Tuning my Carb, or One Part of the System Affects the Whole System


Last year, I did some mods to my DR650 to get a bit more power and better throttle response.  These mods included cutting off the top of the airbox and installing a larger main jet and aftermarket needle in the carburetor.  I used the jet kit from ProCycle.  I wound up using the largest main jet in the kit, and also set my needle at the highest position.  Basically, tuning the motor to run as rich as possible given the items in the kit.  It ran way better than stock.  Even at full throttle, there was change in the motor response with minor throttle changes.  This is opposed to stock, where the last 1/3 of throttle twist did nothing.  Still, I suspected something was still off.   It ran fine, but I had a nagging feeling something could be better.  I let it ride, while I rode.

When I cut the top of my airbox out, I cut the whole thing out.  This dude don’t mess around.  The instructions in the kit called for drilling two large holes in the top.  OK, I’m a little over zealous and I’m probably letting in more air than what the kit accounted for.  OK, It’s time to see what’s up.

Before I get into the steps, I should stop for a second to mention that there are several things to consider in how fuel and air move through the motor.

  • The carb floats determine how high the fuel level in the bowl is.  This should not normally be messed with, but it should be checked to ensure it is within the manufacturer’s tolerance.
  • The main jet mostly controls how much fuel (in proportion to air) is delivered at wide open throttle.
  • The needle shape and vertical position affect fuel delivery at mid throttle.
  • The idle (or pilot jet) affect the fuel delivery at low throttle.
  • The fuel mixture screw more finely tunes low throttle mix.
  • Restrictions on the intake (choked airbox or dirty air filter) affect how much air gets to the carb.
  • Restrictions on the exhaust affect how efficiently burnt fuel-air is expelled.

Anywho, This weekend I went through the laborious procedure of reading my spark plugs, which is as follows:

  1. Place a piece of tape on the throttle cable housing, and another on the twist grip.  Turn the grip just enough to take up the slack in the throttle cable, then mark a line across both pieces of tape.  This is the idle position.
  2. Open the throttle all the way, then mark a line on the twist grip, inline with the mark on the throttle cable housing.  This is full open obviously.
  3. Mark a line on the twist grip half way between idle and full positions.  I’m guessing you’ve figured out this is the half-open position.
  4. Start the bike and ride it until the motor is at normal operating temperature.
  5. Going down a road with no cops, kids, or pets around, put the bike in high gear and get to a speed where the motor is just above idle.
  6. Open the throttle full open and hold it there until the motor is reaching max RPMs, or whatever the speed limit is.  *winks*
  7. Pull the clutch and hit the kill switch at the same time.  The idea is to stop the motor as abruptly as possible as it’s running with the throttle wide open.
  8. Coast to a nice shady place to stop and read the plug.
  9. Now that the bike is stopped, remove both side covers, the seat, detach the fuel line, remove the gas tank, and remove the spark plug without burning your fingers.
  10. The deposits on the plug reveals a lot about how the motor is running.  You can google for more details, but basically, if the electrode is white, the fuel mix is too lean.  If it’s black, the fuel mix is too rich.  We’re looking for a nice medium-light grey.
  11. Install a larger or smaller main jet as required, reassemble, then run through the same steps again.  Replacing a main jet requires digging into the carb, which is a delicate device…  Go slow and keep it clean.
  12. Once the full open position is tuned, which is controlled almost entirely by the size of the main jet, then move on to the half-open test.  This is done the same way, but is done to determine the needle position.  The needle has a  number of slots in it, in which a tiny cir-clip is placed.  The higher the clip sits, the lower the needle sits in the main jet inlet.  A higher needle gives a richer mixture.  So, do the test again, but at half throttle this time.  Adjust the position of the clip as needed.
  13. Once the mid range is good, the next thing to do is set the idle mixture.  This is done by turning the pilot (idle) screw in or out until the motor runs smoothly.  This is a ‘by the ear’ thing.  Turn it too far in, and the motor will die.  Turn it too far out and the motor will run poorly or die.  You may need to tweak the idle speed screw too so the motor stays running at a good idle RPM.  Set the pilot screw half way in between those two extremes.  If you can’t tell a difference after turning the screw out 2 full turns, then you need a bigger pilot jet.

This process is a pain.  I had to take my tools with me, and I had to disassemble my bike out of my shop.  I did make things a bit easier on myself by only installing one bolt where two is the norm, and finger tightening at that.  Still, PITA.

I stopped after doing the full open test.  The plug was light grey, almost white.  I guess I need a bigger main jet because I removed the entire top of my airbox.  It’s not awful, but it ain’t right.

As an aside, I actually cut the top of my airbox out before getting the jet kit.  It didn’t just make a small difference… it made the bike unridable.  The motor was sick.  Not skatehead sick, dog with cancer sick.

Any changes you make to either the intake or the exhaust must be matched with changes to the carb.  I had a WR425 one time that I bought used.  The previous owner had installed a high flow aftermarket exhaust on it and apparently didn’t re-jet the carb accordingly.  It was a pain to get started, and when it did start, the exhaust header would glow red hot if you weren’t rolling.  I tuned the carb and proceeded to have more fun than a drunken sailor at a massage parlor.

So kids, the lesson here is twofold:

  1. Nothing exists unto itself.  A change here will have an effect there.
  2. The right thing to do may not be easy, but the result is worth the effort.
Categories: DR650 Modifications, Motorcycles

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