Small Engines (Lawn Mowers, etc.): Electrical problem., craftsman lawn tractor, briggs stratton

Question
 I have a 21hp Briggs & Stratton Craftsman lawn tractor Mod #407777 and type #0167-E1.At times when I go to start it I have
to turn the key and hold it for about three seconds before it will turn over.I put a new battery in it,cleaned all connections,
checked voltage.When I turn the key on to start I only have 4volts
at the starter for those few seconds then goes up to 12volts.
  Some times I have to boost it but still have 12volt in battery.
  Can you help? Keith.
  PS:The last two times you helped me you was right on the money
and I forgot to thank you so A big Thank You for those.  

Answer
Difficult problem to troubleshoot.   Since you know the starter voltage is only 4 volts sometimes you must have a multimeter.  I came across this link the other night discussing how to measure voltage drop.

http://www.aa1car.com/library/voltage_drop_testing.htm

Sometimes measuring the voltage drop can really help isolate bad/high resistance wiring and/or bad grounds.  Bad grounds are probably the most difficult to track down.

I'd measure the voltage drop and verify the battery cables are good.  

Second, the key switch could be malfunctioning sometimes.  Key switches are relatively cheap and easy to replace...you might consider replacing it just to be safe.

I have had the small solenoid exciter wire not supply enough power/current for the solenoid to fully engage starting circuit.  You can measure the voltage drop between the key switch and solenoid terminal post/wire.  Snapper had a problem a few years back where the solenoid exciter wire was too small.  They published a service note requiring an additional wire to be installed in parallel with the original wire.  I doubt this is your problem but it is worth noting.

To recap, measure the voltage drop to verify cable/wire condition and test/replace the key switch.  

The Briggs link below covers testing key switches...look in the engine replacement guide file.

http://engines.myfaqcenter.com/Answer.aspx?p_faqid=26&body_0$tbKeyword=wiring diagram

Notice the letters on the key switch:

G = Ground
B = battery, usually the main power for the switch
S = Solenoid, usually the exciter tab/post of the solenoid
M = Magneto, goes to the ignition coil, grounds the ignition primary coil to shut-off the engine
L = Lights, usually 12 volts when the key is in the run position;  it is also used for anti-afterfire solenoid on carbs these days

Finally, once is a while, a safety switch connection may have some slight corrosion causing a bad ground.  Many times just unplugging the connections and re-connecting them will fix the problem.  The worst one I had was a seat safety switch on a Lawn Chief tractor...ended up soldering the connection after the customer kept bringing the mower back for the same problem.  After soldering, never had the problem again.

I realize this was a long response but intermittent problems can be the most difficult to fix.

Let me know if this helps.

Eric