Briggs engine pops through the carburetor

My Father-in-law has a Craftsman Riding Mower with a B&S 15hp? (28N707) OHV engine. He says he was cutting the grass and heard a loud pop and the engine would not stay running. He's right! It will start and run rough but continues to pop through the carburetor.(Not so much a backfire but air/fuel) I removed the Valve Cover and could not believe the valve clearance. The exhaust rocker was pretty sloppy but the intake was so loose I could spin it around and remove the push rod.(It was straight) There are no positions where the rocker is this loose... right? I have looked at the specs on the Briggs site and see that clearances should be around .005 and .009. From the parts illustrations this system looks pretty basic; a cam with pushrods and tappets. My question is do you think this has happened over time or has something bigger and worse really happened?

 

It could be a partially sheared flywheel key which will put the timing off and make it backfire/spit. Have a good one. Geo

 

Put piston on top dead center and adjust valves to spec.

 

You need to be concerned that the setscrews that keep the adjustments in place are in good shape. There really is no way, reasonably, to know this other than replacing each one. Has someone been into the valve train and, perhaps, forgotten to tighten each setscrew? No, this amount of play is not normal and there is no position that would allow such play. In order to set the clearances properly, you must position the piston (on the corresponding cylinder you're adjusting) at 1/4" past top dead center (TDC) on the power stroke. This ensures that the MCR (mechanical compression release) mechanism doesn't come in to play and foul up your adjustment. In other words, bring the piston up the bore of the compression stroke to TDC and then continue to rotate the flywheel enough for the piston to move 1/4" down the bore on the next cycle (power).

 

Also you'll need to check and make sure that the pushrods have not bent, and the rocker arm studs have not backed out of the head to create such clearance. Also, make sure the tappet is on the valve stem between the rocker arm and the valve stem.

 

I was able to get to this project yesterday and by using the procedures in this thread the problem was resolved!! It's nice to keep the in-laws happy! Curious to me how much those adjusters had backed out (Those set screws were tight). However, I am sure there was never any maintenance performed on the valve train since this mower was new (10+ years). Thanks again... especially to puey61 and cheese!

 

If the excessive clearance just "occurred" all of a sudden, it's usually due to either the rocker arm stud backing out of the head, or a bent push-rod. I guess you did check those, right?

Glad you got it going!