How to turn a lawnmower into a yard kart

Know the Basics

Your kids aren’t going to think riding around on an old mower is that cool, so some demolition and construction are in the cards. First, though, it helps to have a basic understanding of the components involved.

 

Engines:

Most non-racing go-karts are based on bits and pieces from small engines. Think of the ones powering lawnmowers, snow blowers and other yard implements. We prefer to use four-cycle engines for our karts. 

While there’s always a temptation to go for big power, we’ve found that 3- to 5-horsepower engines offer plenty of oomph in a small, light package. These engines can propel our yard karts to 25 mph, and to be honest that’s fast enough for our tastes. 

Engines come in two basic varieties, with either a horizontal or vertical crankshaft. The configuration determines where the drive output is located—on the side (horizontal) or bottom (vertical) of the engine. 

Vertical shaft engines are much more common—just about every push mower has one—so they’re usually cheaper. They also seem to come with more updated features like overhead valves. However, horizontal shaft engines are often easier to install and set up with a clutch and chain.

Clutch:

Now the power has to be transmitted to the wheels. The easiest way to do this is with a centrifugal clutch mounted on a horizontal shaft engine. Clutches are available in all shaft and chain configurations as well as belt-drive varieties.

The next rung on the ladder is a torque converter setup, which is basically a centrifugal clutch and variable-speed transmission. Torque converters work with cones that move in and out, first letting the belt slip before changing the effective pulley ratio to alter the speed of the final drive. They cost more and require more space and fabrication to install than a centrifugal clutch. However, they’re a good solution if you think you crave a faster kart.

Transmissions and Drive Systems:

Our favorite is the ubiquitous Peerless unit, offered by the company that makes Tecumseh small engines. It’s found in many rear-engine riding lawnmowers as well as walk-behind snow blowers. There are several versions, but most are close-ratio five-speeds with a sequential shift pattern and reverse. 

Newer ones have a neutral safety switch, and some feature a small disc brake. We’ve stayed away from transaxles because they’re bulky and harder to package, but we have seen them used successfully.

While belt drives are possible, a chain is a cheap, easy way to get power to the wheels. If you’re using lawnmower parts, just reuse the chain and gears. You’ll find that you can mix and match gears sourced from junk mowers to change ratios if necessary. 

Chains come in a variety of numbered sizes, and the most common sizes seem to be 35, 40 and 41. Your lawnmower shop or some Web-based suppliers can help sort out the sizing and bits needed for the chain. While a chain-breaking tool can make it easier to lengthen or shorten the chain, we’ve found that a grinder and quick-release connecting links can work well, too.

Differentials:

A lot of cheap go-karts are one-wheel drive, while most racing versions have a solid, locked axle. Riding lawnmowers, on the other hand, have differentials. 

Again, Peerless is the name most commonly found on these units, and we’ve used them several times. Most are traditional gear-type differentials, meaning they’ll allow for a lot of slipping. Even so, that’s still better than one-wheel drive. 

We’ve found that some of the budget-priced riding mowers feature spring-loaded pucks in their differentials, and these actually seem to work as low-buck limited-slip units. If you find one, you’ve got a hotrodded kart in the making.

Brakes:

The most primitive brakes are scrub brakes—metal flaps that rub against the tires. While they work okay, we prefer more elegant solutions such as band, drum and disc brakes. Most riding mowers use one of these setups, and adapting them to a kart is pretty straightforward. 

The low-buck solution: Grab something from the biggest junked mower you can find. If you want to go a more sophisticated route, the mail-order kart supply houses will have some good solutions.

Steering:

We often just adapt lawnmower steering components, retaining the basic geometry with maybe a few tweaks. This step can involve some fabrication, as we typically section the mower’s spindles to lower them. 

This is a great time to discuss chassis setup and steering with your kids. In no time they’ll be tossing around terms like “Ackermann,” “caster” and “camber.”

Wheels: 

Riding lawnmowers use wheels of nearly every size, so you can usually mix and match to get the desired look, handling and speed. Most mowers use a 5/8- or 3/4-inch shaft, so fabrication is rarely part of the mix-and-match routine.

Linkages:

Expect to modify and fabricate the brake linkage. You’ll probably have to tackle the clutch, shifter and throttle linkages, too. We like to use lawnmower leftovers for those first three jobs; for the throttle linkages, a bicycle brake or derailleur cable should work perfectly.

Donor Junk

We’ve found that smaller, rear-engine riding lawnmowers make great donor junk for a grassroots go-kart. We’re especially fond of some of the older Toro and John Deere models that feature simple folded chassis. (We tend to avoid mowers made up of complex stampings.)

Our preferred chassis generally come with engines that are too big and heavy for our tastes, so we often mount smaller engines. As a bonus, most of these mowers already have five-speed Peerless gearboxes. Brakes, steering and even wiring can often be modified and reused, so these mowers give you most of what you need to get started.

If we’re ditching the big engine on a rider, where do we turn? When opting for a centrifugal clutch and no transmission, we look at smaller horizontal shaft snow blowers, leaf blowers, pressure washers and chipper-shredders. If we’re going for a transmission, then just about any vertical shaft push mower will do the job—we prefer examples with overhead valves, though.

A bonus option is an electric start. Some push mowers feature a starter and charging system, though this setup does require a battery. Some snow blowers use similar systems, while others use 110-volt starters that only work with power cords. If you want to spoil your kids—or save your own arm—look for electric start.

There are myriad other options when it comes to finding donor junk. We recommend getting friendly with the local mower junkyard—every town has one—to scout and dream up ideas. The mower junkyard is going to charge more than the real grassroots sources, but the convenience may be worth it.

Finding Said Junk

While the local mower junkyard can be a great parts source, the real bargains are found at garage sales and just out by the curb. We’ve picked up many project starters at garage sales, usually for $5 to $20. If the junk doesn’t run, it’s probably not worth more than $10. 

We’ve also received many things for free when people know the kids are involved. We actually prefer to get stuff that doesn’t run because it’s cheaper and provides more teaching opportunities. 

The fixes are usually pretty simple. Probably nine out 10 non-running engines that we see just need their carburetors cleaned or their gaskets replaced.

Better and Faster

Of course, sooner or later a grassroots go-karter is going to start tweaking the engine to make more power. Whether you’re talking about a small engine or a big-block V8, the best gains come after some careful planning.

First of all, keep in mind that mower engines differ from car engines in that they’re optimized to run at one speed, usually 3600 rpm. Since horsepower equals torque times rpm, an easy way to get more power is to increase the speed of the engine. Some engines will handle more rpm just fine; we’ve had an OHV Briggs engine running past 5000 rpm for several years. Others, however, grenade pretty quickly when they get buzzed to a higher rpm.

The lawnmower bench racing community first recommends removing the governor. We generally agree, but with a few conditions. Remember that the governor’s job is to keep the engine running at 3600 rpm so the mower can handle a lawn’s rough spots.

If you have a younger kid, you may actually find that retaining the governor keeps things smoother. On the other hand, if you want snappier throttle response and a simpler linkage, bypass the governor. 

If you decide to do without the governor, be cautious regarding wide-open throttle. While conventional wisdom says that WOT is desirable, it may over-rev the engine and send you back out to the mower junkyard.

You’ll also need to modify the gear or pulley ratios. Mowers are geared for power at low speeds. Since you’re not cutting grass anymore, you can increase the pulley ratios and speeds without worrying about having enough power. 

If you’re using a centrifugal clutch and a chain, you’ll want to shop for a smaller gear for the axle. (Most centrifugal clutches come with one gear size, so you’ll need to work with the gear on the axle.) 

If a transmission or belt drive is part of your plan, it’s easy to swap around pulleys to get a ratio that delivers the desired speed and power. You can also alter tire size to optimize power and speed, as taller tires give a higher top end. It’s a trade-off, however, as tall tires typically hurt acceleration. 

We always start with a pretty slow ratio to keep our kids safe, then increase slightly as necessary. As a bonus, the kids learn some math as they figure out how pulley ratios and gear ratios affect acceleration and speed.

Get Out There

If it sounds like we’ve overthought grassroots go-karts in an effort to find every possible trick, we plead guilty as charged. We’re always after getting the best stuff for the lowest price, so why shouldn’t that apply to lawnmowers and go-karts?

These simple karts may not win a national points race, but they’re neat, handy tools that can be made out of junk. Now go build.