Tips and Hints from Personal Experience
This section describes a difficulty I had with my 1987 Noma Brute walk-behind mower and the solution I chose.
Having worked well for so many years since purchasing it new, it was a disappointment when my mower was returned and would not start. After checking for a spark at the spark plug and seeing nothing, I reasoned that there was a problem in the ignition system rather than the fuel system or carburetion. I referred to the owner's manual to familiarize myself with the ignition system but there was insufficient detail provided.
I removed as many parts from the mower as required to gain access to the ignition components. Upon inspection, I discovered that the Briggs and Stratton 3.5 HP motor had one of the simplest ignition systems I had ever seen. From past experience with engineering design, I believe, as many others do, that the best engineering is the simplest design that will accomplish the intended purpose. In this case, there are three components to provide ignition - the aluminum flywheel with built-in magnets; the magnetron (coil/pickup); and the spark plug. There is also a safety switch and brake connected to a supplemental handlebar which is intended to both short the ignition to mower ground and stop the rotating flywheel when the handlebar is released.
Discovery and Frustration
As I suspected the safety ignition short as being the problem, which seemed likely, I inspected the wire (the only electrical wire other than the spark plug high tension lead) to find that the insulation had indeed worn from one spot allowing the ignition (magnetron) to be short-circuited to ground all of the time. I promptly cut a length of heat-shrink tubing and covered the entire insulated portion of the wire and shrank it in place. Having faith that I had solved the problem, I checked for a spark at the spark plug as I had done so many times before with older battery/coil/points systems but could still see no spark. Frustrated, I reassembled the mower and tried to start the motor. This was fruitless. I removed the new spark plug that I had bought by this point and smelled the plug for fuel and it was obviously getting fuel.
I then disassembled again and disconnected the shorting circuit wire. I checked for spark in the usual manner by grounding the plug against the cylinder head and could still not see anything. At this point, I took a well-needed break and visited the Briggs and Stratton website (excellent site) and read about the magnetron ignition component. There, I found that the spark from the ignition can not usually be seen without a special tool/indicator. I also found a test impedance/resistance measurement that I could make with my simple multimeter to ensure that the magnetron was functional. I made this measurement and found that the reading of about 5000 ohms was within the specification range.
Although I believed that the magnetron was OK, I was at a loss for what to do next, and opted to buy a new magnetron. After fitting the new magnetron (2700 ohm measurement), and re-assembling yet again, there was still no spark visible and I was losing faith in myself.
The Final Solution
Running out of options and not having a new cable to replace the
worn/bent unit for the safety switch mechanism, I decided to completely
remove all of the safety shut-off/brake system from the mower, thereby
downgrading the mower to one similar to those available when I was a
young boy (i.e., with no safety features). I re-assembled the mower one
more time so that if it didn't
function, I could throw it out in one piece. However, I didn't need to
do this, as the mower started after 4 or 5 pulls on the starter and now starts
after 1 or 2 tries. Great joy was experienced at this point and a
re-affirmation of my capabilities. Incidentally, I do not recommend taking this
approach for a permanent repair but this did simplify the debugging process
and allow me to keep the mower. I will likely replace the "dead-man switch"
on this mower when time permits.
The mower burns a small but invisible quantity of oil (needs new piston rings) and probably could stand a valve grind job but still has good power, is reliable once again and represents excellent value by still doing its job after about 20 years of constant summer use. Kudos to Briggs and Stratton for reliable, simple motors and a good website. The mower is much cleaner now too having parts of it disassembled/reassembled/cleaned at least 8 times over a one week period. I also believe that I have both a good spare sparkplug and magnetron, if ever needed (which seems unlikely now).