On Sat, 9 Sep 2006 05:37:25 -0700, jeffcoslacker
> wrote:

>
>George Wrote:
>> I have an old tiller with a Kohler single-cylinder 4-stroke engine. It
>> lost all power (over about 30 secs), and now won't start. Since then,
>>
>> - Replaced spark plug. (Which does spark, when grounded to the
>> head.)
>>
>> - Points look OK.
>>
>> - Compression looks OK (75 psi).
>>
>> - Cylinder is getting fuel (plug gets wet, and exhaust smells.)
>>
>> - Carb float valve works, and float seems OK.
>>
>> - Coil resistances look OK.
>>
>> - I cranked it in the dark, and didn't see or hear anything bad
>> around the coil.
>>
>> I wonder if I have a weak spark when the plug is installed. But, a new
>> coil costs $75, which is a kind of expensive experiment.
>>
>> To test the coil, I made a 10:1 voltage divider (with a HV 10M
>> resistor), and looked at the spark plug voltage with a scope. With the
>> plug not grounded, I see a 4-6KV pulse. When the plug is grounded, this
>> drops to about 1KV. I get the same 1KV when the plug is in the cylinder
>> (no fuel).
>>
>> Questions:
>> - Is above consistent with a 'weak' coil?
>> - Is there another way to test that?
>>
>>
>> And, any other thoughts would be appreciated.
>>
>> G
>
>75 psi compression doesn't sound like enough for a gas engine to
>run....usually around twice that...

This is a 7 hp pull start, and it has some sort of compression relief to
reduce the starting effort. It works off the crankshaft speed. I
figured it out once, but I don't remember the details.

G