weak ignition coil? (small engine)

[George]

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 suspect that 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

[avid_hiker]

Does it crank and/or sputter at all when you try to start it?

[avid_hiker]

NM....compression gives that away.....sorry. :-)

[spamTHISbrp]

Yeah, 75 is way low.

D

[John Lawrence]

[Chris Lewis]

According to John Lawrence :

Our small engines class said that anything above 50PSI is "acceptable", below that you're looking at something more major.

[George]

That engine has some sort of low-speed compression relief, to make pull-starting easier. (It's 7 hp, IIRC.) I don't remember the details, but some centrifugal thing on the crankshaft. So, I think 75 is OK when cranking.

And, no, it doesn't sputter or anything. No signs of life.

G

[beerguzzler50]

Not sure if this is it or not, but could the condensor be crapping out? I think this will mess with your spark quality and give bad or no performance.

George wrote:

[George]

Well, I thought about that. And, ...

(a) I don't see any arcing at the points, which I kind of think the condensor is supposed to prevent.

(b), when I 'exercise' the condensor with my (analog) VOM, it acts like a capacitor: it shows an initial low resistance, which quickly climbs to a very high one; and, when I switch over to a milliamp scale, I see a quick pulse of current.

So, I kind of think it's OK. Maybe. I'd just try another, but the condensor has a weird mounting/connector scheme, so it's not so easy. But, maybe I should jury-rig something.

G

[Nick Hull]

I had a similiar problem with my Kawasaki 3-wheeler. Worked fine, died & wouldn't start. After MUCH troubleshooting, I decided the engine sucked in a small piece of foam air filter, which dropped compression and poisoned the fire. I fed oxygen directly into the carb and cranked and got a weak pop-pop, too weak to start, but I continued cranking and the pop-pops kept getting stronger until it would run by itself without the started cranking, then in about another 15 seconds it got strong enough I could turn off the Oxy and in another 30 seconds it was running strong and back to normal. Wierd, never saw it before, but check your air filter if foam.

[Stormin Mormon]

First couple things. Remove the points, and clean them with a wire brush wheel on a grinder. Gap to .020 inches at widest piont (check your book to see if that's the spec for your machine).

Take the spark plug box, and tear it open flat. Turn the flywheel so the magnets are lined up with the ignition coil.

Put the now flat spark plug box between the coil and the flywheel. Loosen the bolts, and snug in the coil so it touches the spark plug box. Turn the flywheel by hand untilt he spark plug box comes out. That's about the right gap. Briggs calls for .014, I think, but thin cardboard works OK too.

One side of the coil is a low power wire. Probably ends in a ring terminal, and is under one of the coil mounting bolts. Be sure that's clean, also. Lightly grease the terminal.

[John Lawrence]

[mm]

Old, low compression lawn mowers have advantages. They're easy to start.

Same with cars, although a) I haven't come acrosss any lately, and B) any disadvantage would be worse because cars are driven so much more.

[hallerb]

points a condenser cheap just replace them along with a new plug.

many engines will accept a solid state ignition upgrade, not expensive hotter spark and more reliable too.

[Chris Lewis]

According to mm :

If they have enough compression to fire.

Larger manual start engines have compression reducers to make starting easy. Our instructor wasn't talking about _those_.