Small Engines (Lawn Mowers, etc.): Briggs 18 HP

Question
I have an 18 HP Twin V Briggs
Model 422707
Type  1510-01
Code  91102332

The motor has increasingly become harder to turn over even when jumping with a car.  Currently the motor will barely turn once then stop.  I've been told there is a some type of pressure release valve that needs repaired and others tell me there is not.  Can you help me determine which way to turn to get the motor repaired or if it would be cheaper to get a replacement motor?

Answer
That engine had a mechanical compression release ground on the cam lobe...no repair unless the cam is bad.

Have you checked the valve clearances?
Have you measured the voltage drop across the battery cables to verify their condition?  Both the positive and negative cables.  As old as the mower is the cables may actually be causing the problem.  If you try to jump start it by connecting directly to the starter motor post will the starter spin the engine?

Let me know.
Eric