Help! Mower won't start

[kirkawilson]

Yes. Points and condenser can cause the problems you describe. Best to just pull the flywheel (might need some coaxing if you don't have a puller), check the sheer pin (Woodruff key) and replace it even if its just barely offset (the timing must be exact), replace points and condenser located in round aluminum casing directly under the flywheel. If there are no points and condenser (magneto fired), then your problem is likely the sheer pin (timing).

 

[alchemysa]

I've had 2 instances in the last month of spark plugs appearing to spark (albeit weakly) when grounded but failing to fire correctly in the cylinder. So firstly, buy a new plug to eliminate the possibility of a faulty plug. And make sure the connection to the plug is very good. Sometimes the action of bending the plug lead into an 'unnatural position' to create a ground can create a good connection within the lead that disappears when the lead is pushed back into its correct position. (The fault is usually at the ends of the lead.)

If you have compression, and a strong spark from the screwdriver, but the engine still won't even briefly fire when you spray starter fluid into the carby then it almost has to be the spark plug or the lead. (If it fires but still wont run then you can move on to other possible problems.)

By the way, a simple way to get a good ground to a plug for testing is to use a car jumper lead. Just attach one clamp to the spark plug, and the other clamp to an earth on the mower engine. That way you'll get a good earth and should see a good strong spark if the plug and the associated electrics are OK.

And make sure you can feel 'sucking' at the carby air intake when turning it over with the plug fitted. If the intake valve is stuck shut for some reason (e.g. a broken rocker or pushrod) you won't be getting any fuel or starter fluid into the cylinder.

 

[gocards1177]

Thanks all for the suggestions. I knew i'd get some help here. Looks like I've got some things to try tomorrow...

 

[Don87]

Thanks all for the suggestions. I knew i'd get some help here. Looks like I've got some things to try tomorrow...

The thing that bothers me the most from what you've described is exactly what alchemysa said, and what I've experienced.

If you can't get spark on the plug, between the center electrode, and the curled metal part by touching the threads to the ground, you have a bad plug (ceramic broken).

If you hold the plug at the correct angle so you can see it good, then hold the threads against a good ground, you will probably see the spark going 'sideways' from the ceramic to the outside of the plug(just inside the threads). It will never, ever run that way. And I have had on occasion, where a new plug will do this.........possibly from being dropped at some point I think.

 

[sd455dan]

Yes. Points and condenser can cause the problems you describe. Best to just pull the flywheel (might need some coaxing if you don't have a puller), check the sheer pin (Woodruff key) and replace it even if its just barely offset (the timing must be exact), replace points and condenser located in round aluminum casing directly under the flywheel. If there are no points and condenser (magneto fired), then your problem is likely the sheer pin (timing).

The magnetos coil can also go bad .. and spark plugs can foul do to (glazing of the ceramic insulator ) usually from cracked or old varnished fuel- u would normally be able to smell the bad fuel tho

 

[sox3]

If you have a wire from the coil going under the flywheel, you have points and condenser. If you do, I would cut the wire and replace with newer electronic coil. Either new or used. You could also have a valve stuck partially open
which you can check by turning the motor by hand. If it can be turned past the compression stroke easily,
then valve(s) may be passing enough to hamper starting. A lot of those older units had a problem with the exhaust valve seat coming loose from the block. Usually due to clogged air passages creating overheating. they would run until hot, seat moves or comes out, and it dies. You said it popped through the carb when trying to start once. that sounds like a leaking intake valve.

 

[The kid]

Just a thought here. Magneto and coils. Two items easily overlooks. When I set the gap on my coil. I simply take a piece of printer paper, fold it and place between the magneto and coil. Tighten the coil and rotate the the flywheel to make sure it's clear.

My son left my riding mower outside once for several days. It rained a few times. Although these engines are made to endure weather. Weather can damage them. The mower wouldn't start. Had spark. No start. Pulled the cover and found a bit of rust on the magnetic part of the flywheel. Particles where also magnetized on the coil. Removed the coil. Cleans all debris from it. Sanded the flywheel. Reset the gap with paper as mentioned. Tightened and secured coil. Reassembled and it fired right up. Seems the spark was weak and would cause a weak attempted start. Not saying that this is the problem, but take a look see. Eyeballing something is not always science. -kid

 

[gocards1177]

The thing that bothers me the most from what you've described is exactly what alchemysa said, and what I've experienced.

If you can't get spark on the plug, between the center electrode, and the curled metal part by touching the threads to the ground, you have a bad plug (ceramic broken).

If you hold the plug at the correct angle so you can see it good, then hold the threads against a good ground, you will probably see the spark going 'sideways' from the ceramic to the outside of the plug(just inside the threads). It will never, ever run that way. And I have had on occasion, where a new plug will do this.........possibly from being dropped at some point I think.

I'm pretty sure it's not the plug. I put the plug in question in a different mower and it fires right up.

 

[gocards1177]

If you have a wire from the coil going under the flywheel, you have points and condenser. If you do, I would cut the wire and replace with newer electronic coil. Either new or used. You could also have a valve stuck partially open
which you can check by turning the motor by hand. If it can be turned past the compression stroke easily,
then valve(s) may be passing enough to hamper starting. A lot of those older units had a problem with the exhaust valve seat coming loose from the block. Usually due to clogged air passages creating overheating. they would run until hot, seat moves or comes out, and it dies. You said it popped through the carb when trying to start once. that sounds like a leaking intake valve.

It doesn't spin very easily through the compression. It spins with effort but its not freely spinning.

I have points. Replacing the coil is tempting but given the machine if go used so that may be hard to find here on a Saturday. I'm going to try to get the flywheel off and check/replace the points and condenser.

 

[gocards1177]

Getting ready take flywheel off. But I thought I'd give another update that the shear pin looks partially sheared.