Identifying which Briggs would be a good idea.
Model & Type.

IF it's an OHV engine, the compression release requires the valves to be properly adjusted.

IF the "rider" sat over the winter, your battery is probably low/bad.

B&S 21 HP 540cc Platinum series with XDR OHV
Mower has 1/2 season on it. Started several times during winter with no problems. No indication of problems.
Went to start and the 1/2 crank. Tried to jump with car and still 1/2 crank. Took plug out and cranks fine.
Is that normal for valves to be out of adjustment in one season?? Hoping that all it is. I guess I am just looking for answers... Frustrating to have this with new mower. Old rider lasted years and years with no adjustments.
Thanks for responses.

How many hours is a "season"?
Your numbers don't identify the engine.
Model Type & Code do.
OR
The Sears 917 or 247.xxxxxx number.

There were 2 possibilities I addressed.
Have you charged & load tested the battery?

***"Started several times during winter with no problems"***
How long did you let the engine run each time you started it?
Did you maintain the battery with a battery tender or periodic trickle charge during the whole time the tractor was overwintering?
Starting an engine and letting it run for less than 15 minutes "just to see if it will still start" actually does more harm than good..............because.......the amount of charge you "pull out" of the battery doing so will require at least 15 to 30 minutes of run time for the charging system to replenish.
And if you pull out some charge without replenishing it, you have lowered the battery's overall state of charge and voltage level, which in turn causes the acid in the electrolyte solution to become stronger and begin attacking the battery plates.
And while boosting with a known good battery might rule out the tractor battery as a suspect, not necessarily so.
Even when jumping with booster cables, there are still some unknown factors, such as whether the jumper cables are good and whether or not there are some corroded or poor connections in the tractor battery cables.
ALL connecting points in the battery cable network must be checked to make sure they are clean, corrosion free, and tight. Not just at the battery terminals, but everywhere the cables are fastened to the engine and frame.

Also- IF the battery is severely discharged, the jumper battery is probably having most the current going to the bad battery in an attempt to charge it.

Sorry I wasn't specific enough. Anyway repairman came out and first thing he said was I bet it's the camshaft release is broken. i suggested he check valve adjustment which he did and they were fine. He then gave me the option to replace camshaft or replace with new engine. I took new engine which will be replaced in a week. He has seen lots of these camshafts go bad on this one year old model . Part is made in china. Oh well I am getting a new and better model engine he said.

Well, a broken camshaft release is about as specific as it gets.

Ok this is weird.. Sears is suppose to replace engine on Tue of next week for the bad camshaft. Well i was in garage today and just for heck of it tried to start tractor and it started right up without any hesitation. Now what?
I am afraid same problem will happen again so still go with the new engine? Right?Is it possible for the camshaft release to just get stuck and it just freed itself? When the repairman was here he keep trying to turn by hand and said sometimes that works but it didnt why he was still there. Anyone have any experience with this before?
thanks for input.Engine model number is B&S 331877 type number 2371-G5

There is NO Camshaft RELEASE on that engine!
There is a slight "bump" on the intake cam lobe that VERY slightly raises the intake valve @TDC, relieving compression. (That's why proper valve lash is important for it to operate. Too loose and it doesn't come into play)

When running, things happen so fast that there really isn't time for any significant compression leakage to occur.

Your technician is an idiot!
Let him replace the engine before they replace him.

I agree the "tech" is at least a doofus.
And you watched the guy, did he attempt to adjust the valves on the engine?
How about ambient air temperature on the 2 different days that you tried to start the engine.
Had the weather gotten warmer today than it was a few days ago?
Are you paying for the engine or is it covered by the warranty?

Err, the cam in question has the compression release arm. Is this the question?

Posting the engine model number would help clear up the confusion about whether it has an active or passive compression release. The numbers you posted are not the model number. There are multiple B&S 21 hp engines and while Bill does look ups from the Sears number, I do not. Since two very knowledgeable people disagree on the form of compression release, just post the engine model number off the valve cover so we can see.

Walt Conner

Walt Conner

This post was edited by walt2002 on Sat, Mar 29, 14 at 14:00

These are the numbers directly off of the engine valve cover: Model 331877 Type 2371 code 120105ZD

Tech did look at valves and said were not out of adjustment.
Just said it's easier to change engine than cam.
Tractor is less than 1 year old and IS UNDER WARRANTY
so i opted for new better engine. I am afraid same thing will happen again. It was about 10 degrees warmer on day that I got tractor started without hesitation.
Thoughts

It's possible that 10 degrees warmer temps meant battery had a bit more kick plus the oil was just a bit less viscous on the day you tried it and it started.
Batteries have less energy to offer when cold and oil is stickier when cold.
As far as your new engine perhaps going the same route, no way to predict that.

Maybe this pic will help. I just pulled this one apart to do a failure analysis.