Carb Tuning Illustrated

The carb’s upper lid contains the diaphragm, needle, and diaphragm spring (fig. 1). These can be tuned but it is best you leave these alone for now.  We’ll discuss adjusting this area later. You'll be tuning the main jet, slow jet and idle screw. These are all located in the float bowl of the carb as shown in Figure 5 below.

Figure 1 Figure 2

To access the float bowls: With the carbs removed from the intake manifold and on the work bench, remove the four screws circled in red (fig. 2).   It is a good idea to idle the bike with the fuel petcock off until it dies to use up as much of the gas in the float bowls as possible. Or just remove the carbs and empty the bowls by loosening the bowl drain screws (pictured just left of the green circle in fig. 2.

Figure 3

Figure 4

Once you tip up the bottom of the carb, you'll see the float bowl with its four mounting screws.  They are highlighted with red rings (fig. 1).  The green ring highlights the idle mixture screw. Figure 3 shows the mixture screw removed. This is an air bleed that only cuts in at idle. On new unmodified carbs, this screw is covered by a metal plug that must be removed to access the screw adjustment.  It can be easily popped by drilling the surface and prying it out.  You'll need to adjust this screw as you tune.  You do not need to replace the plug.

Remove the four screws circled in red.  When you do, a little gas may come out of the bowl so take the appropriate precautions.  Sometimes these screws can be really stuck. I suggest using a very good Phillips screwdriver to get them out without stripping their heads.  A hand impact driver works well. Remove all four screws.

Remove the float bowl from the carb. Be careful, there's a gasket under the float bowl.  Pay attention to the way the accelerator pump fits into the black accordion rubber grommet because you'll be putting it back together soon.  Some carbs are not equipped with an accelerator pump. Pumps are found on many Honda carbs.

Figure 5

Figure 6

You should now see the jets inside the carb.  A carb basically is a control system of jets which feeds the engine at a particular throttle position.  At high throttle, it only sips from the main jet, at low throttle it only sips from the slow jet.  In the above photo, you can see the main jet, slow jet and our now uncovered idle screw.  These are your basic mixture "knobs" that you adjust.  If your engine needs fattening or leaning in the high (over 3000 rpm) range, you adjust the size of the main (red) jet.  If your engine requires fattening or leaning in the idle to 3000 rpm range, you adjust the size of your slow jet (yellow).  If you want to just set your idle mixture, you tweak the screw.  Idle mixture adjustment is best done when the bike is running and the whole thing is put together.  You can use a stubby screwdriver to tweak this screw.  Be careful not to burn your knuckles on the crossover pipe. The diagram at the bottom of the page shows how each jet works in conjunction with the throttle.

To 'adjust' the main and slow jets, just remove them with a flat screwdriver and drop in new ones. To adjust the idle screw, just turn it - do not remove it or you'll damage it.

 Remove the main and slow jets with your flat screwdriver. Put them out on the table.  They'll look something like this.

Figure 7

Figure 8

 The slow (not shown) has a little snoot on it but is basically a small version of the main.  Jets are just like water pipes.  They control the flow of fuel by constricting the diameter of the flow path.  A small hole means leaner mixtures while a big hole means richer mixtures.  Keihin jets have numbers on them. When you buy new jets, you need to tell your dealer what number you want.  They usually come in steps of 4.   The slow jets are more sensitive to steps because the smaller hole means a step yields a bigger percentage change in diameter.  Some guys drill out their jets with small drills (drill numbers 55 to 80).  I don't recommend this because you can only adjust richer and it is hard to tell what the jet is afterwards since the number doesn't match the hole anymore.  Save the time and get a dyno-jet kit.  On my Virago 1000 the air screw is set at 4.5 turns out.

So put it all back together and you're ready to go.  Make sure all the cables and hoses aren't pinched.

Figure 9