Briggs 21 H.P won't spin past compression

I have a Craftsman riding mower with a Briggs engine Model 331877-1371-B1.
Code is 100422ZD. With the spark plug in it won't release the compression and allow the engine to spin. With plug out it spins fine. I have read many forums regarding this issue but haven't solved the problem. All of the obvious possibilities are good-battery at 12.88 and 13.4 with charger hooked up.
I have adjusted the valves every which way and it will start if I position the flywheel at the end of the compression stroke, so it's better but not perfect.
I also get a backfire through carb when starting with the choke on. Valves are at .003 intake and .006 exh with piston 1/4 inch past TDC on compression stroke.
Rocker studs are tight. I cannot see that the intake is staying open early in the compression stroke. The only thing I haven't done is take the pushrods out and see if one is slightly bent. Any ideas??

 

Just checked pushrods..all OK

 

Hello Willyp,


With the piston on the bottom of the cylinder in compression stroke stick a small screwdriver into the spark plug hole, Slowly rotate the flywheel buy hand clockwise while watching the intake rocker arm (bottom one).

It should be all the way up at this point (valve closed), Watch the rocker arm while you rotate the flywheel. Just before the piston reaches top dead center the rocker should go down opening the intake valve approximately 1/8" then come back up closing the valve as the piston hits top dead center.

That's the mechanical compression release lever opening the valve back up to bleed off a little compression for easier starting, This release lever wears over time and doesn't open the valve enough or sometimes not at all.

If the valves are adjusted correctly which it sounds like they are, This is likely your issue. If that's the case a new camshaft will need to be installed to correct the issue.


Good Luck

 

I see no movement at all on the intake valve other its normal intake opening and closing.
You did a wonderful job of explaining what to watch for.....much more clear than any other I've read.

Let's go back over valve adjustment in your words. I have seen so many different ways on the various forums, I may have become confused.
I really appreciate your time.

 

Hello Willyp,


Not a problem, Do the same as I explained with checking the compression release lever.

Instead of stopping at top dead center, With the small screwdriver still touching the top of the piston continue to rotate clockwise until the screwdriver goes back down into the spark plug hole 1/4". Both valves should be closed at this point (both rockers up and even).

Now your positioned to adjust the valves, The top valve is the exhaust the bottom is intake.

If you seen no movement in the intake rocker other than normal full open then full close, Looks like your in need of a new camshaft.


Good Luck

 

Again, thanks for your help. Looks like I need a cam, BUT, since I'm broke and since it starts easily if I hand turn the flywheel just past the compression stroke, I'll live with it a while.
I'm surprised that any part would wear out that quick. The engine is 4 years old and the previous owner (Brother in law ) had the thing serviced every spring. The oil looked like new.
Any way, thanks so much for your time and expertise.

Will

 

I might suggest another thing you could check.
It is common for a bad battery to measure 12.8 volts.
A good way to check battery capacity or excessive current draw is to measure voltage while cranking.
Should have no less than 10 volts while cranking.
You could also try booster cables connected to a larger battery.