I’m looking for someone who can help me diagnosis my 9N..

The 9N has been steadily running for the last 11 years, mowing our fields. This fall it has been underpowered. It stalls when mowing tall grass. There’s little difference between 1st and 2nd gear. Once it’s hot, after an hour or so, if it’s stopped there is no way to start it until it cools.

And it has only gotten worse. Now it won’t start at all unless it’s towed. When trying to start, it backfires.

We have checked the gas flow and carburetor. They’re good. We replaced the coil and condenser, checked and filed the points. The wires and spark plugs are new and in good condition.

One possibility is that the camshaft timing gear is broken. Another is that it has a weak spark.

What’s your diagnosis?

What should be my next steps? Thank you.

If you need info on compression testing the N-engines, you can find a few tutorials in our How-To's library:

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I suggest a compression test.

I don't understand why you have a weak spark if all the ignition parts are good.
...Low Battery voltage? ...Ballast resistor?

Next two steps. First...While running, pull one plug wire at a time and put back. Listen for sounds like rpm dropping.

Second... Compression check