Shifter Kart Engines: Opportunities from Spec to Open Classes - Engine Builder Magazine

Shifter Kart Engines: Opportunities from Spec to Open Classes

Like many forms of racing in this economy, shifter kart racing is in a state of change. With substantially more power, a six-speed sequential gearbox and 4-wheel brakes, they certainly delivered "bang for your buck" performance. Probably the biggest testament to the value of racing shifter karts is the career path of Scott Speed who went virtually from shifter kart champion to a ride in F1.

Initially the engine of choice in the U.S. was a 125cc motocross engine with Honda being the predominant brand. After a few years of “Moto” engines the dedicated Italian 125cc kart shifter motors migrated here from European Formula C shifter racing. They became known as ICC engines with brands like TM, Pavesi, Vortex, Maxter and many others. They were nearly race-ready out of the box and they thrived with broad, high rpm power curves. For the last few years these dedicated ICC kart motors have become the backbone of the Pro classes in shifter kart racing.

However, what started out as affordable, entry-level racing soon escalated to a point where fewer and fewer people could afford the top-notch equipment needed to be competitive. A national-level fully prepared ICC engine will cost a racer about $7,000, and if you plan on getting serious about your racing, you’re going to need a spare backup engine too.

A fully modified Moto engine costs about the same amount of money and produces equivalent power to an ICC, however they have much different torque curves. Open Moto engines have fallen out of favor with pro racers because the brutal low and mid-range torque of these engines negatively affect the handling of today’s “softer” shifter kart chassis that are developed mostly in Europe for ICC style engines.

Of that $7,000 engine cost approximately $5,000 is spent on the initial ICC engine purchase leaving about $2,000 for the engine builder. Let’s see…$2,000 to build a one-cylinder engine that you can hold in your hands – it sounds pretty attractive but read on before you sell your CK 10.

As shifter karts grew in popularity with Hollywood stars and NASCAR heros like Ricky Rudd going public about how great they are, many people with no karting or racing experience went out and purchased a shifter kart from a local shop, and took it out once only to realize how far over their heads they were. They immediately put their shifters up for sale and moved on to some form of entertainment a little mellower.

Kart shops now have a much better alternative for the newbies. They are called TAG (touch and go) karts and although they still have 125cc water cooled two-stroke engines, they are fitted with an electric starter and centrifugal clutch instead of a gearbox, and are much more friendly to drive although still extremely fast.

Many of these factors plus the downturn in the economy has spawned the Spec Honda classes that make up the bulk of shifter kart racing today. Spec Honda uses a CR125 Honda engine with few allowable modifications. Yet the racing is fierce, and you need a professionally prepared engine to be competitive. Spec Hondas are more cost effective for the racer for both the initial purchase and the race-to-race cost of maintenance. All successful Honda 125 kart engines begin life as engine kits from Honda.

For approximately $2600 you get a box full of individually packaged engine and transmission components that must be put together into a running assembly. There are basically two model years only that are used, the ’99 cylinder and head assembly and the ’01 cylinder and head assembly. Most of the other parts in the engine kit are interchangeable, but if you are planning on buying an old CR 125 motocross bike and using the engine, it had better be a ’99 or ’01 or you’re just spinning your wheels.

To take a closer look at today’s shifter kart engine technology we visited Darcy DeCoste’s engine shop that is dedicated to shifter kart engines and GP motorcycle two-strokes. Darcy’s shop is located in Santa Paula, California a little northwest of LA. From this location Darcy and his brother Vernon have turned out championship winning shifter kart motors of all kinds for years. Racers like Phil Carlson, Matt Jaskol, Jason Bowles, Brendon and Ryan Phinny and Conner DePhilippe are just a few riding DDR power to victory.

We first wanted to get a grip on all of the power numbers being thrown around the pits to see what engine builders should be shooting for. Darcy has a water brake chassis dyno that once belonged to Yamaha. He likes it because he can do steady state testing, and he can also run the kart through the gears and check the clutch. He claims that his water brake is 10-16% more conservative than most DynoJets, and the power numbers he shoots for with a fully prepped ICC or “open” moto engine is in the 43-44 hp range. Darcy said that both styles of engines make about the same peak power.

The difference is that the moto motor does it with cylinder pressure (hence the massive low and mid-range torque), and the ICC motor does it with rpm. Of course with rpm comes cost – a competitive pro racer will require that the $400 connecting rod in his ICC engine be replaced every one or two races. On the other hand a good competitive Spec Honda will show 35-37 hp on Darcy’s dyno and the bottom end is good for quite a few race weekends.

Let’s look at shifter kart engines from an engine builder’s perspective. First, it should tell you something that there are only a handful of national event winning caliber builders in the nation. I believe the reason for this is that it takes a lot of specialized knowledge to be competitive here. Knowledge gained only through years of experience and testing. Sure, just about anyone can assemble a Honda engine kit and make it run – that’s not the level of sophistication we are talking about here.

Darcy pointed out that you could take an engine from any of the top shops, and if you don’t know how to jet the carburetor and gear it, you will most likely be back in the middle of the pack somewhere. So for these reasons, the importance of tuning and track setup cannot be underestimated in shifter kart racing today.  

For years the traditional ways you increased the power of a two-stroke engine was to raise the compression, modify the squish band in the cylinder head, grind out the transfer and exhaust ports in the cylinder, tweak the ignition timing and modify the reeds and reed cage (in reed-type engines).  To some degree you still do these same mods, but with ICC motors to a much smaller degree.

Darcy says that they change port widths, heights and angles and reshape the porting in the crankcase, but in some cases the ports are just changed .005?. A slip of the grinder can literally kill a cylinder from being competitive. The ICC cylinder heads are pretty good the way they come, but Darcy will machine them to change compression depending upon the driver and track.

Open class Moto motors are different animals. They require extensive cylinder head machining, inlet track port work including splitters and stuffers to increase port velocity. Reed tension and cage design is extremely important and whereas ICC motors are limited to fixed (locked) ignition timing, there are programmable ignition systems for the Moto engines that make a huge difference in performance.

The bulk of Darcy’s work these days is spec Hondas that go out the door for $4,800 for a national level engine and $4,300 for a club-level piece. With the rules being so restrictive to internal engine modifications it’s a wonder that he can increase performance as much as he does. There are a couple of open areas in the rules that Darcy exploits to good advantage.

The rules allow you to remove and plug the power valve (a variable sized exhaust port that broadens the throttle response for trail riding motorcycles). The size and shape of these plugs have a large affect on power output. Another area where Darcy’s expertise pays off is with reed cages and reeds. The rules state that the reed cages must be stock although different year cages can be used. And reeds are totally non-tech items and Darcy has his own reeds manufactured and juggles various reed cages to suite track conditions and driver preferences.

Engine assembly is pretty straight forward only extremely precise. The crankshaft assembly is trued to within .0003?. The crankshaft seals are pressed into the cases perfectly straight because crankcase compression is what determines how much power the engine will produce. There is an allowable ignition modification in the rules that Darcy also makes. The remainder of the spec Honda mods are external with one of Darcy’s jetting kits and attention to detail like the silencers that go onto the spec pipe. After years of testing Darcy recommends and uses the M4 silencer.

To be in the shifter kart engine building business doesn’t require much specialized equipment from what a normal engine shop would have except for a small variable angle grinder that can get into tight places for porting cylinders and cases. Good measuring equipment and a surface plate are must have items, as are some fixtures for assembling crankshafts. However, the key piece of equipment is a quality kart engine dyno. The only way you will ever know what is good and what is bad is to test. Then back your dyno testing up with track testing. When you start to see a correlation between the two, you will be on your way to building the knowledge base necessary to be competitive in the world of shifter kart racing.


Darcy DeCoste Racing

333 S. 11th St.

Santa Paula, CA 93060

Phone: 805-933-2616

Len Emanuelson has spent the past 37 years reporting on the performance and racing market. He enjoys racing his shifter kart whenever he gets the chance.

about 10 years ago shifter kart racing burst onto the american karting scene promising a driving and racing experience more like an open wheel formula car than the traditional 100cc or briggs-powered kart could provide. with substantially more power, a six-speed sequential gearbox and 4-wheel brakes, they certainly delivered. darcy decoste has been building racing two-strokes for many years. his water brake dyno is an essential tool for building race-winning engines. by using a chassis-style dyno he can feel things like clutch engagement and shift quality that would be difficult to feel with an engine dyno.this tm cylinder head is typical. it has a bowl-like combustion chamber and squish band around the outside that provides the quench for good combustion. as simple as it may look, there is plenty of power to be gained with the proper head modifications. here are the two major types of shifter motors in competition today. on the left is a honda cr125 
	</div><!-- .entry-content -->

		<div class=

You May Also Like

The Road to AAPEX Season 2, Ep 3

Just 3,356 Lincoln Blackwoods exist in the world. For comparison, the Ford F-150—the Blackwood’s inspiration—has spawned more than 40 million since its launch in 1948. Guess which one is harder to track down parts for? Babcox Media’s Joe Keene, an ASE-certified technician, has tracked down his fair share of elusive parts, but fixing up a

Just 3,356 Lincoln Blackwoods exist in the world. For comparison, the Ford F-150—the Blackwood’s inspiration—has spawned more than 40 million since its launch in 1948. Guess which one is harder to track down parts for? Babcox Media’s Joe Keene, an ASE-certified technician, has tracked down his fair share of elusive parts, but fixing up a Lincoln Blackwood for a cross-country drive on the Lincoln Highway to AAPEX 2023 is on another level.

The Road to AAPEX Season 2, Ep 2

This year’s Road to AAPEX is a tale of two roads: One metaphorical, paved with questions that face the automotive aftermarket like the impact of EV adoption and sustainability efforts; and one quite literal, that was paved at the start of the 20th century and conceptualized the first transcontinental highway. The Lincoln Highway, which begins

The Road to AAPEX Season 2, Ep 1

Last year, the idea was simple: Find a junker, fix it up with the best from the automotive aftermarket, and drive it to Las Vegas for AAPEX 2022. This year, it’s anything but simple. Related Articles – The Evolution of Pro Mod Diesels – Top Fuel and Funny Car Engines – Race Oils The automotive

What’s a Ford Sidevalve Engine?

It looks like an ordinary inline 4-cylinder flathead engine. Essentially it is, but it has quite a cult following here in the UK.

The Drag & Drive Revolution

Following that first drag-and-drive event back in 2005, spinoffs of Drag Week have been happening all over the country, and the world, both large and small. In recent years, the trend has been completely blowing up!

Other Posts

Facts About Engine Bearings

The experts all agree that cleanliness is the most important factor during installation, and the lack thereof is the most common problem that leads to bearing failure. But measuring is just as critical.

Does Connecting Rod Length Matter?

Over the years, we’ve gotten asked numerous times about connecting rod length and the impact that has on an engine’s horsepower and durability. As it turns out, this question is often overthought. It’s not so much the connecting rod length that matters as much as it is the correct piston pin height. The connecting rod

LTR Engine Build

This Late Model Engines build is centered around Concept Performance’s new LTR block, which is the first aftermarket as-cast aluminum Gen V LT block. 

A Look at Lead Times

Lead times are no longer months upon months as they were in the middle of 2020 and throughout 2021, but the situation is still of some concern, and it’s forced engine builders to get creative at times.