Intermittent or no spark.

[Lubener]

I have been working on an old spare Craftsman GT/18 lawn tractor (ROPER). It has a Briggs 18 HP twin engine model 422437. It won't start because of an erratic, weak or a no spark condition. The coils check good with an ohm meter and are silimilar in readings. . The kill switch wire on the coil is disconnected for test purposes and I use the engine's starter to crank the engine during tests. . One of the two cylinder's spark is worse than the other with this. I have tried two separate new "magna coils" , all behave the same. I set the air gap from .010-.012". I have even flopped the new coils over and tried without success. I have sanded the light coating of rust off the flywheel magnets and the magneto's mounting surface on the block. too. I am starting to lean to a flywheel magnet issue, what do you think?

 

[Oro_O]

I have run through this same problem with 80’s vintage motor cycles. Also a Honda mower last spring.Drives you crazy.

What I learned:

- Put in new plugs. Never trust how they look (HD mc guru taught me that; always helps)
- Coils testing good w/Ohm meter means nothing. Put them under stress to test.

The bottom line is that bench tests of things are not reliable, you need to see what the components do when given resistance like in starting. If the plugs are known good, it’s likely the coil, despite however it tests on the bench and not pushing through the coil(s) and plug(s).

An 18hp twin is going to behave much more like this than a typical small mower.

 

[Soft Cars]

Has the flywheel ever been changed? Those engines are sensitive to flywheels and the exact placement of the magnets.

 

[ka9mnx]

Did you gap the coil to flywheel? Business card works ok.

 

[Lubener]

Has the flywheel ever been changed? Those engines are sensitive to flywheels and the exact placement of the magnets.

It's never been off.

 

[Lubener]

Did you gap the coil to flywheel? Business card works ok.

Used feeler gauge.

 

[zrxkawboy]

I'd lean toward a weak magnet, too. Also, like Oro said, replace the plugs regardless of how they look.

 

[Soft Cars]

Used feeler gauge.

The way I gap those, is I take the feeler gauge out of its holder and lay it across the flywheel magnets. Then I loosen the screws holding the coil and watch the coil get instantly sucked against the feeler gauge and the magnets. My .010 feeler gauge has a slight curve in it from doing that.

 

[Lubener]

I have read some have had their flywheels re-polorized by B&S. I am leaning towards a flywheel issue. I don't know what the chances are of having two bad "new" coils are. I have changed these before so I know the procedure. Thanks for the replies.

 

[Soft Cars]

Are you saying you suspect the magnetism in the flywheel has gotten weaker?

 

[Lubener]

Are you saying you suspect the magnetism in the flywheel has gotten weaker?

Yes. I suspect the coil is not being properly triggered. It is old, being made in the early 80's. There's not much left to suspect.