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Shifter Kart Engines: Opportunities from Spec to Open Classes

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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.

Source:
Darcy DeCoste Racing
333 S. 11th St.
Santa Paula, CA 93060
Phone: 805-933-2616
www.darcydecosteracing.com

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.

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