Small engine advice needed

[Tractors4u]

I have an old Craftsman riding mower with a 12 hp Briggs in it. It was given to me as a fixer upper and I planned on giving it to my brother in law when I got it fixed. Anyway, the thing acts like it wants to start but it backfires through tthe carb but never starts. I was describing the symptoms to a couple of guys and the first one said stuck exhaust valve. The second one, who had a mower just like this one said that he has a nut come off of the rocker arm for the exhaust valve. Today I pulled the valve cover and the vavle isn't stuck, the nut was still on but there was play in the rocker arm. With the valve cover I turned the engine over and the exhaust side doesn't move at all. Any ideas as to what has happened? I was hoping to avoid going all the way into this enginem but I may have to.

Thanks in advance.

 

[Junkman]

Check the key in the fly wheel..... if it is sheared, it will cause it to backfire..... there is also a good group for Briggs & Stratton engines on Yahoo if you need more help.... You will have to join, but it is free... It is a moderated group, so your membership and postings might not show up immediately. http://groups.yahoo.com/group/Briggs_N_Stratton_Engine_Group/

 

[KenNTx]

Here is another Forum for small gas engines..

http://bbs.cartserver.com/bbs/a/5624/index.cgi

 

[Slamfire]

You aren't going to get the exhaust valve to move by playing with the flywheel. Although bad timing can cause a backfire.

 

[jrlichina]

Make sure you turn it over at least 4 times to make sure you have made it through the exhaust stroke. Is the intake valve opening? If the valve doesn't move while rotating the engine the exhaust valve tappet could be broken or the exhuast cam lobe could be worm completely round. You'll need to open it up and see what's wrong.

<font color="blue">You aren't going to get the exhaust valve to move by playing with the flywheel. Although bad timing can cause a backfire.
</font>

?????

The Flywheel is directly connected to the crankshaft(keyed shaft). The crankshaft is connected to the camshaft directly by the gear on the crank and the gear on the cam. If you turn the flywheel the cam WILL turn unless the teeth are stripped off either the cam or the crank. Also, the cam is timed to the crank via the cam gear position. If the cam timing is off it would have had to be assembled that way(incorrectly). You can also check the ignition timing at the flywheel.

 

[MossRoad]

Maybe the cam shaft is broken, so that the part the turns the cam that lifts the exhaust valve isn't turning. Would that be more likely than a cam lobe being broken?

 

[Slamfire]

Since he was specific that the exhaust valve didn't move, I assumed the intake valve did. /forums/images/graemlins/wink.gif

 

[Tractors4u]

The intake valve does move but the exhaust side does not.

 

[jrlichina]

Yeah you definately have some kind of internal failure. At least it's an easy engine to work on.