean slow RPMs correct?
have you tried speeding it up by tightening the spring that goes from the governor link to the tab up front?

Scrubcadet10 said:

$7 carb won't always fix the problem.
over time fuel line can break down and clog up the line it's self or the carb. Either replace or blow it out with compressed air.
And by running slow, you mean slow RPMs correct?
have you tried speeding it up by tightening the spring that goes from the governor link to the tab up front?

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I checked old fuel line from the fuel tank to the carb first thing. Old line was cracked and even though it did not leak fuel was close to this state.
Running slow as very slow RPMs, running very rough like there is not enough fuel getting in the ignition chamber. It goes as if it is almost dying.

Some of those engines are set up with a fixed speed throttle and because of that don't have an idle jet in the carb. they run on the high speed jet only, and below a certain speed will cause surging and poor running. Like Scrub said you need to check the tension on the governor springs by making sure the tab that connects between the throttle link and bendable tab hasn't been bent inward by contact with bushes etc.

I looked at the tab that connects to the throttle link and it is not bend. The spring is pretty tight and I cannot tighten it up with my fingers. Also yesterday the lawnmower was running slow but it was running, today it chokes after a second or less from the start. Used the new spark plug. Tomorrow will change back to the old plug and see if it will make it run, like it was before.

Try running it with the gas cap loose