Riding Lawn Mower Backfires While Running

Today I have a lawn tractor that the customer said it popped and when it was warm, it backfired and also hesitated a bit at low speed. You can tell that the engine doesn’t quite sound right. So what I’m going to do now is start it up to see what is really going on.

I didn’t want to run it too long because I do have the cowling off. You can hear it hesitate a bit when you throttle up and it doesn’t quite run as smooth as it should. Now when you throttle up, you can hear the engine hesitating.

If I did let the engine get warm, it would start to pop and backfire. The customer was adamant that it’s probably a dirty carburetor. I’ve already taken it apart and cleaned it. It’s not the problem, it’s still doing.

The Reasons Why Lawn Mower Backfires

I’ve actually figured out what it is and I’m going to show you that.

What the issue with this engine is that one of the coils is bad. It has intermittent power at low speeds and high speeds. What I’ve done is hooked up the ignition in line tester.  I’ve put one on each cylinder that is exactly how I diagnosed the bad coil. Then start it up and you’re going to see that this one here sometimes does not light up at low speed.

Here is a video about Why Your Lawn Mower Runs Rough, Backfires, Blows Black Smoke:

How To Fix Riding Lawn Mower Backfires While Running

Step 1: Replacing the ignition modules

So you can clearly see that the spark is really weak on one side and it’s really good on the other side. Fỉrst, replace both ignition modules, just to be safe.

There’s no point just replacing one because both are the same age. And the part number for these coils today is 24 space 5/8 for space for 5-s.

Step 2: Checking the wires

Now I’m just going to remove the in-line testers. Once I have the new coils installed, I will put them back on to show you the difference. What you need to do is remove the positive wire from the ignition modules and it’s a good time to check the wires for damage from rodents.

I used to diagnose a lawn tractor that wouldn’t run because of rodent damage to the wires. For safety reasons, you can remove the key from the ignition switch so nobody turns the engine over while you’re doing this repair.

Step 3: Removing The Coils

So now with a 5/16 or 8mm ratchet and socket or nut driver, just remove the coils. You need to take off these two bolts. Remove the one on this side and install the new ignition modules.

Make sure to connect the positive wires on each module, line it up over here and on the bolts. I’m going to put a bit of blue loctite and I’m just going to start the bolts here.

Step 4: Gapping The Ignition Module

And now what I’m going to do to gap the ignition module is just grab a business card. I’m going to insert it between the flywheel and the module. Turn the flywheel so the magnets line up with the module.

You definitely need a gap between the magnets and the coil or module. When you have the module stuck to the magnet, it gives you the  perfect gap with the business card in between.

Step 5: Tightening

Now you can tighten up both bolts and you want to tighten these up evenly, so I’m just going to snug this bolt now. I don’t have the exact torque specs but just use common sense and tighten them up fairly tight. Just don’t overdo it.

Now you can turn the flywheel and remove the business card. It’s going to give you the perfect gap which is usually around 10 to 15 thousands and you can clearly see that gap right there.

Step 6: Repeat With The Other Side

Now repeat the same exact procedure on the other side. First connect the positive wire, and if you’re wondering what this positive wires for, so that when you turn off the ignition it actually cuts the spark from spark plugs thus turning your engine off.

And then just run the wires through the cowling here and I’ve reinstalled the inline spark tester, so that we can see the difference between the old coils and the new ones.

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That’s a huge difference, especially on the bad coil. That makes all the difference, the engine sounds much better, so now I’ll just take these off, replug the spark plug boots on the spark plugs here. Not got the cowling back on the engine. I’m going to start it up again, just so that you guys can hear the difference. Just hear how smoother that engine is with brand new ignition modules.

You can see a huge difference now. That engine is way smoother. It just  throttles up without hesitating no little pops or up and down surging. Like I said, the engine was way worse if you got a warm. I just couldn’t do it earlier because I had the cowling off. But it was just to show that with these simple testers here, you can diagnose a problem quite quickly.

riding lawn mower backfires while running