How to fix a petrol lawnmower that won't start

Published

In this video Matt demonstrates how to fix a petrol lawnmower that won't start:

Video Transcript:

Hi, I’m Matt from eSpares.

Now if you’ve had problems starting your petrol lawnmower, you’ll know just how frustrating that can be. I’m going to run through some of the basic checks we need to go through to make sure your mower starts first time every time. Now I’ve got a Qualcast classic petrol 35s but these checks will be applicable for any type of Qualcast, indeed any type of lawnmower. Now let’s start with fuel. A standard four stroke engine takes ordinary unleaded petrol. Now the thing about petrol is it degrades over time, it goes off.

So if you’ve left petrol in your mower over the winter, the best thing to do is to drain that off first and put some fresh petrol in there. The two things I want to drain; drain the main fuel reservoir just by uncoupling the fuel line there and drain that into a bowl. And then, you can see the carburettor here there is a little release valve there that will just drain the fuel off the carburettor as well, and then make sure we’ve got fresh petrol in. Let’s check the oil; the dipstick on this Qualcast is on the back here. Just give that a wipe and pop it back in. I think it looks alright. Yeah, that’s just over half full that’s fine. If you do need to top up, then top up at the front there. Make sure you’re using proper lawnmower engine oil. Obviously if yours is a two stroke then use two stroke oil and then gets mixed directly into your fuel.

So spark plug next; standard 14 mm lawnmower spark plug, and I’m using a 21 mm socket which is specifically for spark plugs. I’m just going to make sure I don’t damage the porcelain or the top of the engine. We’ll take that out. Spark plugs do have a limited life, it’s typically about 150 hours for this type of spark plug. Now as you can see there’s a spare bit of sooting, fair bit of carbon on the electrodes there. It’s not actually in back nick, I think we’ll just clean that up with a wire brush and get a bit of fine sand paper between the two electrodes, just make sure it’s all nice and clean. I’m actually going to put a brand new on in here. Make sure you’re getting the right spark plug for your particular mower; they’re available for all makes and models on the eSpares website. And then just tighten that up; not too tight, make sure you don’t overdo it. There we go, we’re going to get a good spark every time now. So, a few things I just want to check before I fire it up: just the cables, the belts and the gears. Now if I take off the belt housing cover here, that’s just held into place by these four hex head screws, you can use a torx bit in there. And if I engage the rear roller, just make sure that that’s tautening the belt when it’s in the on position. That’s fine. Likewise if I engage the cutting disc. Yep, that’s putting some tauten in that belt fine. Just have a look at the belts, make sure they’re not fraying, no sign of damage or wear. They’re fine.

And these gears; I don’t know if you can see in there, there’s actually three nylon gears. They can actually break quite easily, and you can see a colleague of mine actually replacing those in another video. By that all looks fine. Now if I just check the throttle; so in the fully off position, you should be able to see that just cuts the power to the spark plug. That’s fine. And then as I open the throttle, you should be able to see it’s just opening the carburettor there. If that needs any adjustment, this little screw here just changes the position at which it opens the carb. Ok, let’s get it started. Let’s push the throttle all the way back, all the way open. I’m not going to give it any choke because it’s quite warm, we’re inside. Obviously if it’s on a cold day you’re going to want to give it a bit of choke. It’s probably worth showing you actually how the choke works. If I take the air filter off here, you can see that’s the air filter. And the choke restricts or chokes the flow of air going into the carb. It just gives us a much more petrol rich mixture going in which is what we need on a cold day because the ait doesn’t mix with the petrol quite so well. It’s a bit of an awkward height with me here on the table but let’s see if we can do it. First time.

Now if yours doesn’t start, the problem is probably going to be in the carburettor. Let’s take a closer look. So before I open up the carb, I just want to disconnect my fuel line and drain the fuel in the tank down into my cam. Bound to spill a little bit. So with my fuel line out of the way, and if I just get the air filter out of the way as well, I just need to drain the remaining fuel just sat in the carb. I can do it on this release valve here. Now I’m going to get as much paper towel under there as possible, and just soak that up. There we go, that’s emptying nicely.

Ok, let’s just open the nut on the bottom of the carburettor now, and see what we’ve got inside. So as I undo the bottom screw, the float chamber comes away there. Just pop those to one side. And here we have the float itself. I’m going to push through this hinge pin and then I can pull the whole of the float mechanism away. So the two things I’m concerned about are this float piston, it sits on top of the float there and rises and fall in it, and that let’s fuel into the float chamber. Now, the bottom screw that we took out has fuel jets in there; we are going to need to clean both of these two things thoroughly. For these jets, I’m just going to run a peace of wire through there and the one through the middle just to make sure we’ve got no debris in there, and give the piston a good clean with compressed air or you can get a special carburettor cleaner. It’s also a good idea to spray that cleaner or the compressed air around the rest of the carburettor. Just check the gasket seal as well, this one’s in perfectly good condition but if that’s looking bit of a mess than it’s probably a good idea to replace that as well.

Ok, Let’s put the carburettor back together. You can see on the float chamber, there is a shallow end and a deep end. Make sure you get the shallow end where the hinge of the float goes and then just put the screw back in. Don’t forget to reconnect the fuel line and, of course, put the air filter back on. There we go, top up with fresh fuel and we’re ready to go.

There we go, that should deal with most of the problems associated with getting your petrol lawnmower to start. Spare parts for Qualcast and lots of other makes of mower are available on the eSpares website.

Thanks for watching.