There are two available engines for the Kid Kart, the Comer C50 and C51. The C51 is the newer of the two. It has been reported by some that the C50 is faster than the C51, however we have no experience with the C51 so I don't have a direct personal comparison.
The Kid Kart carburetor is very sensitive to jetting changes. Jets are available from 54 through 58, smaller numbers are smaller sizes.
It is important to adjust the jet size with temperature and altitude variations, hotter temperatures or higher altitudes mean smaller jets. Also as you increase oil concentrations in your fuel mix you will need larger jets to get more fuel to the engine. Increasing oil concentration leans the fuel to air ratio.
The best way to adjust the jet if you don't have data acquisition is by lap times and spark plug color. When properly adjusted the spark plug should be a tan color, not black. Make sure and check the spark plug after some hard laps on the track, don't let the engine idle any longer than absolutely necessary before checking the plug.
The carburetor must be kept very clean. It has very small passages that easily get clogged with dirt. I recommend installing an in-line fuel filter between the gas tank and the carburetor. Also disassemble the carburetor at least every other race and clean it thoroughly with carburetor cleaner and blow out the passages with compressed air.
The Comer comes with two foam air filter elements installed back to back, both must be used to be legal. After every race remove the filter elements and hand wash them with liquid dish detergent to clean out the oil splash from the engine.
Most organizations specify a minimum combustion chamber volume of 7.5 cc. As the engines are run carbon builds up on the piston top and the combustion chamber causing the volume of the combustion chamber to decrease. Periodically you should measure combustion chamber volume and remove the cylinder and clean the carbon from the piston and combustion chamber.
The carbon can be cleaned with carburetor cleaner and a Scotch-brite pad. Never scrape on the piston or cylinder with a sharp tool, such as a screwdriver.
I found that 87 octane pump gas is by far the best performing fuel for these engines. I tried 93 octane pump gas and Phillips B-32 race fuel. The 87 octane pump gas reduced lap times over 1 second on a 58 second course compared to B-32.
I waited a long time to try this since I had always thought that too much octane was never a bad thing (except for the expense). One theory I have been told since finding this improvement is that lower octane fuels burn quicker, so you get more of the fuel burn while the piston is near the top of the cylinder, where it can do the most work. I don't know if this is true, but it sounds good and correlates with track results.
- Oil
More oil is better. We found significant performance improvements in oil ratios up to 8%. Above this the engine becomes very difficult to start due to spark plug fouling. I ran up to 10% oil concentrations but found little if any improvement in performance compared to 8%.
I ran Maxima Castor and Redline Synthetic. The castor will increase the carbon build up in the engine, thus reducing your combustion chamber volume. Some people use castor to get the combustion chamber volume close to 7.5cc through carbon build up. Redline synthetic doesn't build up as much carbon as the castor. I haven't seen a performance variation between the two, with the same combustion chamber volumes and gas/oil concentrations. We ended up running castor and checking the cc's periodically.
- Heat
These engines run best when hot. One trick to increase engine temperature is covering the flywheel air inlets with tape. We did this on the front and bottom, during cooler weather.
- Exhaust
Stock exhaust must be used, no modifications are allowed. The exhaust can bolts are prone to loosening, you should check tightness periodically, or drill and safety wire.
The exhaust gasket should be checked periodically especially if you remove the exhaust can. Make sure it isn't cracked or leaking. A leaking exhaust gasket can cause you to fail technical inspection.
One idea I had but never tried is to buy a spare exhaust can and install an EGT probe. Use this can during practice to optimize the jet size, then replace with the unmodified can for the race.
- Data Acquisition
We didn't have this at all. All of our testing was done with a stopwatch. Buy the time I learned enough to appreciate the value of data acquisition it was too late in the season to justify the cost.
If we were going to race kid kart another year I would have bought a data acquisition system that recorded RPM, Lap Times and one temperature. If you plan to move on to Junior 1 it would not be unrealistic to spend the money on a system that records wheel speed and g's. Having just begun using a MyChron Pro and Race Studio Software (which has all of the above) on our Junior 1 kart, I realize how much I was missing.
- Run It!
These engines run better the looser they get. Friction is your enemy.
- Sealing
Make sure you don't have any leaks in the engine, especially around the cylinder base gasket. If it appears that other karts are getting off the line from a standing start faster than you this is the first place to check. Look for oil around the mating surface between the cylinder and crankcase - you shouldn't see any. To be sure wipe the area clean and re-inspect.
If you do have a leaking gasket it is easy to replace. Remove the cylinder by first removing the gas tank, carburetor, exhaust can and then the four cylinder bolts. Lift straight up on the cylinder carefully guiding the cylinder off of the piston and rings. Clean the surface and install a new gasket, use a very slight amount of silicon gasket sealer if you like.
To reassemble make sure that the piston ring gaps are aligned with the locating pins in the piston ring grooves. Squeeze the rings together with your fingers while sliding the cylinder over the piston and rings. Lubricating first with Marvel Mystery Oil will make installation easier.
Once the cylinder is on but before you install the cylinder bolts rotate the engine so the piston is near top dead center. Then carefully tighten the bolts in a criss-cross pattern, making sure that the cylinder stays centered on the piston as best you can.
Reinstall the carb, exhaust can, and gas tank.
- Blueprinting
Don't waste your money. It is illegal in most organizations and from what I have seen it is merely a way for some less than reputable kart shops to rip you off. What some call blueprinting is merely cheating by illegally modifying the engine. It is not rebuilding the engine very close to factory specifications, as you might expect.
Some of the illegal modifications that are called "blueprinting" are:
- Cutting a slot in the exhaust gasket to allow leaks.
- Removing the key on the crankshaft that locates the flywheel and then rotating the flywheel to modify the ignition timing. The key can also be modified (undersized) and achieve the same affect.
- Installing stiffer clutch springs to increase the stall speed.
- Machining the clutch shoes to a smaller size.
As far as I know these are illegal modifications in every organization that has a rulebook.