1. Home
  2. Automotive Technologies
  3. weak ignition? (small engine)

weak ignition? (small engine)

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
Reply to
George
Loading thread data ...

75 psi compression doesn't sound like enough for a gas engine to run....usually around twice that...
Reply to
jeffcoslacker

I seached on a lawn equipment site I post on sometimes...the consensus seemed to be a small engine won't run very strong below 120 psi, and probably wouldn't start below 90 psi...new compression spec on that motor is probably around 160 psi.

Reply to
jeffcoslacker

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
Reply to
George

See if it will start with a dry plug and the air cleaner removed.

Let me know.

I did see Kohler has a compression release, I was wondering if you figured that into the compression reading...

Your spark, fat and blue or thin and yellow/white?

Reply to
jeffcoslacker

No. Tried that, early on.

No - that was just a raw reading while I pulled it.

I'd call it violet and thin (watching it in the dark). It's not inspiring, but it is there. (That's spinning the crank with a drill.)

G
Reply to
George

I think I'd put a new points and condensor in just for laughs and see where that gets you...

Reply to
jeffcoslacker

The mechansim is actually on the camshaft, not the crank. When the cam is below a certain speed, a tab moves onto the back of the exhaust valve lobe so that the exhaust valve lifts partway through the compression stroke and reduces compression dramatically. Once the engine starts, the tab moves out of the way and the exhaust valve stays closed on the compression stroke.

Here's a manual for the K161 (7-horse) and k181 (8-horse) engine families:

formatting link

Reply to
Steve

MotorsForum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.