Honda Fourtrax 300 How to Repair TRX300

This post covers Honda Fourtrax 300 How to Repair TRX300.  Here we troubleshoot the Honda Fourtrax 300 engine and repair it.  We also explain how I used multimeter to aid in the troubleshooting process.

Honda Fourtrax 300 How to Repair TRX300

Honda Fourtrax 300 How to Repair TRX300

The Troubleshooting Process

The first thing I try to do on a simple motor like on the Honda Fourtrax is to break it down to either an electrical issue or a fuel issue.

The easiest way that I have found to do this is to remove the spark plug and then try and start the engine.  Now hold the sparkplug up to the frame of the Honda and try starting it, you should see spark.  If you do, you likely have a fuel issue.  If you do not, you likely have an electrical issue.

Honda Fourtrax 300 How to Repair TRX300

Honda Fourtrax 300 How to Repair TRX300

Fuel Issues

At the same time as you are turning the engine with the spark plug out, give a smell and see if you are getting a gas smell out of the spark plug hole.  If you smell gas, that means your fuel line is not clogged.  If you do have spark and you smell fuel, then the fuel mixture could be off.  At this point you need to do a deeper dive into the carburetor and adjust according to the manual or do a carburetor rebuild.  That is not the problem with my Honda today, so we will save carburetor issues for another day.

No Spark

The issue with my Honda is no spark.  There are several things we can check out, but everything is under the gas tank, so we will need to remove that.  It is removed by simply unscrewing one bold underneath the seat and removing the fuel line.  Make sure the tank fuel switch is set to the off position.

Tank Off

Honda Fourtrax 300 How to Repair TRX300

I removed my tank and immediately saw the likely cause of my issues.  There was a great big wad of pink insulation in my wiring harness.  A mouse had decided to make a nest under my gas tank which is on top of my coil and wiring harness.  Now I have to hope that the little critter didn’t chew through the wiring harness, as that would be a big old pain to repair.

Honda Fourtrax 300 How to Repair TRX300

Nest Removed

I removed the nest and inspected the wiring harness.  He did chew through the outer insulation of the wiring harness, but did not chew through any wires.  That is good, but I still have some kind of electrical issue.

The Coil

There is mouse urine on the coil, which could be causing the spark to short out before the spark plug.  But I need to be sure.  So I removed the coil and checked the system out with a multimeter.  The coil has two wires going into the back side of it.  A green wire which is ground and a black wire which gets 12 volts applied at a certain point as the engine turns.


The multimeter I own came from Harbor Freight Tools.  I had a coupon that let me get the multimeter for free if I bought one other item.  I don’t have a problem finding things to buy at this store, so it was free.  Even if you don’t have a coupon, you can pick one up for under ten dollars.


The first thing I did was to check that I had a good connection to ground on the green wire.  Make sure the coil is connected up to the frame with the wire on the screw.  That is your connection to ground.  Then remove the green wire from the back of the coil.  You will check that wire for connectivity to ground.

Set your voltmeter to Ohms and put one lead on the green wire you removed from the coil and touch the other lead to the frame of the Honda.  You should see a direct short.  Depending on which multimeter you have, you will either see a needle move or you will see the meter reading go down near zero.  If you have a good reading here, connect the green wire back up.


The next step is to remove the black wire from the coil.  Here we are checking for voltage, so change your multimeter to the DC voltage setting at 20V DC.  You will need to connect one wire of the multimeter to the frame of the Honda and the other to the black wire.  Then with everything in the “On” position on the Honda, push the start switch.  You should see some voltage on the meter.  It will not get up to 12V unless you have a really good multimeter.  It is just a quick 12V spike, so it usually doesn’t measure the full voltage.  Mine only showed between 2 and 4 volts.  Connect the black wire back up.

Coil Issue

If you see voltage there and have a good ground, your coil has everything it needs to generate a spark.  At this point you need to look at the coil as the issue.  In my case, I had a good ground and voltage.  It was a coil issue.  Fortunately, I found a new coil online for about $16.  I ordered that and will install it as well as a new spark plug and give a general tune up on the Honda.


I’m a little impatient and didn’t want to wait for the new coil in the mail to prove that was the issue.  So I removed the coil, cleaned up the mouse urine, and reinstalled the coil.  The Honda started right up.  I’m still going to replace it because it is so cheap, but I just wanted to make sure there wasn’t another problem as well.

Happy Homesteader

With this project near complete, with the troubleshooting done at least, I’m a happy homesteader.  Time to move on to another project.

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