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Comer Carb Tuning

Kid Kart carbs and tuning tend to be the weakness of most teams. There are some basics we can share. These basics should significantly improve the tuning you do throughout the weekend so your jetting is perfect for the main event. 

The basics - The Kid Kart carb made by Dellorto is a very simply float style carb. It has very few parts.  Tuning it consists of changing the jets and that is about it.  You'll read all sorts of internet lore about tilting it one way or another, or spacing it off the engine on the manifold a quarter inch or so, etc.  Don't worry about any of this. Here is what you do need to worry about. And I will cover all of these things in detail below:

  • Keep it clean, there is nothing worse than some debris in the carb affecting performance.  It is a good idea to take it apart and clean it before every important session like qualifying, heat races and finals. 
  • Jet it properly - we'll discuss this in detail below, but make sure you have an appropriate jet for the conditions
  • Have it secured - so many people forget to tighten the retaining bolt and the carb falls off.  Don't be this tuner! 
  • Keep you K&N Filter clean

If you have covered these basics then there really isn't much more you can do for the carb. It should be dialed in.  If these basics are covered and it is acting funny there are a few things to look for.  One is your gas cap, is the vent working in it?  Sometimes a gas cap vent will lock up essentially locking your fuel supply system so the engine starves of fuel.  Make sure your vent is working.  The other common reason that makes a carb not work is a bad float needle.  The float needle can form a ridge after a while (like a month) and stop working correctly.  This is a good item to replace regularly.  Finally, check the internal filter screen on the carb for debris. Clean it.  If you carb still doesn't work after all this it may be time to retire that carb for the weekend and send it off to an expert.  

Carb Cleaning - Cleaning Comer carb is a simple 10 minute task.  Turn off your fuel shutoff valve. Remove your K&N Filter, loosen the carb retaining bolt, remove the two screws holding the black throttle cable/slider assembly to the top and remove the assembly.  Remove the fuel line, then pull off the carb.  You can turn it upside down at this point to drain out the excess gas. Next you will remove the two screws holding the black bowl at the bottom.  You'll see a little pin holding the white float in place.  Use something small like the end of a paperclip to slide that out.  Now pull the white float, and float needle out.  Next you can remove the jet.  Finally, you can remove the fuel inlet cap to expose the filter screen under it, and remove the filter screen.  Now your carb is completely apart.  Using brake cleaner (easier on plastic than carb cleaner) and air you can begin by blasting out every orifice in the carb with brake cleaner then blasting it clean with air, then doing the same to each part as you reassemble the carb. When you put the carb back on the manifold tighten the retaining bolt just enough so that the carb is very hard to twist on the manifold, but you still should be able to twist it.  We see people every year breaking carbs from being overtightened.  Don't overtighten it.  In fact, buy the PKT Carb Retaining Bracket and then you'll have to tighten the carb even less, which is less risk of breaking it. 

Proper Jetting - Here is some info that will likely change your entire outlook and effectiveness on carb jetting.  First, did you know that the stamped number on top of a jet is practically meaningless?  I honestly don't even know why they bother stamping them, they are rarely accurate. So what do you do?  You buy a Jet Holder like this, then you go to Mcmaster-Carr and buy a set of pin gauges.  What are pin gauges?  These are basically little pins in various sizes you poke through the jet to figure out it's exact size. WKA and Kid Kart Nationals use a "No Go" size of .0260".  This is just a tiny but smaller than a .61mm.  So according to the rules you can use a 60 size jet, and actually a large 60 that is just a but smaller than 61.  So I suggest buying a .0260" Pin Gauge.  You can throw away any jet you have that this Pin Gauge will poke through.  Whatever is left are your legal jets.  So order these size Pin Gauges from McMaster:  

  • .0260" - This is your "no go" jet
  • .60mm
  • .59mm
  • .58mm
  • .57mm
  • .56mm
  • .55mm
  • .54mm

Now you can measure jets until you find jets that fit your pins from .54mm to .60mm, and these will be your jets.  Throw the rest away or put duplicates down the other side of the PKT Jet Holder.  The jets you now have in your PKT holder are all you'll ever need.  Now, how do you use them?  

Tuners like to monitor all sorts of environmental factors when evaluating which jet to use.  Some by temp, some by humidity, some people just say "the air feels heavy today".  This is all great but there is only one true measurement that takes everything into account and is a consistent means by which to tune.  It is "Air Density".  Air Density takes into account temperature, pressure, and humidity.  Air Density can be shown in a simple and easy measurement to understand called "Density Altitude".  Density Altitude is the equivalent altitude you are in right now.  So you may be standing at sea level, but if it is very hot and very humid, with low pressure, then the air is actual similar to the air you might be in if you were on top of a mountain.  Of course on top of a mountain you would use a very lean jet.  The effectiveness and simplicity of the Density Altitude usage with jets is so simple it is actually hard to explain. But all you need to know is that you can use Density Altitude to pick the right jet.  You'll need to make up your own chart, but it will look something like this:

  • Altitude = Negative 1500' = 60 Jet
  • Altitude = Negative 500' = 59 Jet
  • Altitude = 0' = 58 Jet
  • Altitude = 500' = 57 Jet
  • Altitude = 1500' = 56 Jet
  • Altitude = 2500' = 55 Jet
  • Altitude = 3500' = 54 Jet

Of course there are other factors. The main one is track type.  A track with elevation changes may need to be jetted a little rich to get up the hills better.  But otherwise once you make your chart based on your own testing, and use Density Altitude you should be almost right on the money every time with jetting.

So how do you get "Density Altitude"?  With the Kestrel Racing Weather Gauge.  We sell these here.  For Kid Karts this will be one of the most important tools in your tool box.  

K&N Filter Cleaning - The K&N Filter needs to have as little restriction as possible.  Don't oil it.  Keep it dry.  Clean it out a few nights before you race with brake cleaner.  Clean the inside and the outside. Then use an air nozzle to dry it out. You'll want it bone dry come race day. It is a good idea to have several of them all with this treatment and switch them out over the course of a race weekend. 

This sums up carb treatment.   The carb is one of the parts of your kart that should get the most attention between sessions.  The tips above should keep you tuned up!