Murray 42" riding mower runs at low rpm's and backfires almost constantly

[timberhunter]

I have a 2008 Murray 42'' riding mower (42576X92B) that has been sitting up a while and now I am trying to get it running. The mower had been running fine the last couple of days, (would run rough for about 2 mins then smooth out), but when I finally got the hydrostatic transmission to engage yesterday and went to push the gas pedal down the rpms dropped and it started running rough with a small puff of black smoke about every second or two. Up until then when I had not engaged the transmission it had done fine. I have cleaned the carburetor in and out twice, put a new air filter on, new spark plug and I have ethanol-free 93 octane running through it. Gas flows freely through gas filter and through the line. Oil looks good, does have the original oil filter though. The motor now will run but never does smooth out and every time it backfires or detonates (actually think the backfire is happening inside the muffler), it shakes the whole motor. I also took the air filter off and let it run for a minute to see if there was a blockage, but it was still acting up. I have run out of things to check. Any one know what might be causing this?

Also here is the motor info:

16.5HP Briggs and Stratton
model:313777
type:0134-E1
code:000328ZE

Thanks

 

[ILENGINE]

Remove the valve cover and check the valve clearances. Also while checking valve clearances, roll the engine over a couple of times, and make sure both valves are opening the same amount.

Problem could be related the valve clearances, or possibly a worn camshaft lobe, preventing valve from opening fully, and for the full length of time.

 

[timberhunter]

Thanks for the info, I'll check it.

 

[timberhunter]

I pulled the valve cover off and there was quite a bit of oil in the bottom of the valve cover and you could tell some had been on the valves. I rolled the motor over and the valves seemed to be opening and closing fine. The top rocker arm is a tad loose, but the bottom one is snug. I planned on adjusting them a little but wanted to see if the oil in the valve cover is normal or if there is something else that needs to be done before I go to adjusting the valve clearances?

Thanks

 

[Fish]

Oil is normal up there. As you rotate the engine, look and see if the valves come out the same distance, the one that is not loose is possibly your problem area