Small Engine Repair Question
 

So my Son has been messing with this one riding mower he got a few years ago - a few might remember. He mod'd it up. Anyway...he decided to clean up the engine so he broke it down and cleaned it up including polishing the valves and getting all the soot deposits off the pistons. His auto shop teacher helped him with the valve and lifter clearances. he asked me to help him set it on the frame and fire it up. Basically I watched him and drank a beer while he did all the work. He bolts it down, gets all the wiring hooked up, puts the fuel line on, charges it with oil, turns it some to get the oil in and around the workings. He hooks up the battery and hits the ignition. The starter engages but no kick over. Doesn't cough or anything. I ask him if he is getting a spark. So he unscrews the plug and has me turn off the lights. He presses the threads of the plug to the engine as a ground and turns it over. No spark visible. So maybe its the plug? I told hm he should check with a meter. So he puts the red to the tip of the plug and the black to the housing, sets it to 2.5V DC and turns it. Reads above 6 v but less than 10 v. Its a 10 V battery. I think the battery isn't completely changed but the meter on it reads 10 v. Am I missing something here?

Pull the lead off the plug and see if you get an arc from it.

Work forward or backward depending on result.

Are you saying you measured across the spark gap of the plug with a multi meter? :confused: :eek:

It may be the distance (gap) of the coil from the flywheel. It should be close enough that it doesn't scrape, but not so far away, that it has trouble indicating a signal....not real sure on what dimension it actually needs. Also make sure the flywheel, and the coil itself are free of rust and/or cracked wires. It also could be nothing more than a fouled plug (they tend to that occasionally).

Shoot a little gas in the plug hole before trying to start it next time. It should fire away instantly, if it doesn't keep running, then it is a fuel/carb adjustment.

What is the ignition system? If it is a solid state device make sure the gap to the flywheel is set correct. Also, they fail a lot...voice of experience here.

Jam a screwdriver into the lead and see if you can get a spark to ground, a bad plug is possible, but unlikely.

A normal business card is a good shim for setting the gap.

10v battery ?????
what motor on what mower uses a 10v system
old ones were 6 newer all 12volts

now a dead cell on a 12v will read 10v
or a near dead 12v battery will read 10v

This is the part that has me going "??":

"the starter engages but no kick over. Doesn't cough or anything. I ask him if he is getting a spark."

clarify please?

Old riding mowers lose a lot of voltage between the battery and the starter. Cause can be the many safety switches (seat, clutch, blade engage) and even the ignition switch itself. A good ground can also be a voltage issue.

A fully charged battery should be at least 12.8 volts. Run a jumper directly from battery to solenoid and report back?

I think he means that the starter is working properly, but the engine just spins over under the power of the starter, and doesn't try to get fully running, possible indicating that the starter has juice, but the spark plug doesn't.

If the starter engages, it won't be a safety switch (those switches keep the starter from engaging), it will be somewhere between the battery and the sparkplug (including the coil).

If the starter just turns and does not extend outwards to engage the ring gear, it will be the bendix in the starter (responsible for moving the spinning starter gear outwards). If the starter just makes a clicking noise with no spinning, it is probably the solenoid. You can jump across the leads of the solenoid to see if the starter works properly. Another thing I would do is to tap on the starter housing, and manually turn the starter gear (with the key off). and then try to start again.

I would first try just pulling on the rope to turn the engine over. If no spark, then it could be the magneto or more likely, the coil. If there is a gap required for the coil, then check the gap.

+1

Probably doesn't apply in this case, but if the flywheel has sat for some time, the magnets might need remagnetizing. I had that happen once & had to find a guy to do that.

Starter spins the fly wheel but there is no combustion indication occurring

made the gad the width of Emory paper

12V battery but reads 10V. He has it on a charger now to get it a full charge. Its worked before...

OK, that was not clear before. I take back what I said about low voltage to starter. Now, check for spark at the plug. If you have spark, a shot of starting fluid in the intake will tell if you have fuel problem or not....

check for spark with a new plug. If the spark is weak, or no spark, then I would normally suspect that the coil is bad, but in this case, why is the battery weak? Is there a way to have a shop look at the magneto?

Silly question but what is the easiest way to check for a spark? I just take it out, keep the lead on the + and hold the plug threads to the block to act like the ground and turn her over. Is there a better way?

Use a known good or brand new plug. Connect the wire (make sure it's tight), touch the spark plug base to the engine head, and pull on the starter rope. Wear heavy gloves. Look for a fat blue spark. You can plug in a spark plug or coil tester as well. They sell them at FLAPS, but they are not as reliable as just using a spark plug.