Small Engine Repair

In the middle of fixing  the "fire engine" bike

Whether you enjoy working in the garage or not, it is not uncommon to find yourself with a gas engine that refuses to start. Even if it is just that lawnmower that worked fine last year, sat through the winter, and now you are wearing out your arm trying to bring it back to life. I thought it would be fun to put together a little tutorial on small engine repair. I have found that a small sampling of the basics will cover a large percentage of the problems you will face.

  Over the last year I have had a couple friends bring 4 wheelers to the orange garage for repair. The great thing about these small engines is that they are infinitely more simple than fuel injected cars. The first one I fixed was a lawnmower engine off of a little buggy. I worked slow and took lots of pictures so I could put it back together. After bumbling my way through it I was surprised to find that some simple fixes were actually able to get it running. Since then I have repaired gokarts, lawnmowers, weedwackers, ATV's and motorcycles and for the most part they all have the same parts and the same problems.

To run, an engine needs a spark, compression, and good carbureted gas and air. When I troubleshoot something it is all about eliminating variables so I will start with the spark. 1) remove wire from top of spark plug 2) remove spark plug 3) With it still out, put the wire back over the top 4) touch the plug to the engine block or anything where it will be grounded 5) crank the engine

 If you see the spark then you don't need to spend any more time worrying about gap lengths, wires or coils. Hand tighten the plug back in place, wrench it no more than a quarter turn past that, and make sure the wire seats all the way back down over the top.

Before you actually replace the sparkplug you will want to take a look at the compression. Most people don't have a compression test kit at home but many auto parts places will loan them out, I got mine on the cheap at HarborFreight. While the spark plug is out you simply screw in the attachment in its place and crank the engine.

If you dig around online or in a good service manual, you can find out what psi you should be getting. A lot of these smaller engines use an automatic mechanical decompression. Basically the exhaust valve purposely doesn't seat all the way when starting. This makes the engine much easier to start so you don't throw your back out ( I watched my dad do that trying to start a snowmobile once) but it will also give you a reading that seems too low. Even if your engine has automatic mechanical decompression, it should still give you at least 15 psi. This one reads 60 psi so I am going to call it good and move on.  If it was below 15 or if you can find a PSI value from the manufacturer and you feel it is reading low, then I would do a leak down test as follows: 1) manually put engine into TDC ( piston all the way up with exhaust and intake valves in closed position) 2) attach air compressor 3) listen to see where air is 'leaking down'
If it comes out exhaust then the exhaust valve is not seating. If it comes out at the air filter than the intake valve is not seating. If it comes out of the oil cap then the piston ring needs to be replaced. It may also come out around the head gasket. Poorly seated valves may just need to be cleaned and you can often visibly see large carbon deposits on them.

These first two tests go quick because they usually are not the problem. Most of the engines I have looked at were not getting fuel because of a dirty carburetor. First, a simple crash course in how a carburetor works. The car you drive is probably fuel injected, meaning it actually sprays fuel right into the cylinder, as opposed to most small engines (and older cars) which are carbureted. Basically the carburetor is in charge of mixing gas and air together right before it enters the cylinder. To do so the carburetor literally sucks the gas through tiny holes called jets. As the gas is sucked through the jets it turns into a mist so that it will burn well once it flows into the engine. Unfortunately the sucking power of the carburetor is limited so it can't suck it all the way from the gas tank. To make carburetor's job easier, it has what is called a fuel bowl sitting directly below it. The fuel bowl works very much like the tank on your toilet. It has a float valve that allows gas to fill the fuel bowl until its floats close the valve. This maintains a consistent amount of gas for the carburetor to conveniently suck from. The bottom of the fuel valve will often have a tube coming out where excess gas can leak out (like the overfill hole near the lip of your sink) and will also have a screw or bolt which you can open to drain gas from the bottom (a good idea for storing the engine,draining bad gas, or testing to see if gas is making it in at all)

The carburetor is found in between the engine cylinder (on the right with fins) and the air filter (to the left out of the picture). You will commonly see one or two throttle cables, a choke cable, fuel line, excess fuel drain tube, intake tube (from air filter), and exhaust tube (going to the cyliner) coming off of the carburetor. Lawn mower engines often have no cables but will have the choke and throttle adjustments directly on the carburetor with the air filter directly on top.

The carburetor often sits below the fuel tank, you may have to rip it out for better access. Usually not that big of a deal. You are going to want to drain the old gas anyway and this will make the job easier.

To be thorough you will have to remove the carburetor. Take a couple pictures of where things go, be brave, and yank her out. The fuel bowl can be see here below the level of my thumb nail.

With the cover for the fuel bowl removed you can see the floats and their little valve that they operate. It is not uncommon for debri to get caught in this valve causing continually leaking gas (which is why it is not a bad idea to clean things before you take them apart, being particularly careful with fuel lines that have collected oil and grime on the outside). I have also seen these get stuck in the closed position which prevents the fuel bowl from filling which will stop an engine in its tracks. With the carburetor out and fuel bowl removed, I like to poor gas in the fuel line and test this valve, moving the floats up and down.

In between the floats you can usually see two different tubes which you can screw out (only one is visible here). You will want to remove both.

With them removed you can now see the actual jets. These holes are often the size of a needle head. This is the weakest link in getting fuel to the engine. If you skim this article here is the most important part, YOUR ENGINE PROBABLY WILL NOT START BECAUSE THESE HOLES ARE CLOGGED.

Get yourself some carb cleaner to blast and soak the jets. While you have the carburetor out take the time to also blast its many different nooks and crannies, both from the fuel bowl side and from the venturi (main channel) of the carburetor. You may also find one or several diaphragm gaskets on more advanced carburetors. If they appear damaged the engine will probably still start but may idle funny and or be sluggish or even die when you punch the gas. These are often fairly cheap to replace.

It is also worth the time to take a look at the air filter and its housing to make sure air can easily flow through (and only air). This one had a puddle of water, gas, and oil sloshing around the bottom...not an ideal set up.



With all that done, put her back together and cross your fingers. In most cases the engine should be able to start up but may need some tuning. If you follow the throttle cable to the carburetor  you may be able to find a set screw which limits the motion of the throttles rest position. This is the idle set screw. Play with the set screw until the engine RPMs at idle are a healthy margin higher from where the engine dies. On the side of the carburetor you will also find another screw that adjusts how rich or lean the fuel mixture is.

This carburetor has its fuel mixture screw just above the MIKUNI lettering. The simplest way to describe this adjustment is to just play with it until you like the sound, probably something worth YouTubing if you would like to actually listen to an adjustment.






I described how to be thorough and break down the carburetor but if the engine is not so bad off you may be able to take some shortcuts that don't involve you actually taking the carburetor and gas tank out. A good abbreviated approach would be to drain the old gas (be sure to not just drain the tank but also the fuel bowl), add fresh gas with SeaFoam added to it, remove the air filter and spray engine starter down the hatch and try to start the engine that way. It is also possible to pump up carb cleaner through the fuel bowl drain tube with carburetor in place, just be careful not to flood the engine in the process.




Sometimes the biggest obstacle is just being afraid that you will break something or never be able to put it back together. That usually isn't the case and 9 times out of 10 you won't regret the "just go for it" attitude. If I waited for a mentor every time I tried something new in the garage I would never get anything done, so hopefully I have inspired someone to tackle that dusty lawnmower in the corner of the garage instead of putting it on the curb or paying someone else to take care of it. And if I didn't then feel free to give me your "irreparable" four wheeler that you aren't using anymore...

I tried to keep this crash course tutorial brief so if you have questions or get stuck halfway through a project feel free ask any questions and I will try my best to answer!