My neighbor purchased a new riding mower, and has basically given me is old one, which is about 9 years old. I think it has a B&S engine but I am not 100% certain.

The issue is that when cold it cranks slowly like it has a bad battery and will not start. If you jump it off it will start and run fine, and if you turn it off and come back within a few minutes it will crank usually.

This sounded like a battery, but he just purchased a new battery and that didnt solve the problem, and that is why he was tired of dealing with it. He indicated it might be the alternator, but in my experience (on cars) if the altenator is bad it wont run at all once the battery charge is used up. That doesnt seem to be the problem here.

Any thoughts? Could something discharge the battery while it sits?

The alternator can be bad but that won't keep the engine from running. The alternator on most of these engines are not used to operate the ignition or the engine. They are just there to maintain the battery in a charged state, and possibly operate some accessories like the head lights or an electric pto clutch.

Check the valve lash on this engine, I would bet that the clearance is greater then it should be and the compression release is not working properly. This will make an engine harder to crank over and start.

Do as 30yearTec recommends, also check all your battery cables, and wire brush all connections. I have seen battery cables corrode on the inside and this draws to much ampage from the battery, and the starter can not turn at full speed. May be a bad starter also. Just my 2 cent worth.

To check the charging system put a test meter acrossed battery posts with engine not running should read 12v or 12.5. Now start mower and with engine running at high speed check battery again with meter acrossed posts and it should 13 to 16 volts.

what is the cold cranking amps of the battery he put in it , and what is the weight of the oil, also might be hydraulicking , pull the dip stick and smell it does it smell like gas . ? also make sure the deck belt is not draging , does it have and electric clutch ?

Thanks to all of you for the replies.

So this behavior is consistent with it being a bad altenator. I will also check all cables and connections to makes sure they all work. If I do the charging system test that tommyj3 suggests, and the volts do not increase when it is running, is that conclusive of a bad altenator?

I am not smart enough to understand the valve lash suggestion. I will have to read up on what and where that is.

Regarding the new battery, I am not sure of the specs. The original owner is a good guy, so I assume he bought the appropriate one. The mower works fine aside from the fact that you have to jump it off to start it. He mostly bought a new one because he has money and wanted a fancier mower. Its behaving the same before and after the battery replacement. It does need an oil change, and having lighter/cleaner oil might help it crank when cold. I will see if the oil smells like gas.

And yes, I think it does have an electric clutch. I can change speeds without using the clutch.

Thank you again for the advice.

No, if your alternator is bad it will not keep the engine from starting.

Yes, if you do not register an increase in voltage with the engine running then there is some kind of problem with your charging system.

If your battery is low, or you have a lot of resistance in the electrical connections, this could cause your starter to drag. It's possible there is a problem with the starter itself.

The size of the battery should not make any difference if its good and fully charged, even the smallest tractor battery supplies sufficient amperage to crank over a lawn tractor engine if everything is in good working condition.

Try cranking over the engine with the spark plug removed, see if any fuel or oil blows out the spark plug hole when you crank it over. It's possible that if fluid has accumulated in the cylinder then the compression would be too great for the starter motor to overcome.

The valve lash is basically the intake and exhaust valve clearance. After these engines have some run time on them the valves need to be periodically adjusted. Too much clearance and the automatic compression release no longer operates and the compression is too great for the starter to overcome.

This is very common next to dirty or loose connections and bad batteries.

Thank you again. I will do the electrical tests and if that doesnt show something conclusive move on to the engine issues. I guess if a fully charged battery cranks it then it is most likely something to do with the battery not being recharged. If not then its something specific to the engine that I might need help with.

I appreciate your time.

Just a thought on the battery. I worked on my sons lawn tractor about a month ago and it was acting just like what you are talking about. He had put a new battery in it. (Walmart Everlast) I put the battery on a charger and it showed it was down just a little, so I charged it up to full. Put battery back in tractor and no go. Ended up the battery was bad. Put another new battery in it and it started great. The battery showed good voltage but didn't have the ampage to turn starter.